Transcriber's note: Text enclosed by underscores is in italics (_italics_).
A single underscore NH_3 introduces a subscript,
and a caret 30^a a superscript.
* * * * *
THE PRINCIPLES OF
BIOLOGY
BY
HERBERT SPENCER
[Illustration]
IN TWO VOLUMES
VOLUME I
NEW YORK AND LONDON
D. APPLETON AND COMPANY
1910
COPYRIGHT, 1866, 1898,
BY D. APPLETON AND COMPANY.
PREFACE
TO THE REVISED AND ENLARGED EDITION.
Rapid in all directions, scientific progress has during the last generation
been more rapid in the direction of Biology than in any other; and had this
work been one dealing with Biology at large, the hope of bringing it up to
date could not have been rationally entertained. But it is a work on the
_Principles_ of Biology; and to bring an exposition of these up to date,
seemed not impossible with such small remnant of energy as is left me.
Slowly, and often interrupted by ill-health, I have in the course of the
last two years, completed this first volume of the final edition.
Numerous additions have proved needful. What was originally said about
vital changes of matter has been supplemented by a chapter on "Metabolism."
Under the title "The Dynamic Element in Life," I have added a chapter which
renders less inadequate the conception of Life previously expressed. A gap
in preceding editions, which should have been occupied by some pages on
"Structure," is now filled up. Those astonishing actions in cell-nuclei
which the microscope has of late revealed, will be found briefly set forth
under the head of "Cell-Life and Cell-Multiplication." Further evidence and
further thought have resulted in a supplementary chapter on "Genesis,
Heredity, and Variation"; in which certain views enunciated in the first
edition are qualified and developed. Various modern ideas are considered
under the title "Recent Criticisms and Hypotheses." And the chapter on "The
Arguments from Embryology" has been mainly rewritten. Smaller increments
have taken the shape of new sections incorporated in pre-existing chapters.
They are distinguished by the following section-marks:--§ 8a, § 46a, § 87a,
§ 100a, § 113a, § 127a, §§ 130a-130d. There should also be mentioned a
number of foot-notes of some significance not present in preceding
editions. Of the three additional appendices the two longer ones have
already seen the light in other shapes.
After these chief changes have now to be named the changes necessitated by
revision. In making them assistance has been needful. Though many of the
amendments have resulted from further thought and inquiry, a much larger
number have been consequent on criticisms received from gentlemen whose aid
I have been fortunate enough to obtain: each of them having taken a
division falling within the range of his special studies. The part
concerned with Organic Chemistry and its derived subjects, has been looked
through by Mr. W. H. Perkin, Ph.D., F.R.S., Professor of Organic Chemistry,
Owens College, Manchester. Plant Morphology and Physiology have been
overseen by Mr. A. G. Tansley, M.A., F.L.S., Assistant Professor of Botany,
University College, London. Criticisms upon parts dealing with Animal
Morphology, I owe to Mr. E. W. MacBride, M.A., Fellow of St. John's
College, Cambridge, Professor of Zoology in the McGill University,
Montreal, and Mr. J. T. Cunningham, M.A., late Fellow of University
College, Oxford. And the statements included under Animal Physiology have
been checked by Mr. W. B. Hardy, M.A., Fellow of Gonville and Caius
College, Cambridge, Demonstrator of Physiology in the University. Where the
discoveries made since 1864 have rendered it needful to change the text,
either by omissions or qualifications or in some cases by additions, these
gentlemen have furnished me with the requisite information.
Save in the case of the preliminary portion, bristling with the
technicalities of Organic Chemistry (including the pages on "Metabolism"),
I have not submitted the proofs, either of the new chapters or of the
revised chapters, to the gentlemen above named. The abstention has resulted
partly from reluctance to trespass on their time to a greater extent than
was originally arranged, and partly from the desire to avoid complicating
my own work. During the interval occupied in the preparation of this volume
the printers have kept pace with me, and I have feared adding to the
entailed attention the further attention which correspondence and
discussion would have absorbed: feeling that it was better to risk minor
inaccuracies than to leave the volume unfinished: an event which at one
time appeared probable. I make this statement because, in its absence, one
or other of these gentlemen might be held responsible for some error which
is not his but mine.
Yet another explanation is called for. Beyond the exposition of those
general truths constituting the Principles of Biology as commonly accepted,
the original edition of this work contained sundry views for which
biological opinion did not furnish any authority. Some of these have since
obtained a certain currency; either in their original forms or in modified
forms. Misinterpretations are likely to result. Readers who have met with
them in other works may, in the absence of warning, suppose, to my
disadvantage, that I have adopted them without acknowledgment. Hence it
must be understood that where no indication to the contrary is given the
substance is unchanged. Beyond the corrections which have been made in the
original text, there are, in some cases, additions to the evidence or
amplifications of the argument; but in all sections not marked as new, the
essential ideas set forth are the same as they were in the original edition
of 1864.
BRIGHTON,
_August, 1898_.
PREFACE.
The aim of this work is to set forth the general truths of Biology, as
illustrative of, and as interpreted by, the laws of Evolution: the special
truths being introduced only so far as is needful for elucidation of the
general truths.
For aid in executing it, I owe many thanks to Prof. Huxley and Dr. Hooker.
They have supplied me with information where my own was deficient;[1] and,
in looking through the proof-sheets, have pointed out errors of detail into
which I had fallen. By having kindly rendered me this valuable assistance,
they must not, however, be held committed to any of the enunciated
doctrines that are not among the recognized truths of Biology.
The successive instalments which compose this volume, were issued to the
subscribers at the following dates:--No. 7 (pp. 1-80) in January, 1863; No.
8 (pp. 81-160) in April, 1863; No. 9 (pp. 161-240) in July, 1863; No. 10
(pp. 241-320) in January, 1864; No. 11 (pp. 321-400) in May, 1864; and No.
12 (pp. 401-476) in October, 1864.
_London, September 29th, 1864._
CONTENTS OF VOL. I.
----
CHAPTER PAGE
I. ORGANIC MATTER 3
II. THE ACTIONS OF FORCES ON ORGANIC MATTER 27
III. THE RE-ACTIONS OF ORGANIC MATTER ON FORCES 45
III^A. METABOLISM 62
IV. PROXIMATE CONCEPTION OF LIFE 78
V. THE CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES 91
VI. THE DEGREE OF LIFE VARIES AS THE DEGREE OF 101
CORRESPONDENCE
VI^A. THE DYNAMIC ELEMENT IN LIFE 111
VII. THE SCOPE OF BIOLOGY 124
PART II.--THE INDUCTIONS OF BIOLOGY.
I. GROWTH 135
II. DEVELOPMENT 162
II^A. STRUCTURE 181
III. FUNCTION 197
IV. WASTE AND REPAIR 213
V. ADAPTATION 227
VI. INDIVIDUALITY 244
VI^A. CELL-LIFE AND CELL-MULTIPLICATION 252
VII. GENESIS 269
VIII. HEREDITY 301
IX. VARIATION 320
X. GENESIS, HEREDITY, AND VARIATION 336
X^A. GENESIS, HEREDITY, AND VARIATION--_Concluded_ 356
XI. CLASSIFICATION 374
XII. DISTRIBUTION 395
PART III.--THE EVOLUTION OF LIFE.
I. PRELIMINARY 415
II. GENERAL ASPECTS OF THE SPECIAL-CREATION-HYPOTHESIS 417
III. GENERAL ASPECTS OF THE EVOLUTION-HYPOTHESIS 431
IV. THE ARGUMENTS FROM CLASSIFICATION 441
V. THE ARGUMENTS FROM EMBRYOLOGY 450
VI. THE ARGUMENTS FROM MORPHOLOGY 468
VII. THE ARGUMENTS FROM DISTRIBUTION 476
VIII. HOW IS ORGANIC EVOLUTION CAUSED? 490
IX. EXTERNAL FACTORS 499
X. INTERNAL FACTORS 508
XI. DIRECT EQUILIBRATION 519
XII. INDIRECT EQUILIBRATION 529
XIII. THE CO-OPERATION OF THE FACTORS 549
XIV. THE CONVERGENCE OF THE EVIDENCES 554
XIV^A. RECENT CRITICISMS AND HYPOTHESES 559
APPENDICES.
A. THE GENERAL LAW OF ANIMAL FERTILITY 577
B. THE INADEQUACY OF NATURAL SELECTION, ETC. 602
C. THE INHERITANCE OF FUNCTIONALLY-WROUGHT MODIFICATIONS: 692
A SUMMARY
D. ON ALLEGED "SPONTANEOUS GENERATION" AND ON THE 696
HYPOTHESIS OF PHYSIOLOGICAL UNITS
PART I.
THE DATA OF BIOLOGY.
CHAPTER I.
ORGANIC MATTER.
§ 1. Of the four chief elements which, in various combinations, make up
living bodies, three are gaseous under all ordinary conditions and the
fourth is a solid. Oxygen, hydrogen, and nitrogen are gases which for many
years defied all attempts to liquefy them, and carbon is a solid except
perhaps at the extremely high temperature of the electric arc. Only by
intense pressures joined with extreme refrigerations have the three gases
been reduced to the liquid form.[2] There is much significance in this.
When we remember how those redistributions of Matter and Motion which
constitute Evolution, structural and functional, imply motions in the units
that are redistributed; we shall see a probable meaning in the fact that
organic bodies, which exhibit the phenomena of Evolution in so high a
degree, are mainly composed of ultimate units having extreme mobility. The
properties of substances, though destroyed to sense by combination, are not
destroyed in reality. It follows from the persistence of force, that the
properties of a compound are _resultants_ of the properties of its
components--_resultants_ in which the properties of the components are
severally in full action, though mutually obscured. One of the leading
properties of each substance is its degree of molecular mobility; and its
degree of molecular mobility more or less sensibly affects the molecular
mobilities of the various compounds into which it enters. Hence we may
infer some relation between the gaseous form of three out of the four chief
organic elements, and that comparative readiness displayed by organic
matters to undergo those changes in the arrangement of parts which we call
development, and those transformations of motion which we call function.
Considering them chemically instead of physically, it is to be remarked
that three out of these four main components of organic matter, have
affinities which are narrow in their range and low in their intensity.
Hydrogen, it is true, may be made to combine with a considerable number of
other elements; but the chemical energy which it shows is scarcely at all
shown within the limits of the organic temperatures. Of carbon it may
similarly be said that it is totally inert at ordinary heats; that the
number of substances with which it unites is not great; and that in most
cases its tendency to unite with them is but feeble. Lastly, this chemical
indifference is shown in the highest degree by nitrogen--an element which,
as we shall hereafter see, plays the leading part in organic changes.
Among the organic elements (including under the title not only the four
chief ones, but also the less conspicuous remainder), that capability of
assuming different states called allotropism, is frequent. Carbon presents
itself in the three unlike conditions of diamond, graphite, and charcoal.
Under certain circumstances, oxygen takes on the form in which it is called
ozone. Sulphur and phosphorus (both, in small proportions, essential
constituents of organic matter) have allotropic modifications. Silicon,
too, is allotropic; while its oxide, silica, which is an indispensable
constituent of many lower organisms, exhibits the analogue of
allotropism--isomerism. No other interpretation being possible we are
obliged to regard allotropic change as some change of molecular
arrangement. Hence this frequency of its occurrence among the components of
organic matter is significant as implying a further kind of molecular
mobility.
One more fact, that is here of great interest for us, must be set down.
These four elements of which organisms are almost wholly composed, exhibit
certain extreme unlikenesses. While between two of them we have an
unsurpassed contrast in chemical activity; between one of them and the
other three, we have an unsurpassed contrast in molecular mobility. While
carbon, until lately supposed to be infusible and now volatilized only in
the electric arc, shows us a degree of atomic cohesion greater than that of
any other known element, hydrogen, oxygen, and nitrogen show the least
atomic cohesion of all elements. And while oxygen displays, alike in the
range and intensity of its affinities, a chemical energy exceeding that of
any other substance (unless fluorine be considered an exception), nitrogen
displays the greatest chemical inactivity. Now on calling to mind one of
the general truths arrived at when analyzing the process of Evolution, the
probable significance of this double difference will be seen. It was shown
(_First Principles_, § 163) that, other things equal, unlike units are more
easily separated by incident forces than like units are--that an incident
force falling on units that are but little dissimilar does not readily
segregate them; but that it readily segregates them if they are widely
dissimilar. Thus, the substances presenting these two extreme contrasts,
the one between physical mobilities, and the other between chemical
activities, fulfil, in the highest degree, a certain further condition to
facility of differentiation and integration.
§ 2. Among the diatomic combinations of the three elements, hydrogen,
nitrogen and oxygen, we find a molecular mobility much less than that of
these elements themselves; at the same time that it is much greater than
that of diatomic compounds in general. Of the two products formed by the
union of oxygen with carbon, the first, called carbonic oxide, which
contains one atom[3] of carbon to one of oxygen (expressed by the symbol
CO) is a gas condensible only with great difficulty; and the second,
carbonic acid, containing an additional atom of oxygen (CO_{2}) assumes a
liquid form also only under a pressure of about forty atmospheres. The
several compounds of oxygen with nitrogen, present us with an instructive
gradation. Nitrous oxide (N_{2}O), is a gas condensible only under a
pressure of some fifty atmospheres; nitric oxide (NO) is a gas which
although it has been liquefied does not condense under a pressure of 270
atmospheres at 46.4° F. (8° C.): the molecular mobility remaining
undiminished in consequence of the volume of the united gases remaining
unchanged. Nitrogen trioxide (N_{2}O_{3}) is gaseous at ordinary
temperatures, but condenses into a very volatile liquid at the zero of
Fahrenheit; nitrogen tetroxide (N_{2}O_{4}) is liquid at ordinary
temperatures and becomes solid at the zero of Fahrenheit; while nitrogen
pentoxide (N_{2}O_{5}) may be obtained in crystals which melt at 85° and
boil at 113°. In this series we see, though not with complete uniformity, a
decrease of molecular mobility as the weights of the compound molecules are
increased. The hydro-carbons illustrate the same general truth still
better. One series of them will suffice. Marsh gas (CH_{4}) is gaseous
except under great pressure and at very low temperatures. Olefiant gas
(C_{2}H_{4}) and ethane (C_{2}H_{6}) may be readily liquefied by pressure.
Propane (C_{3}H_{8}) becomes liquid without pressure at the zero of
Fahrenheit. Hexane (C_{5}H_{12}) is a liquid which boils at 160°. And the
successively higher multiples, heptane (C_{7}H_{16}), octane (C_{8}H_{18}),
and nonane (C_{9}H_{20}) are liquids which boil respectively at 210°, 257°,
and 302°. Pentadecan (C_{15}H_{32}) is a liquid which boils at 270°, while
paraffin-wax, which contains the still higher multiples, is solid. There
are three compounds of hydrogen and nitrogen that have been obtained in a
free state--ammonia (NH_{3}) is gaseous, but liquefiable by pressure, or by
reducing its temperature to -40° F., and it solidifies at -112° F.;
hydrazine (NH_{2}--NH_{2}) is liquid at ordinary temperatures, but
hydrozoic acid (N_{3}H) has so far only been obtained in the form of a
highly explosive gas. In cyanogen, which is composed of carbon and
nitrogen, (CN)_{2}, we have a gas that becomes liquid at a pressure of four
atmospheres and solid at -30° F. And in paracyanogen, formed of the same
proportions of these elements in higher multiples, we have a solid which
does not fuse or volatilize at ordinary temperatures. Lastly, in the most
important member of this group, water (H_{2}O), we have a compound of two
difficultly-condensible gases which assumes both the fluid state and the
solid state within ordinary ranges of temperature; while its molecular
mobility is still such that its fluid or solid masses are continually
passing into the form of vapour, though not with great rapidity until the
temperature is raised to 212°.
Considering them chemically, it is to be remarked of these diatomic
compounds of the four chief organic elements, that they are, on the
average, less stable than diatomic compounds in general. Water, carbonic
oxide, and carbonic acid, are, it is true, difficult to decompose. But
omitting these, the usual strength of union among the elements of the
above-named substances is low considering the simplicity of the substances.
With the exception of acetylene and possibly marsh gas, the various
hydro-carbons are not producible by directly combining their elements; and
the elements of most of them are readily separable by heat without the aid
of any antagonistic affinity. Nitrogen and hydrogen do not unite with each
other immediately save under very exceptional circumstances; and the
ammonia which results from their union, though it resists heat, yields to
the electric spark. Cyanogen is stable: not being resolved into its
components below a bright red heat. Much less stable, however, are several
of the oxides of nitrogen. Nitrous oxide, it is true, does not yield up its
elements below a red heat; but nitrogen tetroxide cannot exist if water be
added to it; nitrous acid is decomposed by water; and nitric acid not only
readily parts with its oxygen to many metals, but when anhydrous,
spontaneously decomposes. Here it will be well to note, as having a bearing
on what is to follow, how characteristic of most nitrogenous compounds is
this special instability. In all the familiar cases of sudden and violent
decomposition, the change is due to the presence of nitrogen. The explosion
of gunpowder results from the readiness with which the nitrogen contained
in the nitrate of potash, yields up the oxygen combined with it. The
explosion of gun-cotton, which also contains nitrogen, is a substantially
parallel phenomenon. The various fulminating salts are all formed by the
union with metals of a certain nitrogenous acid called fulminic acid; which
is so unstable that it cannot be obtained in a separate state.
Explosiveness is a property of nitro-mannite, and also of nitro-glycerin.
Iodide of nitrogen detonates on the slightest touch, and often without any
assignable cause. And the bodies which explode with the most tremendous
violence of any known, are the chloride of nitrogen (NCl_{3}) and hydrazoic
acid (N_{3}H). Thus these easy and rapid decompositions, due to the
chemical indifference of nitrogen, are characteristic. When we come
hereafter to observe the part which nitrogen plays in organic actions, we
shall see the significance of this extreme readiness shown by its compounds
to undergo changes. Returning from these facts parenthetically introduced,
we have next to note that though among the diatomic compounds of the four
chief organic elements, there are a few active ones, yet the majority of
them display a smaller degree of chemical energy than the average of
diatomic compounds. Water is the most neutral of bodies: usually producing
little chemical alteration in the substances with which it combines; and
being expelled from most of its combinations by a moderate heat. Carbonic
acid is a relatively feeble acid: the carbonates being decomposed by the
majority of other acids and by ignition. The various hydro-carbons are but
narrow in the range of their comparatively weak affinities. The compounds
formed by ammonia have not much stability: they are readily destroyed by
heat, and by the other alkalies. The affinities of cyanogen are tolerably
strong, though they yield to those of the chief acids. Of the several
oxides of nitrogen, it is to be remarked that, while those containing the
smaller proportions of oxygen are chemically inert, the one containing the
greatest proportion of oxygen (nitric acid) though chemically active, in
consequence of the readiness with which one part of it gives up its oxygen
to oxidize a base with which the rest combines, is nevertheless driven from
all its combinations by a red heat.
These diatomic compounds, like their elements, are to a considerable degree
characterized by the prevalence among them of allotropism; or, as it is
more usually called when displayed by compound bodies--isomerism. Professor
Graham finds reason for thinking that a change in atomic arrangements of
this nature, takes place in water, at or near the melting point of ice. In
the various series of hydro-carbons, differing from each other only in the
ratios in which the elements are united, we find not simply isomerism but
polymerism occurring to an almost infinite extent. In some series of
hydro-carbons, as, for example, the terpenes, we find isomerism and at the
same time a great tendency to undergo polymerisation. And the relation
between cyanogen and paracyanogen is, as we saw, a polymeric one.
There is one further fact respecting these diatomic compounds of the chief
organic elements, which must not be overlooked. Those of them which form
parts of the living tissues of plants and animals (excluding water which
has a mechanical function, and carbonic acid which is a product of
decomposition) belong for the most part to one group--the
carbo-hydrates.[4] And of this group, which is on the average characterized
by comparative instability and inertness, these carbo-hydrates found in
living tissues are among the most unstable and inert.
§ 3. Passing now to the substances which contain three of these chief
organic elements, we have first to note that along with the greater atomic
weight which mostly accompanies their increased complexity, there is, on
the average, a further marked decrease of molecular mobility. Scarcely any
of them maintain a gaseous state at ordinary temperatures. One class of
them only, the alcohols and their derivatives, evaporate under the usual
atmospheric pressure; but not rapidly unless heated. The fixed oils, though
they show that molecular mobility implied by an habitually liquid state,
show this in a lower degree than the alcoholic compounds; and they cannot
be reduced to the gaseous state without decomposition. In their allies, the
fats, which are solid unless heated, the loss of molecular mobility is
still more marked. And throughout the whole series of the fatty acids, in
which to a fixed proportion of oxygen there are successively added higher
equimultiples of carbon and hydrogen, we see how the molecular mobility
decreases with the increasing sizes of the molecules. In the amylaceous and
sugar-group of compounds, solidity is the habitual state: such of them as
can assume the liquid form, doing so only when heated to 300° or 400° F.;
and decomposing when further heated, rather than become gaseous. Resins and
gums exhibit general physical properties of like character and meaning.
In chemical stability these triatomic compounds, considered as a group, are
in a marked degree below the diatomic ones. The various sugars and kindred
bodies, decompose at no very high temperatures. The oils and fats also are
readily carbonized by heat. Resinous and gummy substances are easily made
to render up some of their constituents. And the alcohols, with their
allies, have no great power of resisting decomposition. These bodies,
formed by the union of oxygen, hydrogen, and carbon, are also, as a class,
chemically inactive. Formic and acetic are doubtless energetic acids; but
the higher members of the fatty-acid series are easily separated from the
bases with which they combine. Saccharic acid, too, is an acid of
considerable power; and sundry of the vegetable acids possess a certain
activity, though an activity far less than that of the mineral acids. But
throughout the rest of the group, there is shown but a small tendency to
combine with other bodies; and such combinations as are formed have usually
little permanence.
The phenomena of isomerism and polymerism are of frequent occurrence in
these triatomic compounds. Starch and dextrine are probably polymeric.
Fruit-sugar and grape-sugar, mannite and sorbite, cane-sugar and
milk-sugar, are isomeric. Sundry of the vegetal acids exhibit similar
modifications. And among the resins and gums, with their derivatives,
molecular re-arrangements of this kind are not uncommon.
One further fact respecting these compounds of carbon, oxygen and hydrogen,
should be mentioned; namely, that they are divisible into two classes--the
one consisting of substances that result from the destructive decomposition
of organic matter, and the other consisting of substances that exist as
such in organic matter. These two classes of substances exhibit, in
different degrees, the properties to which we have been directing our
attention. The lower alcohols, their allies and derivatives, which possess
greater molecular mobility and chemical stability than the rest of these
triatomic compounds, are rarely found in animal or vegetal bodies. While
the sugars and amylaceous substances, the fixed oils and fats, the gums and
resins, which have all of them much less molecular mobility, and are,
chemically considered, more unstable and inert, are components of the
living tissues of plants and animals.
§ 4. Among compounds containing all the four chief organic elements, a
division analogous to that just named may be made. There are some which
result from the decomposition of living tissues; there are others which
make parts of living tissues in their state of integrity; and these two
groups are contrasted in their properties in the same way as are the
parallel groups of triatomic compounds.
Of the first division, certain products found in the animal excretions are
the most important, and the only ones that need be noted; such, namely, as
urea, kreatine, kreatinine. These animal-bases exhibit much less molecular
mobility than the average of the substances treated of in the last section:
being solid at ordinary temperatures, fusing, where fusible at all, at
temperatures above that of boiling water, and having no power to assume a
gaseous state. Chemically considered, their stability is low, and their
activity but small, in comparison with the stabilities and activities of
the simpler compounds.
It is, however, the nitrogenous constituents of living tissues, that
display most markedly those characteristics of which we have been tracing
the growth. Albumen, fibrin, casein, and their allies, are bodies in which
that molecular mobility exhibited by three of their components in so high a
degree is reduced to a minimum. These substances are known only in the
solid state. That is to say, when deprived of the water usually mixed with
them, they do not admit of fusion, much less of volatilization. To which
add, that they have not even that molecular mobility which solution in
water implies; since, though they form viscid mixtures with water, they do
not dissolve in the same perfect way as do inorganic compounds. The
chemical characteristics of these substances are instability and inertness
carried to the extreme. How rapidly albumenoid matters decompose under
ordinary conditions, is daily seen: the difficulty of every housewife being
to prevent them from decomposing. It is true that when desiccated and kept
from contact with air, they may be preserved unchanged for long periods;
but the fact that they can be only thus preserved, proves their great
instability. It is true, also, that these most complex nitrogenous
principles are not absolutely inert, since they enter into combinations
with some bases; but their unions are very feeble.
It should be noted, too, of these bodies, that though they exhibit in the
lowest degree that kind of molecular mobility which implies facile
vibration of the molecules as wholes, they exhibit in high degrees that
kind of molecular mobility resulting in isomerism, which implies permanent
changes in the positions of adjacent atoms with respect to each other. Each
of them has a soluble and an insoluble form. In some cases there are
indications of more than two such forms. And it appears that their
metamorphoses take place under very slight changes of conditions.
In these most unstable and inert organic compounds, we find that the
molecular complexity reaches a maximum: not only since the four chief
organic elements are here united with small proportions of sulphur and
sometimes phosphorus; but also since they are united in high multiples. The
peculiarity which we found characterized even diatomic compounds of the
organic elements, that their molecules are formed not of single equivalents
of each component, but of two, three, four, and more equivalents, is
carried to the greatest extreme in these compounds, which take the leading
part in organic actions. According to Lieberkühn, the formula of albumen is
C_{72}H_{112}SN_{18}O_{22}. That is to say, with the sulphur there are
united seventy-two atoms of carbon, one hundred and twelve of hydrogen,
eighteen of nitrogen, and twenty-two of oxygen: the molecule being thus
made up of more than two hundred ultimate atoms.
§ 5. Did space permit, it would be useful here to consider in detail the
interpretations that may be given of the peculiarities we have been
tracing: bringing to their solution, the general mechanical principles
which are now found to hold true of molecules as of masses. But it must
suffice briefly to indicate the conclusions which such an inquiry promises
to bring out.
Proceeding on these principles, it may be argued that the molecular
mobility of a substance must depend partly on the inertia of its molecules;
partly on the intensity of their mutual polarities; partly on their mutual
pressures, as determined by the density of their aggregation; and (where
the molecules are compound) partly on the molecular mobilities of their
component molecules. Whence it is to be inferred that any three of these
remaining constant, the molecular mobility will vary as the fourth. Other
things equal, therefore, the molecular mobility of molecules must decrease
as their masses increase; and so there must result that progression we have
traced, from the high molecular mobility of the uncombined organic
elements, to the low molecular mobility of those large-moleculed substances
into which they are ultimately compounded.
Applying to molecules the mechanical law which holds of masses, that since
inertia and gravity increase as the cubes of the dimensions while cohesion
increases as their squares, the self-sustaining power of a body becomes
relatively smaller as its bulk becomes greater; it might be argued that
these large, aggregate molecules which constitute organic substances, are
mechanically weak--are less able than simpler molecules to bear, without
alteration, the forces falling on them. That very massiveness which renders
them less mobile, enables the physical forces acting on them more readily
to change the relative positions of their component atoms; and so to
produce what we know as re-arrangements and decompositions.
Further, it seems a not improbable conclusion, that this formation of large
aggregates of elementary atoms and resulting diminution of self-sustaining
power, must be accompanied by a decrease of those dimensional contrasts to
which polarity is ascribable. A sphere is the figure of equilibrium which
any aggregate of units tends to assume, under the influence of simple
mutual attraction. Where the number of units is small and their mutual
polarities are decided, this proclivity towards spherical grouping will be
overcome by the tendency towards some more special form, determined by
their mutual polarities. But it is manifest that in proportion as an
aggregate molecule becomes larger, the effects of simple mutual attraction
must become relatively greater; and so must tend to mask the effects of
polar attraction. There will consequently be apt to result in highly
compound molecules like these organic ones, containing hundreds of
elementary atoms, such approximation to the spherical form as must involve
a less distinct polarity than in simpler molecules. If this inference be
correct, it supplies us with an explanation both of the chemical inertness
of these most complex organic substances, and of their inability to
crystallize.
§ 6. Here we are naturally introduced to another aspect of our subject--an
aspect of great interest. Professor Graham has published a series of
important researches, which promise to throw much light on the constitution
and changes of organic matter. He shows that solid substances exist under
two forms of aggregation--the _colloid_ or jelly-like, and the
_crystalloid_ or crystal-like. Examples of the last are too familiar to
need specifying. Of the first may be named such instances as "hydrated
silicic acid, hydrated alumina, and other metallic peroxides of the
aluminous class, when they exist in the soluble form; with starch, dextrine
and the gums, caramel, tannin, albumen, gelatine, vegetable and animal
extractive matters." Describing the properties of colloids, Professor
Graham says:--"Although often largely soluble in water, they are held in
solution by a most feeble force. They appear singularly inert in the
capacity of acids and bases, and in all the ordinary chemical relations." *
* * "Although chemically inert in the ordinary sense, colloids possess a
compensating activity of their own arising out of their physical
properties. While the rigidity of the crystalline structure shuts out
external impressions, the softness of the gelatinous colloid partakes of
fluidity, and enables the colloid to become a medium of liquid diffusion,
like water itself." * * * "Hence a wide sensibility on the part of colloids
to external agents. Another and eminently characteristic quality of
colloids is their mutability." * * * "The solution of hydrated silicic
acid, for instance, is easily obtained in a state of purity, but it cannot
be preserved. It may remain fluid for days or weeks in a sealed tube, but
is sure to gelatinize and become insoluble at last. Nor does the change of
this colloid appear to stop at that point; for the mineral forms of silicic
acid, deposited from water, such as flint, are often found to have passed,
during the geological ages of their existence, from the vitreous or
colloidal into the crystalline condition (H. Rose). The colloid is, in
fact, a dynamical state of matter, the crystalloidal being the statical
condition. The colloid possesses _energia_. It may be looked upon as the
primary source of the force appearing in the phenomena of vitality. To the
gradual manner in which colloidal changes take place (for they always
demand time as an element) may the characteristic protraction of
chemico-organic changes also be referred."
The class of colloids includes not only all those most complex nitrogenous
compounds characteristic of organic tissues, and sundry of the
carbo-hydrates found along with them; but, significantly enough, it
includes several of those substances classed as inorganic, which enter into
organized structures. Thus silica, which is a component of many plants, and
constitutes the spicules of sponges as well as the shells of many
foraminifera and infusoria, has a colloid, as well as a crystalloid,
condition. A solution of hydrated silicic acid passes in the course of a
few days into a solid jelly that is no longer soluble in water; and it may
be suddenly thus coagulated by a minute portion of an alkaline carbonate,
as well as by gelatine, alumina, and peroxide of iron. This last-named
substance, too--peroxide of iron--which is an ingredient in the blood of
mammals and composes the shells of certain _Protozoa_, has a colloid
condition. "Water containing about one per cent. of hydrated peroxide of
iron in solution, has the dark red colour of venous blood." * * * "The red
solution is coagulated in the cold by traces of sulphuric acid, alkalies,
alkaline carbonates, sulphates, and neutral salts in general." * * * "The
coagulum is a deep red-coloured jelly, resembling the clot of blood, but
more transparent. Indeed, the coagulum of this colloid is highly suggestive
of that of blood, from the feeble agencies which suffice to effect the
change in question, as well as from the appearance of the product." The
jelly thus formed soon becomes, like the last, insoluble in water. Lime
also, which is so important a mineral element in living bodies, animal and
vegetal, enters into a compound belonging to this class. "The well-known
solution of lime in sugar forms a solid coagulum when heated. It is
probably, at a high temperature, entirely colloidal."
Generalizing some of the facts which he gives, Professor Graham says:--"The
equivalent of a colloid appears to be always high, although the ratio
between the elements of the substance may be simple. Gummic acid, for
instance, may be represented by C^{12}H^{22}O^{11}; but, judging from the
small proportions of lime and potash which suffice to neutralize this acid,
the true numbers of its formula must be several times greater. It is
difficult to avoid associating the inertness of colloids with their high
equivalents, particularly where the high number appears to be attained by
the repetition of a small number. The inquiry suggests itself whether the
colloid molecule may not be constituted by the grouping together of a
number of smaller crystalloid molecules, and whether the basis of
colloidality may not really be this composite character of the molecule."
§ 7. A further contrast between colloids and crystalloids is equally
significant in its relations to vital phenomena. Professor Graham points
out that the marked differences in volatility displayed by different
bodies, are paralleled by differences in the rates of diffusion of
different bodies through liquids. As alcohol and ether at ordinary
temperatures, and various other substances at higher temperatures, diffuse
themselves in a gaseous form through the air; so, a substance in aqueous
solution, when placed in contact with a mass of water (in such way as to
avoid mixture by circulating currents) diffuses itself through this mass of
water. And just as there are various degrees of rapidity in evaporation, so
there are various degrees of rapidity in diffusion: "the range also in the
degree of diffusive mobility exhibited by different substances appears to
be as wide as the scale of vapour-tensions." This parallelism is what might
have been looked for; since the tendency to assume a gaseous state, and the
tendency to spread in solution through a liquid, are both consequences of
molecular mobility. It also turns out, as was to be expected, that
diffusibility, like volatility, has, other things equal, a relation to
molecular weight--other things equal, we must say, because molecular
mobility must, as pointed out in § 5, be affected by other properties of
atoms, besides their inertia. Thus the substance most rapidly diffused of
any on which Professor Graham experimented, was hydrochloric acid--a
compound which is of low molecular weight, is gaseous save under a pressure
of forty atmospheres, and ordinarily exists as a liquid, only in
combination with water. Again, "hydrate of potash may be said to possess
double the velocity of diffusion of sulphate of potash, and sulphate of
potash again double the velocity of sugar, alcohol, and sulphate of
magnesia,"--differences which have a general correspondence with
differences in the massiveness of their molecules.
But the fact of chief interest to us here, is that the relatively
small-moleculed crystalloids have immensely greater diffusive power than
the relatively large-moleculed colloids. Among the crystalloids themselves
there are marked differences of diffusibility; and among the colloids
themselves there are parallel differences, though less marked ones. But
these differences are small compared with that between the diffusibility of
the crystalloids as a class, and the diffusibility of the colloids as a
class. Hydrochloric acid is seven times as diffusible as sulphate of
magnesia; but it is fifty times as diffusible as albumen, and a hundred
times as diffusible as caramel.
These differences of diffusibility manifest themselves with nearly equal
distinctness, when a permeable septum is placed between the solution and
the water. The result is that when a solution contains substances of
different diffusibilities, the process of dialysis, as Professor Graham
calls it, becomes a means of separating the mixed substances: especially
when such mixed substances are partly crystalloids and partly colloids. The
bearing of this fact on the interpretation of organic processes will be
obvious. Still more obvious will its bearing be, on joining with it the
remarkable fact that while crystalloids can diffuse themselves through
colloids nearly as rapidly as through water, colloids can scarcely diffuse
themselves at all through other colloids. From a mass of jelly containing
salt, into an adjoining mass of jelly containing no salt, the salt spread
more in eight days than it spread through water in seven days; while the
spread of "caramel through the jelly appeared scarcely to have begun after
eight days had elapsed." So that we must regard the colloidal compounds of
which organisms are built, as having, by their physical nature, the ability
to separate colloids from crystalloids, and to let the crystalloids pass
through them with scarcely any resistance.
One other result of these researches on the relative diffusibilities of
different substances has a meaning for us. Professor Graham finds that not
only does there take place, by dialysis, a separation of _mixed_ substances
which are unlike in their molecular mobilities; but also that _combined_
substances between which the affinities are feeble, will separate on the
dialyzer, if their molecular mobilities are strongly contrasted. Speaking
of the hydrochloride of peroxide of iron, he says, "such a compound
possesses an element of instability in the extremely unequal diffusibility
of its constituents;" and he points out that when dialyzed, the
hydrochloric acid gradually diffuses away, leaving the colloidal peroxide
of iron behind. Similarly, he remarks of the peracetate of iron, that it
"may be made a source of soluble peroxide, as the salt referred to is
itself decomposed to a great extent by diffusion on the dialyzer." Now this
tendency to separate displayed by substances which differ widely in their
molecular mobilities, though usually so far antagonized by their affinities
as not to produce spontaneous decomposition, must, in all cases, induce a
certain readiness to change which would not else exist. The unequal
mobilities of the combined atoms must give disturbing forces a greater
power to work transformations than they would otherwise have. Hence the
probable significance of a fact named at the outset, that while three of
the chief organic elements have the greatest atomic mobilities of any
elements known, the fourth, carbon, has the least atomic mobility of known
elements. Though, in its simple compounds, the affinities of carbon for the
rest are strong enough to prevent the effects of this great difference from
clearly showing themselves; yet there seems reason to think that in those
complex compounds composing organic bodies--compounds in which there are
various cross affinities leading to a state of chemical tension--this
extreme difference in the molecular mobilities must be an important aid to
molecular re-arrangements. In short, we are here led by concrete evidence
to the conclusion which we before drew from first principles, that this
great unlikeness among the combined units must facilitate differentiations.
§ 8. A portion of organic matter in a state to exhibit those phenomena
which the biologist deals with, is, however, something far more complex
than the separate organic matters we have been studying; since a portion of
organic matter in its integrity, contains several of these.
In the first place no one of those colloids which make up the mass of a
living body, appears capable of carrying on vital changes by itself: it is
always associated with other colloids. A portion of animal-tissue, however
minute, almost always contains more than one form of protein-substance:
different chemical modifications of albumen and gelatine are present
together, as well as, probably, a soluble and insoluble modification of
each; and there is usually more or less of fatty matter. In a single
vegetal cell, the minute quantity of nitrogenous colloid present, is
imbedded in colloids of the non-nitrogenous class. And the microscope makes
it at once manifest, that even the smallest and simplest organic forms are
not absolutely homogeneous.
Further, we have to contemplate organic tissue, formed of mingled colloids
in both soluble and insoluble states, as permeated throughout by
crystalloids. Some of these crystalloids, as oxygen,[5] water, and perhaps
certain salts, are agents of decomposition; some, as the saccharine and
fatty matters, are probably materials for decomposition; and some, as
carbonic acid, water, urea, kreatine, and kreatinine, are products of
decomposition. Into the mass of mingled colloids, mostly insoluble and
where soluble of very low molecular mobility or diffusive power, we have
constantly passing, crystalloids of high molecular mobility or diffusive
power, that are capable of decomposing these complex colloids, or of
facilitating decompositions otherwise caused; and from these complex
colloids, when decomposed, there result other crystalloids (the two chief
ones extremely simple and mobile, and the rest comparatively so) which
diffuse away as rapidly as they are formed.
And now we may clearly see the necessity for that peculiar composition
which we find in organic matter. On the one hand, were it not for the
extreme molecular mobility possessed by three out of the four of its chief
elements; and were it not for the consequently high molecular mobility of
their simpler compounds; there could not be this quick escape of the waste
products of organic action; and there could not be that continuously active
change of matter which vitality implies. On the other hand, were it not for
the union of these extremely mobile elements into immensely complex
compounds, having relatively vast molecules which are made comparatively
immobile by their inertia, there could not result that mechanical fixity
which prevents the components of living tissue from diffusing away along
with the effete matters produced by decomposition.
§ 8a. Let us not omit here to note the ways in which the genesis of these
traits distinguishing organic matter conforms to the laws of evolution as
expressed in its general formula.
In pursuance of the belief now widely entertained by chemists that the
so-called elements are not elements, but are composed of simpler matters
and probably of one ultimate form of matter (for which the name "protyle"
has been suggested by Sir W. Crookes), it is to be concluded that the
formation of the elements, in common with the formation of all those
compounds of them which Nature presents, took place in the course of Cosmic
Evolution. Various reasons for this inference the reader will find set
forth in the Addenda to an essay on "The Nebular Hypothesis" (see _Essays_,
vol. I, p. 155). On tracing out the process of compounding and
re-compounding by which, hypothetically, the elements themselves and
afterwards their compounds and re-compounds have arisen, certain cardinal
facts become manifest.
1. Considered as masses, the units of the elements are the smallest, though
larger than the units of the primordial matter. Later than these, since
they are composed of them, and since they cannot exist at temperatures so
high as those at which the elements can exist, come the diatomic
compounds--oxides, chlorides, and the rest--necessarily larger in their
molecules. Above these in massiveness come the molecules of the
multitudinous salts and kindred bodies. When associated, as these commonly
are, with molecules of water, there again results in each case increase of
mass; and unable as they are to bear such high temperatures, these
molecules are necessarily later in origin than those of the anhydrous
diatomic compounds. Within the general class of triatomic compounds, more
composite still, come the carbohydrates, which, being able to unite in
multiples, form still larger molecules than other triatomic compounds.
Decomposing as they do at relatively low temperatures, these are still more
recent in the course of chemical evolution; and with the genesis of them
the way is prepared for the genesis of organic matter strictly so called.
This includes the various forms of protein-substance, containing four chief
elements with two minor ones, and having relatively vast molecules.
Unstable as these are in presence of heat and surrounding affinities, they
became possible only at a late stage in the genesis of the Earth. Here,
then, in that chemical evolution which preceded the evolution of life, we
see displayed that process of integration which is the primary trait of
evolution at large.
2. Along with increasing integration has gone progress in heterogeneity.
The elements, regarding them as compound, are severally more heterogeneous
than "protyle." Diatomic molecules are more heterogeneous than these
elements; triatomic more heterogeneous than diatomic; and the molecules
containing four elements more heterogeneous than those containing three:
the most heterogeneous of them being the proteids, which contain two other
elements. The hydrated forms of all these compounds are more heterogeneous
than are the anhydrous forms. And most heterogeneous of all are the
molecules which, besides containing three, four, or more elements, also
exhibit the isomerism and polymerism which imply unions in multiples.
3. This formation of molecules more and more heterogeneous during
terrestrial evolution, has been accompanied by increasing heterogeneity in
the aggregate of compounds of each kind, as well as an increasing number of
kinds; and this increasing heterogeneity is exemplified in an extreme
degree in the compounds, non-nitrogenous and nitrogenous, out of which
organisms are built. So that the classes, orders, genera, and species of
chemical substances, gradually increasing as the Earth has assumed its
present form, increased in a transcendent degree during that stage which
preceded the origin of life.
§ 9. Returning now from these partially-parenthetic observations, and
summing up the contents of the preceding pages, we have to remark that in
the substances of which organisms are composed, the conditions necessary to
that re-distribution of Matter and Motion which constitutes Evolution, are
fulfilled in a far higher degree than at first appears.
The mutual affinities of the chief organic elements are not active within
the limits of those temperatures at which organic actions take place; and
one of these elements is especially characterized by its chemical
indifference. The compounds formed by these elements in ascending grades of
complexity, become progressively less stable. And those most complex
compounds into which all these four elements enter, together with small
proportions of two other elements which very readily oxidize, have an
instability so great that decomposition ensues under ordinary atmospheric
conditions.
Among these elements out of which living bodies are built, there is an
unusual tendency to unite in multiples; and so to form groups of products
which have the same chemical elements in the same proportions, but,
differing in their modes of aggregation, possess different properties. This
prevalence among them of isomerism and polymerism, shows, in another way,
the special fitness of organic substances for undergoing re-distributions
of their components.
In those most complex compounds that are instrumental to vital actions,
there exists a kind and degree of molecular mobility which constitutes the
plastic quality fitting them for organization. Instead of the extreme
molecular mobility possessed by three out of the four organic elements in
their separate states--instead of the diminished, but still great,
molecular mobility possessed by their simpler combinations, the gaseous and
liquid characters of which unfit them for showing to any extent the process
of Evolution--instead of the physical properties of their less simple
combinations, which, when not made unduly mobile by heat, assume the unduly
rigid form of crystals; we have in these colloids, of which organisms are
mainly composed, just the required compromise between fluidity and
solidity. They cannot be reduced to the unduly mobile conditions of liquid
and gas; and yet they do not assume the unduly fixed condition usually
characterizing solids. The absence of power to unite together in polar
arrangement, leaves their molecules with a certain freedom of relative
movement, which makes them sensitive to small forces, and produces
plasticity in the aggregates composed of them.
While the relatively great inertia of these large and complex organic
molecules renders them comparatively incapable of being set in motion by
the ethereal undulations, and so reduced to less coherent forms of
aggregation, this same inertia facilitates changes of arrangement among
their constituent molecules or atoms; since, in proportion as an incident
force impresses but little motion on a mass, it is the better able to
impress motion on the parts of the mass in relation to one another. And it
is further probable that the extreme contrasts in molecular mobilities
among the components of these highly complex molecules, aid in producing
modifiability of arrangement among them.
Lastly, the great difference in diffusibility between colloids and
crystalloids, makes possible in the tissues of organisms a specially rapid
re-distribution of matter and motion; both because colloids, being easily
permeable by crystalloids, can be chemically acted on throughout their
whole masses, instead of only on their surfaces; and because the products
of decomposition, being also crystalloids, can escape as fast as they are
produced: leaving room for further transformations. So that while the
composite molecules of which organic tissues are built up, possess that low
molecular mobility fitting them for plastic purposes, it results from the
extreme molecular mobilities of their ultimate constituents, that the waste
products of vital activity escape as fast as they are formed.
To all which add that the state of warmth, or increased molecular
vibration, in which all the higher organisms are kept, increases these
various facilities for re-distribution: not only as aiding chemical
changes, but as accelerating the diffusion of crystalloid substances.
CHAPTER II.
THE ACTIONS OF FORCES ON ORGANIC MATTER.
§ 10. To some extent, the parts of every body are changed in their
arrangement by any incident mechanical force. But in organic bodies, and
especially in animal bodies, the changes of arrangement produced by
mechanical forces are usually conspicuous. It is a distinctive mark of
colloids that they readily yield to pressures and tensions, and that they
recover, more or less completely, their original shapes, when the pressures
or tensions cease. Evidently without this pliability and elasticity, most
organic actions would be impossible. Not only temporary but also permanent
alterations of form are facilitated by this colloid character of organic
matter. Continued pressure on living tissue, by modifying the processes
going on in it (perhaps retarding the absorption of new material to replace
the old that has decomposed and diffused away), gradually diminishes and
finally destroys its power of resuming the outline it had at first. Thus,
generally speaking, the substances composing organisms are modifiable by
arrested momentum or by continuous strain, in far greater degrees than are
inorganic substances.
§ 11. Sensitiveness to certain forces which are quasi-mechanical, if not
mechanical in the usual sense, is seen in two closely-related peculiarities
displayed by organic matter as well as other matter which assumes the same
state of molecular aggregation.
Colloids take up by a power called "capillary affinity," a large quantity
of water: undergoing at the same time great increase of bulk with change of
form. Conversely, with like readiness, they give up this water by
evaporation; resuming, partially or completely, their original states.
Whether resulting from capillarity, or from the relatively great
diffusibility of water, or from both, these changes are to be here noted as
showing another mode in which the arrangements of parts in organic bodies
are affected by mechanical actions.
In what is termed osmose, we have a further mode of an allied kind. When on
opposite sides of a permeable septum, and especially a septum of colloidal
substance, are placed miscible solutions of different densities, a double
transfer takes place: a large quantity of the less dense solution finds its
way through the septum into the more dense solution; and a small quantity
of the more dense finds its way into the less dense--one result being a
considerable increase in the bulk of the more dense at the expense of the
less dense. This process, which appears to depend on several conditions, is
not yet fully understood. But be the explanation what it may, the process
is one that tends continually to work alterations in organic bodies.
Through the surfaces of plants and animals, transfers of this kind are ever
taking place. Many of the conspicuous changes of form undergone by organic
germs, are due mainly to the permeation of their limiting membranes by the
surrounding liquids.
It should be added that besides the direct alterations which the imbibition
and transmission of water and watery solutions by colloids produce in
organic matter, they produce indirect alterations. Being instrumental in
conveying into the tissues the agents of chemical change, and conveying out
of them the products of chemical change, they aid in carrying on other
re-distributions.
§ 12. As elsewhere shown (_First Principles_, § 100) heat, or a raised
state of molecular vibration, enables incident forces more easily to
produce changes of molecular arrangement in organic matter. But besides
this, it conduces to certain vital changes in so direct a way as to become
their chief cause.
The power of the organic colloids to imbibe water, and to bring along with
it into their substance the materials which work transformations, would not
be continuously operative if the water imbibed were to remain. It is
because it escapes, and is replaced by more water containing more
materials, that the succession of changes is maintained. Among the higher
animals and higher plants its escape is facilitated by evaporation. And the
rate of evaporation is, other things equal, determined by heat. Though the
current of sap in a tree is partly dependent on some action, probably
osmotic, that goes on in the roots; yet the loss of water from the surfaces
of the leaves, and the consequent absorption of more sap into the leaves by
capillary attraction, must be a chief cause of the circulation. The
drooping of a plant when exposed to the sunshine while the earth round its
roots is dry, shows us how evaporation empties the sap-vessels; and the
quickness with which a withered slip revives on being placed in water,
shows us the part which capillary action plays. In so far, then, as the
evaporation from a plant's surface helps to produce currents of sap through
the plant, we must regard the heat which produces this evaporation as a
part-cause of those re-distributions of matter which these currents effect.
In terrestrial animals, heat, by its indirect action as well as by its
direct action, similarly aids the changes that are going on. The exhalation
of vapour from the lungs and the surface of the skin, forming the chief
escape of the water that is swallowed, conduces to the maintenance of those
currents through the tissues without which the functions would cease. For
though the vascular system distributes nutritive liquids in ramified
channels through the body; yet the absorption of these liquids into
tissues, partly depends on the escape of liquids which the tissues already
contain. Hence, to the extent that such escape is facilitated by
evaporation, and this evaporation facilitated by heat, heat becomes an
agent of re-distribution in the animal organism.[6]
§ 13. Light, which is now known to modify many inorganic compounds--light,
which works those chemical changes utilized in photography, causes the
combinations of certain gases, alters the molecular arrangements of many
crystals, and leaves traces of its action even on substances that are
extremely stable,--may be expected to produce marked effects on substances
so complex and unstable as those which make up organic bodies. It does
produce such effects; and some of them are among the most important that
organic matter undergoes.
The molecular changes wrought by light in animals are of but secondary
moment. There is the darkening of the skin that follows exposure to the
Sun's rays. There are those alterations in the retina which cause in us
sensations of colours. And on certain eyeless creatures that are
semi-transparent, the light permeating their substance works some effects
evinced by movements. But speaking generally, the opacity of animals limits
the action of light to their surfaces; and so renders its direct
physiological influence but small.[7] On plants, however, the solar rays
that produce in us the impression of yellow, are the immediate agents of
those molecular changes through which are hourly accumulated the materials
for further growth. Experiments have shown that when the Sun shines on
living leaves, they begin to exhale oxygen and to accumulate carbon and
hydrogen--results which are traced to the decomposition, by the solar rays,
of the carbonic acid and water absorbed. It is now an accepted conclusion
that, by the help of certain classes of the ethereal undulations
penetrating their leaves, plants are enabled to separate from the
associated oxygen those two elements of which their tissues are chiefly
built up.
This transformation of ethereal undulations into certain molecular
re-arrangements of an unstable kind, on the overthrow of which the
stored-up forces are liberated in new forms, is a process that underlies
all organic phenomena. It will therefore be well if we pause a moment to
consider whether any proximate interpretation of it is possible. Researches
in molecular physics give us some clue to its nature.
The elements of the problem are these:--The atoms[8] of several ponderable
matters exist in combination: those which are combined having strong
affinities, but having also affinities less strong for some of the
surrounding atoms that are otherwise combined. The atoms thus united, and
thus mixed among others with which they are capable of uniting, are exposed
to the undulations of a medium that is so rare as to seem imponderable.
These undulations are of numerous kinds: they differ greatly in their
lengths, or in the frequency with which they recur at any given point. And
under the influence of undulations of a certain frequency, some of these
atoms are transferred from atoms for which they have a stronger affinity,
to atoms for which they have a weaker affinity. That is to say, particular
orders of waves of a relatively imponderable matter, remove particular
atoms of ponderable matter from their attachments, and carry them within
reach of other attachments. Now the discoveries of Bunsen and Kirchoff
respecting the absorption of particular luminiferous undulations by the
vapours of particular substances, joined with Prof. Tyndall's discoveries
respecting the absorption of heat by gases, show very clearly that the
atoms of each substance have a rate of vibration in harmony with ethereal
waves of a certain length, or rapidity of recurrence. Every special kind of
atom can be made to oscillate by a special order of ethereal waves, which
are absorbed in producing its oscillations; and can by its oscillations
generate this same order of ethereal waves. Whence it appears that immense
as is the difference in density between ether and ponderable matter, the
waves of the one can set the atoms of the other in motion, when the
successive impacts of the waves are so timed as to correspond with the
oscillations of the atoms. The effects of the waves are, in such case,
cumulative; and each atom gradually acquires a momentum made up of
countless infinitesimal momenta. Note, further, that unless the members of
a chemically-compound molecule are so bound up as to be incapable of any
relative movements (a supposition at variance with the conceptions of
modern science) we must conceive them as severally able to vibrate in
unison or harmony with those same classes of ethereal waves that affect
them in their uncombined states. While the compound molecule as a whole
will have some new rate of oscillation determined by its attributes as a
whole; its components will retain their original rates of oscillation,
subject only to modifications by mutual influence. Such being the
circumstances of the case we may partially understand how the Sun's rays
can effect chemical decompositions. If the members of a diatomic molecule
stand so related to the undulations falling on them, that one is thrown
into a state of increased oscillation and the other not; it is manifest
that there must arise a tendency towards the dislocation of the two--a
tendency which may or may not take effect, according to the weakness or
strength of their union, and according to the presence or absence of
collateral affinities. This inference is in harmony with several
significant facts. Dr. Draper remarks that "among metallic substances
(compounds) those first detected to be changed by light, such as silver,
gold, mercury, lead, have all high atomic weights; and such as sodium and
potassium, the atomic weights of which are low, appeared to be less
changeable." As here interpreted, the fact specified amounts to this; that
the compounds most readily decomposed by light, are those in which there is
a marked contrast between the atomic weights of the constituents, and
probably therefore a marked contrast between the rapidities of their
vibrations. The circumstance, too, that different chemical compounds are
decomposed or modified in different parts of the spectrum, implies that
there is a relation between special orders of undulations and special
orders of molecules--doubtless a correspondence between the rates of these
undulations and the rates of oscillation which some of the components of
such molecules will assume. Strong confirmation of this view may be drawn
from the decomposing actions of those longer ethereal waves which we
perceive as heat. On contemplating the whole series of diatomic compounds,
we see that the elements which are most remote in their atomic weights, as
hydrogen and the noble metals generally, will not combine at all, or do so
with great difficulty: their vibrations are so unlike that they cannot keep
together under any conditions of temperature. If, again, we look at a
smaller group, as the metallic oxides, we see that whereas those metals
which have atoms nearest in weight to the atoms of oxygen, cannot be
separated from oxygen by heat, even when it is joined by a powerful
collateral affinity; those metals which differ more widely from oxygen in
their atomic weights, can be de-oxidized by carbon at high temperatures;
and those which differ from it most widely combine with it very
reluctantly, and yield it up if exposed to thermal undulations of moderate
intensity. Here indeed, remembering the relations among the atomic weights
in the two cases, may we not suspect a close analogy between the
de-oxidation of a metallic oxide by carbon under the influence of the
longer ethereal waves, and the de-carbonization of carbonic acid by
hydrogen under the influence of the shorter ethereal waves?
These conceptions help us to some dim notion of the mode in which changes
are wrought in light in the leaves of plants. Among the several elements
concerned, there are wide differences in molecular mobility, and probably
in the rates of molecular vibration. Each is combined with one of the
others, but is capable of forming various combinations with the rest. And
they are severally in presence of a complex compound into which they all
enter, and which is ready to assimilate with itself the new compound
molecules they form. Certain of the ethereal waves falling on them when
thus arranged, cause a detachment of some of the combined atoms and a union
of the rest. And the conclusion suggested is that the induced vibrations
among the various atoms as at first arranged, are so incongruous as to
produce instability, and to give collateral affinities the power to work a
rearrangement which, though less stable under other conditions, is more
stable in the presence of these particular undulations. There seems,
indeed, no choice but to conceive the matter thus. An atom united with one
for which it has a strong affinity, has to be transferred to another for
which it has a weaker affinity. This transfer implies motion. The motion is
given by the waves of a medium that is relatively imponderable. No one wave
of this imponderable medium can give the requisite motion to this atom of
ponderable matter: especially as the atom is held by a positive force
besides its inertia. The motion required can hence be given only by
successive waves; and that these may not destroy each other's effects, it
is needful that each shall strike the atom just when it has completed the
recoil produced by the impact of previous ones. That is, the ethereal
undulations must coincide in rate with the oscillations of the atom,
determined by its inertia and the forces acting on it. It is also requisite
that the rate of oscillation of the atom to be detached, shall differ from
that of the atom with which it is united; since if the two oscillated in
unison the ethereal waves would not tend to separate them. And, finally,
the successive impacts of the ethereal waves must be accumulated until the
resulting oscillations have become so wide in their sweep as greatly to
weaken the cohesion of the united atoms, at the same time that they bring
one of them within reach of other atoms with which it will combine. In this
way only does it seem possible for such a force to produce such a transfer.
Moreover, while we are thus enabled to conceive how light may work these
molecular changes, we also gain an insight into the method by which the
insensible motions propagated to us from the Sun, are treasured up in such
ways as afterwards to generate sensible motions. By the accumulation of
infinitesimal impacts, atoms of ponderable matter are made to oscillate.
The quantity of motion which each of them eventually acquires, effects its
transfer to a position of unstable equilibrium, from which it can
afterwards be readily dislodged. And when so dislodged, along with other
atoms similarly and simultaneously affected, there is suddenly given out
all the motion which had been before impressed on it.
Speculation aside, however, that which it concerns us to notice is the
broad fact that light is an all-important agent of molecular changes in
organic substances. It is not here necessary for us to ascertain _how_
light produces these compositions and decompositions. It is necessary only
for us to observe that it _does_ produce them. That the characteristic
matter called chlorophyll, which gives the green colour to leaves, makes
its appearance whenever the blanched shoots of plants are exposed to the
Sun; that the petals of flowers, uncoloured while in the bud, acquire their
bright tints as they unfold; and that on the outer surfaces of animals,
analogous changes are induced; are wide inductions which are enough for our
present purpose.
§ 14. We come next to the agency of chief importance among those that work
changes in organic matter; namely, chemical affinity. How readily vegetal
and animal substances are modified by other substances put in contact with
them, we see daily illustrated. Besides the many compounds which cause the
death of an organism into which they are put, we have the much greater
number of compounds which work those milder effects termed
medicinal--effects implying, like the others, molecular re-arrangements.
Indeed, most soluble chemical compounds, natural and artificial, produce,
when taken into the body, alterations that are more or less manifest in
their results.
After what was shown in the last chapter, it will be manifest that this
extreme modifiability of organic matter by chemical agencies, is the chief
cause of that active molecular re-arrangement which organisms, and
especially animal organisms, display. In the two fundamental functions of
nutrition and respiration, we have the means by which the supply of
materials for this active molecular re-arrangement is maintained.
The process of animal nutrition consists partly in the absorption of those
complex substances which are thus highly capable of being chemically
altered, and partly in the absorption of simpler substances capable of
chemically altering them. The tissues always contain small quantities of
alkaline and earthy salts, which enter the system in one form and are
excreted in another. Though we do not know specifically the parts which
these salts play, yet from their universal presence, and from the
transformations which they undergo in the body, it may be safely inferred
that their chemical affinities are instrumental in working some of the
metamorphoses ever going on.
The inorganic substance, however, on which mainly depend these
metamorphoses in organic matter, is not swallowed along with the solid and
liquid food, but is absorbed from the surrounding medium--air or water, as
the case may be. Whether the oxygen taken in, either, as by the lowest
animals, through the general surface, or, as by the higher animals, through
respiratory organs, is the immediate cause of those molecular changes which
are ever going on throughout the living tissues; or whether the oxygen,
playing the part of scavenger, merely aids these changes by carrying away
the products of decompositions otherwise caused; it equally remains true
that these changes are maintained by its instrumentality. Whether the
oxygen absorbed and diffused through the system effects a direct oxidation
of the organic colloids which it permeates, or whether it first leads to
the formation of simpler and more oxidized compounds, which are afterwards
further oxidized and reduced to still simpler forms, matters not, in so far
as the general result is concerned. In any case it holds good that the
substances of which the animal body is built up, enter it in either an
unoxidized or in a but slightly oxidized and highly unstable state; while
the great mass of them leave it in a fully oxidized and stable state. It
follows, therefore, that, whatever the special changes gone through, the
general process is a falling from a state of unstable chemical equilibrium
to a state of stable chemical equilibrium. Whether this process be direct
or indirect, the total molecular re-arrangement and the total motion given
out in effecting it, must be the same.
§ 15. There is another species of re-distribution among the component
matters of organisms, which is not immediately effected by the affinities
of the matters concerned, but is mediately effected by other affinities;
and there is reason to think that the re-distribution thus caused is
important in amount, if not indeed the most important. In ordinary cases of
chemical action, the two or more substances concerned themselves undergo
changes of molecular arrangement; and the changes are confined to the
substances themselves. But there are other cases in which the chemical
action going on does not end with the substances at first concerned, but
sets up chemical actions, or changes of molecular arrangement, among
surrounding substances that would else have remained quiescent. And there
are yet further cases in which mere contact with a substance that is itself
quiescent, will cause other substances to undergo rapid metamorphoses. In
what we call fermentation, the first species of this communicated chemical
action is exemplified. One part of yeast, while itself undergoing molecular
change, will convert 100 parts of sugar into alcohol and carbonic acid; and
during its own decomposition, one part of diastase "is able to effect the
transformation of more than 1000 times its weight of starch into sugar." As
illustrations of the second species, may be mentioned those changes which
are suddenly produced in many colloids by minute portions of various
substances added to them--substances that are not undergoing manifest
transformations, and suffer no appreciable effects from the contact. The
nature of the first of these two kinds of communicated molecular change,
which here chiefly concerns us, may be rudely represented by certain
visible changes communicated from mass to mass, when a series of masses has
been arranged in a special way. The simplest example is that furnished by
the child's play of setting bricks on end in a row, in such positions that
when the first is overthrown it overthrows the second, the second the
third, the third the fourth, and so on to the end of the row. Here we have
a number of units severally placed in unstable equilibrium, and in such
relative positions that each, while falling into a state of stable
equilibrium, gives an impulse to the next sufficient to make the next,
also, fall from unstable to stable equilibrium. Now since, among mingled
compound molecules, no one can undergo change in the arrangement of its
parts without a molecular motion that must cause some disturbance all
round; and since an adjacent molecule disturbed by this communicated
motion, may have the arrangement of its constituent atoms altered, if it is
not a stable arrangement; and since we know, both that the molecules which
are changed by this so-called catalysis _are_ unstable, and that the
molecules resulting from their changes are _more_ stable; it seems probable
that the transformation is really analogous, in principle, to the familiar
one named. Whether thus interpretable or not, however, there is good reason
for thinking that to this kind of action is due a large amount of vital
metamorphosis. Let us contemplate the several groups of facts which point
to this conclusion.[9]
In the last chapter (§ 2) we incidentally noted the extreme instability of
nitrogenous compounds in general. We saw that sundry of them are liable to
explode on the slightest incentive--sometimes without any apparent cause;
and that of the rest, the great majority are very easily decomposed by
heat, and by various substances. We shall perceive much significance in
this general characteristic when we join it with the fact that the
substances capable of setting up extensive molecular changes in the way
above described are all nitrogenous ones. Yeast consists of vegetal cells
containing nitrogen,--cells that grow by assimilating the nitrogenous
matter contained in wort. Similarly, the "vinegar-plant," which greatly
facilitates the formation of acetic acid from alcohol, is a fungoid growth
that is doubtless, like others of its class, rich in nitrogenous compounds.
Diastase, by which the transformation of starch into sugar is effected
during the process of malting, is also a nitrogenous body. So too is a
substance called synaptase--an albumenous principle contained in almonds,
which has the power of working several metamorphoses in the matters
associated with it. These nitrogenized compounds, like the rest of their
family, are remarkable for the rapidity with which they decompose; and the
extensive changes produced by them in the accompanying carbo-hydrates, are
found to vary in their kinds according as the decompositions of the
ferments vary in their stages. We have next to note, as having here a
meaning for us, the chemical contrasts between those organisms which carry
on their functions by the help of external forces, and those which carry on
their functions by forces evolved from within. If we compare animals and
plants, we see that whereas plants, characterized as a class by containing
but little nitrogen, are dependent on the solar rays for their vital
activities; animals, the vital activities of which are not thus dependent,
mainly consist of nitrogenous substances. There is one marked exception to
this broad distinction, however; and this exception is specially
instructive. Among plants there is a considerable group--the Fungi--many
members of which, if not all, can live and grow in the dark; and it is
their peculiarity that they are very much more nitrogenous than other
plants. Yet a third class of facts of like significance is disclosed when
we compare different portions of the same organism. The seed of a plant
contains nitrogenous substance in a far higher ratio than the rest of the
plant; and the seed differs from the rest of the plant in its ability to
initiate, in the absence of light, extensive vital changes--the changes
constituting germination. Similarly in the bodies of animals, those parts
which carry on active functions are nitrogenous; while parts that are
non-nitrogenous--as the deposits of fat--carry on no active functions. And
we even find that the appearance of non-nitrogenous matter throughout
tissues normally composed almost wholly of nitrogenous matter, is
accompanied by loss of activity: what is called fatty degeneration being
the concomitant of failing vitality. One more fact, which serves to make
still clearer the meaning of the foregoing ones, remains--the fact, namely,
that in no part of any organism where vital changes are going on, is
nitrogenous matter wholly absent. It is common to speak of plants--or at
least all parts of plants but the seeds--as non-nitrogenous. But they are
only relatively so; not absolutely. The quantity of albumenoid substance in
the tissues of plants, is extremely small compared with the quantity
contained in the tissues of animals; but all plant-tissues which are
discharging active functions have some albumenoid substance. In every
living vegetal cell there is a certain part that includes nitrogen as a
component. This part initiates those changes which constitute the
development of the cell. And if it cannot be said that it is the worker of
all subsequent changes undergone by the cell, it nevertheless continues to
be the part in which the independent activity is most marked.
Looking at the evidence thus brought together, do we not get an insight
into the actions of nitrogenous matter as a worker of organic changes? We
see that nitrogenous compounds in general are extremely prone to decompose:
their decomposition often involving a sudden and great evolution of energy.
We see that the substances classed as ferments, which, during their own
molecular changes, set up molecular changes in the accompanying
carbo-hydrates, are all nitrogenous. We see that among classes of
organisms, and among the parts of each organism, there is a relation
between the amount of nitrogenous matter present and the amount of
independent activity. And we see that even in organisms and parts of
organisms where the activity is least, such changes as do take place are
initiated by a substance containing nitrogen. Does it not seem probable,
then, that these extremely unstable compounds have everywhere the effect of
communicating to the less unstable compounds associated with them,
molecular movements towards a stable state, like those they are themselves
undergoing? The changes which we thus suppose nitrogenous matter to produce
in the body, are clearly analogous to those which we see it produce out of
the body. Out of the body, certain carbo-hydrates in continued contact with
nitrogenous matter, are transformed into carbonic acid and alcohol, and
unless prevented the alcohol is transformed into acetic acid: the
substances formed being thus more highly oxidized and more stable than the
substances destroyed. In the body, these same carbo-hydrates, in continued
contact with nitrogenous matter, are transformed into carbonic acid and
water: substances which are also more highly oxidized and more stable than
those from which they result. And since acetic acid is itself resolved by
further oxidation into carbonic acid and water; we see that the chief
difference between the two cases is, that the process is more completely
effected in the body than it is out of the body. Thus, to carry further the
simile used above, the molecules of carbo-hydrates contained in the tissues
are, like bricks on end, not in the stablest equilibrium; but still in an
equilibrium so stable, that they cannot be overthrown by the chemical and
thermal forces which the body brings to bear on them. On the other hand,
being like similarly-placed bricks that have very narrow ends, the
nitrogenous molecules contained in the tissues are in so unstable an
equilibrium that they cannot withstand these forces. And when these
delicately-poised nitrogenous molecules fall into stable arrangements, they
give impulses to the more firmly-poised non-nitrogenous molecules, which
cause them also to fall into stable arrangements. It is a curious and
significant fact that in the arts, we not only utilize this same principle
of initiating extensive changes among comparatively stable compounds, by
the help of compounds much less stable, but we employ for the purpose
compounds of the same general class. Our modern method of firing a gun is
to place in close proximity with the gunpowder which we wish to decompose
or explode, a small portion of fulminating powder, which is decomposed or
exploded with extreme facility, and which, on decomposing, communicates the
consequent molecular disturbance to the less-easily decomposed gunpowder.
When we ask what this fulminating powder is composed of, we find that it is
a nitrogenous salt.[10]
Thus, besides the molecular re-arrangements produced in organic matter by
direct chemical action, there are others of kindred importance produced by
indirect chemical action. Indeed, the inference that some of the leading
transformations occurring in the animal organism, are due to this so-called
catalysis, appears necessitated by the general aspect of the facts, apart
from any such detailed interpretations as the foregoing. We know that
various amylaceous and saccharine matters taken as food do not appear in
the excreta, and must therefore be decomposed in their course through the
body. We know that these matters do not become components of the tissues,
but only of the contained liquids and solids; and that thus their
metamorphosis is not a direct result of tissue-change. We know that their
stability is such that the thermal and chemical forces to which they are
exposed in the body, cannot alone decompose them. The only explanation open
to us, therefore, is that the transformation of these carbo-hydrates into
carbonic acid and water, is due to communicated chemical action.
§ 16. This chapter will have served its purpose if it has given a
conception of the extreme modifiability of organic matter by surrounding
agencies. Even were it possible, it would be needless to describe in detail
the immensely varied and complicated changes which the forces from moment
to moment acting on them, work in living bodies. Dealing with biology in
its general principles, it concerns us only to notice how specially
sensitive are the substances of which organisms are built up to the varied
influences that act upon organisms. Their special sensitiveness has been
made sufficiently manifest in the several foregoing sections.
CHAPTER III.
THE RE-ACTIONS OF ORGANIC MATTER ON FORCES.
§ 17. Re-distributions of Matter imply concomitant re-distributions of
Motion. That which under one of its aspects we contemplate as an alteration
of arrangement among the parts of a body, is, under a correlative aspect,
an alteration of arrangement among certain momenta, whereby these parts are
impelled to their new positions. At the same time that a force, acting
differently on the different units of an aggregate, changes their relations
to one another; these units, reacting differently on the different parts of
the force, work equivalent changes in the relations of these to one
another. Inseparably connected as they are, these two orders of phenomena
are liable to be confounded together. It is very needful, however, to
distinguish between them. In the last chapter we took a rapid survey of the
re-distributions which forces produce in organic matter; and here we must
take a like survey of the simultaneous re-distributions undergone by the
forces.
At the outset we are met by a difficulty. The parts of an inorganic mass
undergoing re-arrangement by an incident force, are in most cases
passive--do not complicate those necessary re-actions that result from
their inertia, by other forces which they themselves originate. But in
organic matter the re-arranged parts do not re-act in virtue of their
inertia only. They are so constituted that an incident force usually sets
up in them other actions which are much more important. Indeed, what we may
call the indirect reactions thus caused, are so great in their amounts
compared with the direct re-actions, that they quite obscure them.
The impossibility of separating these two kinds of reaction compels us to
disregard the distinction between them. Under the above general title, we
must include both the immediate re-actions and those re-actions mediately
produced, which are among the most conspicuous of vital phenomena.
§ 18. From organic matter, as from all other matter, incident forces call
forth that re-action which we know as heat. More or less of molecular
vibration necessarily results when, to the forces at work among the
molecules of any aggregate, other forces are added. Experiment abundantly
demonstrates this in the case of inorganic masses; and it must equally hold
in the case of organic masses. In both cases the force which, more
markedly than any other, produces this thermal re-action, is that which
ends in the union of different substances. Though inanimate bodies admit of
being greatly heated by pressure and by the electric current, yet the
evolutions of heat, thus induced are neither so common, nor in most cases
so conspicuous, as those resulting from chemical combination. And though in
animate bodies there are certain amounts of heat generated by other
actions, yet these are secondary to the heat generated by the action of
oxygen on the substances composing the tissues and the substances contained
in them. Here, however, we see one of the characteristic distinctions
between inanimate and animate bodies. Among the first there are but few
which ordinarily exist in a condition to evolve the heat caused by chemical
combination; and such as are in this condition soon cease to be so when
chemical combination and genesis of heat once begin in them. Whereas, among
the second there universally exists the ability, more or less decided, thus
to evolve heat; and the evolution of heat, in some cases very slight and in
no cases very great, continues as long as they remain animate bodies.
The relation between active change of matter and re-active genesis of
molecular vibration, is clearly shown by the contrasts between different
organisms, and between different states and parts of the same organism. In
plants the genesis of heat is extremely small, in correspondence with their
extremely small production of carbonic acid: those portions only, as
flowers and germinating seeds, in which considerable oxidation is going on,
having decidedly raised temperatures. Among animals we see that the
hot-blooded are those which expend much force and respire actively. Though
insects are scarcely at all warmer than the surrounding air when they are
still, they rise several degrees above it when they exert themselves; and
in mammals, which habitually maintain a temperature much higher than that
of their medium, exertion is accompanied by an additional production of
heat.
This molecular agitation accompanies the falls from unstable to stable
molecular combinations; whether they be those from the most complex to the
less complex compounds, or whether they be those ultimate falls which end
in fully oxidized and relatively simple compounds; and whether they be
those of the nitrogenous matters composing the tissues or those of the
non-nitrogenous matters diffused through them. In the one case as in the
other, the heat must be regarded as a concomitant. Whether the
distinction, originally made by Liebig, between nitrogenous substances as
tissue-food and non-nitrogenous substances as heat-food, be true or not in
a narrower sense, it cannot be accepted in the sense that tissue-food is
not also heat-food. Indeed he does not himself assert it in this sense. The
ability of carnivorous animals to live and generate heat while consuming
matter that is almost exclusively nitrogenous, suffices to prove that the
nitrogenous compounds forming the tissues are heat-producers, as well as
the non-nitrogenous compounds circulating among and through the tissues: a
conclusion which is indeed justified by the fact that nitrogenous
substances out of the body yield heat, though not a large amount, during
combustion. But most likely this antithesis is not true even in the more
restricted sense. The probability is that the hydrocarbons and
carbo-hydrates which, in traversing the system, are transformed by
communicated chemical action, evolve, during their transformation, not heat
alone but also other kinds of force. It may be that as the nitrogenous
matter, while falling into more stable molecular arrangements, generates
both that molecular agitation called heat and such other molecular
movements as are resolved into forces expended by the organism; so, too,
does the non-nitrogenous matter. Or perhaps the concomitants of this
metamorphosis of non-nitrogenous matter vary with the conditions. Heat
alone may result when it is transformed while in the circulating fluids,
but partly heat and partly another force when it is transformed in some
active tissue that has absorbed it; just as coal, though producing little
else but heat as ordinarily burnt, has its heat partially transformed into
mechanical motion if burnt in a steam-engine furnace. In such case the
antithesis of Liebig would be reduced to this--that whereas nitrogenous
substance is tissue-food _both_ as material for building-up tissue and as
material for its function; non-nitrogenous substance is tissue-food _only_
as material for function.
There can be no doubt that this thermal re-action which chemical action
from moment to moment produces in the body, is from moment to moment an aid
to further chemical action. We before saw (_First Principles_, § 100) that
a state of raised molecular vibration is favourable to those
re-distributions of matter and motion which constitute Evolution. We saw
that in organisms distinguished by the amount and rapidity of such
re-distributions, this raised state of molecular vibration is conspicuous.
And we here see that this raised state of molecular vibration is itself a
continuous consequence of the continuous molecular re-distributions it
facilitates. The heat generated by each increment of chemical change makes
possible the succeeding increment of chemical change. In the body this
connexion of phenomena is the same as we see it to be out of the body. Just
as in a burning piece of wood, the heat given out by the portion actually
combining with oxygen, raises the adjacent portion to a temperature at
which it also can combine with oxygen; so, in a living animal, the heat
produced by oxidation of each portion of organized or unorganized
substance, maintains the temperature at which the unoxidized portions can
be readily oxidized.
§ 19. Among the forces called forth from organisms by re-action against the
actions to which they are subject, is Light. Phosphorescence is in some few
cases displayed by plants--especially by certain fungi. Among animals it is
comparatively common. All know that there are several kinds of luminous
insects; and many are familiar with the fact that luminosity is a
characteristic of various marine creatures.
Much of the evidence is supposed to imply that this evolution of light,
like the evolution of heat, is consequent on oxidation of the tissues or of
matters contained in them. Light, like heat, is the expression of a raised
state of molecular vibration: the difference between them being a
difference in the rates of vibration. Hence it seems inferable that by
chemical action on substances contained in the organism, heat or light may
be produced, according to the character of the resulting molecular
vibrations. Some experimental evidence supports this view. In
phosphorescent insects, the continuance of the light is found to depend on
the continuance of respiration; and any exertion which renders respiration
more active, increases the brilliancy of the light. Moreover, by separating
the luminous matter, Prof. Matteucci has shown that its emission of light
is accompanied by absorption of oxygen and escape of carbonic acid. The
phosphorescence of marine animals has been referred to other causes than
oxidation; but it may perhaps be explicable without assuming any more
special agency. Considering that in creatures of the genus _Noctiluca_, for
example, to which the phosphorescence most commonly seen on our own coasts
is due, there is no means of keeping up a constant circulation, we may
infer that the movements of aerated fluids through their tissues, must be
greatly affected by impulses received from without. Hence it may be that
the sparkles visible at night when the waves break gently on the beach, or
when an oar is dipped into the water, are called forth from these creatures
by the concussion, not because of any unknown influence it excites, but
because, being propagated through their delicate tissues, it produces a
sudden movement of the fluids and a sudden increase of chemical action.
Nevertheless, in other phosphorescent animals inhabiting the sea, as in the
_Pyrosoma_ and in certain _Annelida_, light seems to be produced otherwise
than by direct re-action on the action of oxygen. Indeed, it needs but to
recall the now familiar fact that certain substances become luminous in the
dark after exposure to sunlight, to see that there are other causes of
light-emission.
§ 20. The re-distributions of inanimate matter are habitually accompanied
by electrical disturbances; and there is abundant evidence that electricity
is generated during those re-distributions of matter that are ever taking
place in organisms. Experiments have shown "that the skin and most of the
internal membranes are in opposite electrical states;" and also that
between different internal organs, as the liver and the stomach, there are
electrical contrasts: such contrasts being greatest where the processes
going on in the compared parts are most unlike. It has been proved by du
Bois-Reymond that when any point in the longitudinal section of a muscle is
connected by a conductor with any point in its transverse section, an
electric current is established; and further, that like results occur when
nerves are substituted for muscles. The special causes of these phenomena
have not yet been determined. Considering that the electric contrasts are
most marked where active secretions are going on--considering, too, that
they are difficult to detect where there are no appreciable movements of
liquids--considering, also, that even when muscles are made to contract
after removal from the body, the contraction inevitably causes movements of
the liquids still contained in its tissues; it may be that they are due
simply to the friction of heterogeneous substances, which is universally a
cause of electric disturbance. But whatever be the interpretation, the fact
remains the same:--there is throughout the living organism, an unceasing
production of differences between the electric states of different parts;
and, consequently, an unceasing restoration of electric equilibrium by the
establishment of currents among these parts.
Besides these general, and not conspicuous, electrical phenomena common to
all organisms, vegetal as well as animal, there are certain special and
strongly marked ones. I refer, of course, to those which have made the
_Torpedo_ and the _Gymnotus_ objects of so much interest. In these
creatures we have a genesis of electricity which is not incidental on the
performance of their different functions by the different organs; but one
which is itself a function, having an organ appropriate to it. The
character of this organ in both these fishes, and its largely-developed
connexions with the nervous centres, have raised in some minds the
suspicion that in it there takes place a transformation of what we call
nerve-force into the force known as electricity. Perhaps, however, the true
interpretation may rather be that by nervous stimulation there is set up in
these animal-batteries that particular transformation of molecular motion
which it is their function to produce.
But whether general or special, and in whatever manner produced, these
evolutions of electricity are among the reactions of organic matter called
forth by the actions to which it is subject. Though these re-actions are
not direct, but seem to be remote consequences of changes wrought by
external agencies on the organism, they are yet incidents in that general
re-distribution of motion which these external agencies initiate; and as
such must here be noticed.
§ 21. To these known modes of motion, has next to be added an unknown one.
Heat, Light, and Electricity are emitted by inorganic matter when
undergoing changes, as well as by organic matter. But there is manifested
in some classes of living bodies a kind of force which we cannot identify
with any of the forces manifested by bodies that are not alive,--a force
which is thus unknown, in the sense that it cannot be assimilated to any
otherwise-recognized class. I allude to what is called nerve-force.
This is habitually generated in all animals, save the lowest, by incident
forces of every kind. The gentle and violent mechanical contacts, which in
ourselves produce sensations of touch and pressure--the additions and
abstractions of molecular vibration, which in ourselves produce sensations
of heat and cold, produce in all creatures that have nervous systems,
certain nervous disturbances: disturbances which, as in ourselves, are
either communicated to the chief nervous centre, and there arouse
consciousness, or else result in mere physical processes set going
elsewhere in the organism. In special parts distinguished as organs of
sense, other external actions bring about other nervous re-actions, that
show themselves either as special sensations or as excitements which,
without the intermediation of distinct consciousness, beget actions in
muscles or other organs. Besides neural discharges following the direct
incidence of external forces, others are ever being caused by the incidence
of forces which, though originally external, have become internal by
absorption into the organism of the agents exerting them. For thus may be
classed those neural discharges which result from modifications of the
tissues wrought by substances carried to them in the blood. That the
unceasing change of matter which oxygen and other agents produce throughout
the system, is accompanied by production of nerve-force, is shown by
various facts;--by the fact that nerve-force is no longer generated if
oxygen be withheld or the blood prevented from circulating; by the fact
that when the chemical transformation is diminished, as during sleep with
its slow respiration and circulation, there is a diminution in the quantity
of nerve-force; by the fact that an excessive expenditure of nerve-force
involves excessive respiration and circulation, and excessive waste of
tissue. To these proofs that nerve-force is evolved in greater or less
quantity, according as the conditions to rapid molecular change throughout
the body are well or ill fulfilled, may be added proofs that certain
special molecular actions are the causes of these special re-actions. The
effects of the vegeto-alkalies put beyond doubt the inference that the
overthrow of molecular equilibrium by chemical affinity, when it occurs in
certain parts, causes excitement in the nerves proceeding from those parts.
Indeed, looked at from this point of view, the two classes of nervous
changes--the one initiated from without and the other from within--are seen
to merge into one class. Both of them may be traced to metamorphosis of
tissue. The sensations of touch and pressure are doubtless consequent on
accelerated changes of matter, produced by mechanical disturbance of the
mingled fluids and solids composing the parts affected. There is abundant
evidence that the gustatory sensation is due to the chemical actions set up
by particles which find their way through the membrane covering the nerves
of taste; for, as Prof. Graham points out, sapid substances belong to the
class of crystalloids, which are able rapidly to permeate animal tissue,
while the colloids which cannot pass through animal tissue are insipid.
Similarly with the sense of smell. Substances which excite this sense are
necessarily more or less volatile; and their volatility being the result of
their molecular mobility, implies that they have, in a high degree, the
power of getting at the olfactory nerves by penetrating their mucous
investment. Again, the facts which photography has familiarized us with,
show that those nervous impressions called colours, are primarily due to
certain changes wrought by light in the substance of the retina. And
though, in the case of hearing, we cannot so clearly trace the connexion of
cause and effect, yet as we see that the auditory apparatus is one fitted
to intensify those vibrations constituting sound, and to convey them to a
receptacle containing liquid in which nerves are immersed, it can scarcely
be doubted that the sensation of sound proximately results from molecular
re-arrangements caused in these nerves by the vibrations of the liquid:
knowing, as we do, that the re-arrangement of molecules is in all cases
aided by agitation. Perhaps, however, the best proof that nerve-force,
whether peripheral or central in origin, results from chemical change, lies
in the fact that most of the chemical agents which powerfully affect the
nervous system, affect it whether applied at the centre or at the
periphery. Various mineral acids are tonics--the stronger ones being
usually the stronger tonics; and this which we call their acidity implies a
power in them of acting on the nerves of taste, while the tingling or pain
following their absorption through the skin, implies that the nerves of the
skin are acted on by them. Similarly with certain vegeto-alkalies which are
peculiarly bitter. By their bitterness these show that they affect the
extremities of the nerves, while, by their tonic properties, they show that
they affect the nervous centres: the most intensely bitter among them,
strychnia, being the most powerful nervous stimulant.[11] However true it
may be that this relation is not a regular one, since opium, hashish, and
some other drugs, which work marked effects on the brain, are not
remarkably sapid--however true it may be that there are relations between
particular substances and particular parts of the nervous system; yet such
instances do but qualify, without negativing, the general proposition. The
truth of this proposition can scarcely be doubted when, to the facts above
given, is added the fact that various condiments and aromatic drugs act as
nervous stimulants; and the fact that anæsthetics, besides the general
effects they produce when inhaled or swallowed, produce local effects of
like kind--first stimulant and then sedative--when absorbed through the
skin; and the fact that ammonia, which in consequence of its extreme
molecular mobility so quickly and so violently excites the nerves beneath
the skin, as well as those of the tongue and the nose, is a rapidly-acting
stimulant when taken internally.
Whether a nerve is merely a conductor, which delivers at one of its
extremities an impulse received at the other, or whether, as some now
think, it is itself a generator of force which is initiated at one
extremity and accumulates in its course to the other extremity, are
questions which cannot yet be answered. All we know is that agencies
capable of working molecular changes in nerves are capable of calling forth
from them manifestations of activity. And our evidence that nerve-force is
thus originated, consists not only of such facts as the above, but also of
more conclusive facts established by direct experiments on
nerves--experiments which show that nerve-force results when the cut end of
a nerve is either mechanically irritated, or acted on by some chemical
agent, or subject to the galvanic current--experiments which prove that
nerve-force is generated by whatever disturbs the molecular equilibrium of
nerve-substance.
§ 22. The most important of the re-actions called forth from organisms by
surrounding actions, remains to be noticed. To the various forms of
insensible motion thus caused, we have to add sensible motion. On the
production of this mode of force more especially depends the possibility of
all vital phenomena. It is, indeed, usual to regard the power of generating
sensible motion as confined to one out of the two organic sub-kingdoms; or,
at any rate, as possessed by but few members of the other. On looking
closer into the matter, however, we see that plant-life as well as
animal-life, is universally accompanied by certain manifestations of this
power; and that plant-life could not otherwise continue.
Through the humblest, as well as through the highest, vegetal organisms,
there are ever going on certain re-distributions of matter. In Protophytes
the microscope shows us an internal transposition of parts, which, when not
immediately visible, is proved to exist by the changes of arrangement that
become manifest in the course of hours and days. In the individual cells of
many higher plants, an active movement among the contained granules may be
witnessed. And well-developed cryptogams, in common with all phanerogams,
exhibit this genesis of mechanical motion still more conspicuously in the
circulation of sap. It might, indeed, be concluded _a priori_, that through
plants displaying much differentiation of parts, an internal movement must
be going on; since, without it, the mutual dependence of organs having
unlike functions would be impossible. Besides keeping up these motions of
liquids internally, plants, especially of the lower orders, move their
external parts in relation to each other, and also move about from place to
place. There are countless such illustrations as the active locomotion of
the zoospores of many _Algæ_, the rhythmical bendings of the _Oscillatoræ_,
the rambling progression of the _Diatomaceæ_. In fact many of these
smallest vegetals, and many of the larger ones in their early stages,
display a mechanical activity not distinguishable from that of the simplest
animals. Among well-organized plants, which are never locomotive in their
adult states, we still not unfrequently meet with relative motions of
parts. To such familiar cases as those of the Sensitive plant and the
Venus' fly-trap, many others may be added. When its base is irritated the
stamen of the Berberry flower leans over and touches the pistil. If the
stamens of the wild _Cistus_ be gently brushed with the finger, they spread
themselves: bending away from the seed-vessel. And some of the
orchid-flowers, as Mr. Darwin has shown, shoot out masses of pollen on to
the entering bee, when its trunk is thrust down in search of honey.
Though the power of moving is not, as we see, a characteristic of animals
alone, yet in them, considered as a class, it is manifested to an extent so
marked as practically to become their most distinctive trait. For it is by
their immensely greater ability to generate mechanical motion, that animals
are enabled to perform those actions which constitute their visible lives;
and it is by their immensely greater ability to generate mechanical motion,
that the higher orders of animals are most obviously distinguished from the
lower orders. Though, on remembering the seemingly active movements of
infusoria, some will perhaps question this last-named contrast, yet, on
comparing the quantities of matter propelled through given spaces in given
times, they will see that the momentum evolved is far less in the
_Protozoa_ than in the _Metazoa_. These sensible motions of animals are
effected in sundry ways. In the humblest forms, and even in some of the
more developed forms which inhabit the water, locomotion results from the
oscillations of whip-like appendages, single or double, or from the
oscillations of cilia: the contractility resides in these waving hairs that
grow from the surface. In many _Coelenterata_ certain elongations or tails
of ectodermal or endodermal cells shorten when stimulated, and by these
rudimentary contractile organs the movements are effected. In all the
higher animals, however, and to a smaller degree in many of the lower,
sensible motion is generated by a special tissue, under a special
excitement. Though it is not strictly true that such animals show no
sensible motions otherwise caused, since all of them have certain ciliated
membranes, and since the circulation of liquids in them is partially due to
osmotic and capillary actions; yet, generally speaking, we may say that
their movements are effected solely by muscles which contract solely
through the agency of nerves.
What special transformations of force generate these various mechanical
changes, we do not, in most cases, know. Those re-distributions of liquid,
with the alterations of form sometimes caused by them, that result from
osmose, are not, indeed, incomprehensible. Certain motions of plants which,
like those of the "animated oat," follow contact with water, are easily
interpreted; as are also such other vegetal motions as those of the
Touch-me-not, the Squirting Cucumber, and the _Carpobolus_. But we are
ignorant of the mode in which molecular movement is transformed into the
movement of masses, in animals. We cannot refer to known causes the
rhythmical action of a Medusa's disc, or that slow decrease of bulk which
spreads throughout the mass of an _Alcyonium_ when one of its component
individuals has been irritated. Nor are we any better able to say how the
insensible motion transmitted through a nerve, gives rise to sensitive
motion in a muscle. It is true that Science has given to Art several
methods of changing insensible into sensible motion. By applying heat to
water we vaporize it, and the movement of its expanding vapour we transfer
to solid matter; but evidently the genesis of muscular movement is in no
way analogous to this. The force evolved in a galvanic battery or by a
dynamo, we communicate to a soft iron magnet through a wire coiled round
it; and it would be possible, by placing near to each other several magnets
thus excited, to obtain, through the attraction of each for its neighbours,
an accumulated movement made up of their separate movements, and thus
mechanically to imitate a muscular contraction. But from what we know of
organic matter there is no reason to suppose that anything analogous to
this takes place in it. We can, however, through one kind of molecular
change, produce sensible changes of aggregation such as possibly might,
when occurring in organic substance, cause sensible motion in it. I refer
to change that is allotropic or isomeric. Sulphur, for example, assumes
different crystalline and non-crystalline forms at different temperatures,
and may be made to pass backwards and forwards from one form to another, by
slight variations of temperature: undergoing each time an alteration of
bulk. We know that this allotropism, or rather its analogue isomerism,
prevails among colloids--inorganic and organic. We also know that some of
these metamorphoses among colloids are accompanied by visible
re-arrangements: instance hydrated silicic acid, which, after passing from
its soluble state to the state of an insoluble jelly, begins, in a few
days, to contract and to give out part of its contained water. Now
considering that such isomeric changes of organic as well as inorganic
colloids, are often rapidly produced by very slight causes--a trace of a
neutral salt or a degree or two rise of temperature--it seems not
impossible that some of the colloids constituting muscle may be thus
changed by a nervous discharge: resuming their previous condition when the
discharge ceases. And it is conceivable that by structural arrangements,
minute sensible motions so caused may be accumulated into large sensible
motions.
§ 23. But the truths which it is here our business especially to note, are
independent of hypotheses or interpretations. It is sufficient for the ends
in view, to observe that organic matter _does_ exhibit these several
conspicuous reactions when acted on by incident forces. It is not requisite
that we should know _how_ these re-actions originate.
In the last chapter were set forth the several modes in which incident
forces cause re-distributions of organic matter; and in this chapter have
been set forth the several modes in which is manifested the motion
accompanying this re-distribution. There we contemplated, under its several
aspects, the general fact that, in consequence of its extreme instability,
organic matter undergoes extensive molecular re-arrangements on very slight
changes of conditions. And here we have contemplated, under its several
aspects, the correlative general fact that, during these extensive
molecular re-arrangements, there are evolved large amounts of energy. In
the one case the components of organic matter are regarded as falling from
positions of unstable equilibrium to positions of stable equilibrium; and
in the other case they are regarded as giving out in their falls certain
momenta--momenta that may be manifested as heat, light, electricity,
nerve-force, or mechanical motion, according as the conditions determine.
I will add only that these evolutions of energy are rigorously dependent on
these changes of matter. It is a corollary from the primordial truth which,
as we have seen, underlies all other truths, (_First Principles_, §§ 62,
189,) that whatever amount of power an organism expends in any shape, is
the correlate and equivalent of a power which was taken into it from
without. On the one hand, it follows from the persistence of force that
each portion of mechanical or other energy which an organism exerts,
implies the transformation of as much organic matter as contained this
energy in a latent state. And on the other hand, it follows from the
persistence of force that no such transformation of organic matter
containing this latent energy can take place, without the energy being in
one shape or other manifested.
CHAPTER III^{A.}
METABOLISM.
§ 23a. In the early forties the French chemist Dumas pointed out the
opposed actions of the vegetal and animal kingdoms: the one having for its
chief chemical effect the decomposition of carbon-dioxide, with
accompanying assimilation of its carbon and liberation of its oxygen, and
the other having for its chief chemical effect the oxidation of carbon and
production of carbon-dioxide. Omitting those plants which contain no
chlorophyll, all others de-oxidize carbon; while all animals, save the few
which contain chlorophyll, re-oxidize carbon. This is not, indeed, a
complete account of the general relation; since it represents animals as
wholly dependent on plants, either directly or indirectly through other
animals, while plants are represented as wholly independent of animals; and
this last representation though mainly true, since plants can obtain direct
from the inorganic world certain other constituents they need, is in some
measure not true, since many with greater facility obtain these materials
from the decaying bodies of animals or from their _excreta_. But after
noting this qualification the broad antithesis remains as alleged.
How are these transformations brought about? The carbon contained in
carbon-dioxide does not at a bound become incorporated in the plant, nor
does the substance appropriated by the animal from the plant become at a
bound carbon-dioxide. It is through two complex sets of changes that these
two ultimate results are brought about. The materials forming the tissues
of plants as well as the materials contained in them, are progressively
elaborated from the inorganic substances; and the resulting compounds,
eaten and some of them assimilated by animals, pass through successive
changes which are, on the average, of an opposite character: the two sets
being constructive and destructive. To express changes of both these
natures the term "metabolism" is used; and such of the metabolic changes as
result in building up from simple to compound are distinguished as
"anabolic," while those which result in the falling down from compound to
simple are distinguished as "katabolic." These antithetical names do not
indeed cover all the molecular transformations going on. Many of them,
known as isomeric, imply neither building up nor falling down: they imply
re-arrangement only. But those which here chiefly concern us are the two
opposed kinds described.
A qualification is needful. These antithetic changes must be understood as
characterizing plant-life and animal-life in general ways rather than in
special ways--as expressing the transformations in their totalities but not
in their details. For there are katabolic processes in plants, though they
bear but a small ratio to the anabolic ones; and there are anabolic
processes in animals, though they bear but a small ratio to the katabolic
ones.
From the chemico-physical aspect of these changes we pass to those
distinguished as vital; for metabolic changes can be dealt with only as
changes effected by that living substance called protoplasm.
§ 23b. On the evolution-hypothesis we are obliged to assume that the
earliest living things--probably minute units of protoplasm smaller than
any the microscope reveals to us--had the ability to appropriate directly
from the inorganic world both the nitrogen and the materials for
carbo-hydrates without both of which protoplasm cannot be formed; since in
the absence of preceding organic matter there was no other source. The
general law of evolution as well as the observed actions of _Protozoa_ and
_Protophyta_, suggest that these primordial types simultaneously displayed
animal-life and plant-life. For whereas the developed animal-type cannot
form from its inorganic surroundings either nitrogenous compounds or
carbo-hydrates; and whereas the developed plant-type, able to form
carbo-hydrates from its inorganic surroundings, depends for the formation
of its protoplasm mainly, although indirectly, on the nitrogenous compounds
derived from preceding organisms, as do also most of the plants devoid of
chlorophyll--the fungi; we are obliged to assume that in the beginning,
along with the expending activities characterizing the animal-type, there
went the accumulating activities characterizing both of the vegetal
types--forms of activity by-and-by differentiated.
Though the successive steps in the artificial formation of organic
compounds have now gone so far that substances simulating proteids, if not
identical with them, have been produced, yet we have no clue to the
conditions under which proteids arose; and still less have we a clue to the
conditions under which inert proteids became so combined as to form active
protoplasm. The essential fact to be recognized is that living matter,
originated as we must assume during a long stage of progressive cooling in
which the infinitely varied parts of the Earth's surface were slowly
passing through appropriate physical conditions, possessed from the outset
the power of assimilating to itself the materials from which more living
matter was formed; and that since then all living matter has arisen from
its self-increasing action. But now, leaving speculation concerning these
anabolic changes as they commenced in the remote past, let us contemplate
them as they are carried on now--first directing our attention to those
presented in the vegetal world.
§ 23c. The decomposition of carbon-dioxide (§ 13)--the separation of its
carbon from the combined oxygen so that it may enter into one or other form
of carbo-hydrate,--is not now ordinarily effected, as we must assume it
once was, by the undifferentiated protoplasm; but is effected by a
specialized substance, chlorophyll, imbedded in the protoplasm and
operating by its instrumentality. The chlorophyll-grain is not simply
immersed in protoplasm but is permeated throughout its substance by a
protoplasmic network or sponge-work apparently continuous with the
protoplasm around; or, according to Sachs, consists of protoplasm holding
chlorophyll-particles in suspension: the mechanical arrangement
facilitating the chemical function. The resulting abstraction of carbon
from carbon-dioxide, by the aid of certain ethereal undulations, appears to
be the first step in the building up of organic compounds--the first step
in the primary anabolic process. We are not here concerned with details.
Two subsequent sets of changes only need here to be noted--the genesis of
the passive materials out of which plant-structure is built up, and the
genesis of the active materials by which these are produced and the
building up effected.
The hydrated carbon which protoplasm, having the chlorophyll-grain as its
implement, produces from carbonic acid and water, appears not to be of one
kind only. The possible carbo-hydrates are almost infinite in number.
Multitudes of them have been artificially made, and numerous kinds are made
naturally by plants. Though perhaps the first step in the reduction of the
carbon from its dioxide may be always the same, yet it is held probable
that in different types of plants different types of carbo-hydrates
forthwith arise, and give differential characters to the compounds
subsequently formed by such types: sundry of the changes being katabolic
rather than anabolic. Of leading members in the group may be named dextrin,
starch, and the various sugars characteristic of various plants, as well as
the cellulose elaborated by further anabolism. Considered as the kind of
carbo-hydrate in which the products of activity are first stored up, to be
subsequently modified for divers purposes, starch is the most important of
these; and the process of storage is suggested by the structure of the
starch-grain. This consists of superposed layers, implying intermittent
deposits: the probability being that the variations of light and heat
accompanying day and night are associated now with arrest of the deposit
and now with recommencement of it. Like in composition as this stored-up
starch is with sugar of one or other kind, and capable of being deposited
from sugar and again assuming the sugar form, this substance passes, by
further metabolism, here into the cellulose which envelopes each of the
multitudinous units of protoplasm, there into the spiral fibres, annuli, or
fenestrated tubes which, in early stages of tissue-growth, form channels
for the sap, and elsewhere into other components of the general structure.
The many changes implied are effected in various ways: now by that simple
re-arrangement of components known as isomeric change; now by that taking
from a compound one of its elements and inserting one of another kind,
which is known as substitution; and now by oxidation, as when the
oxy-cellulose which constitutes wood-fibre, is produced.
Besides elaborating building materials, the protoplasm elaborates
itself--that is, elaborates more of itself. It is chemically distinguished
from the building materials by the presence of nitrogen. Derived from
atmospheric ammonia, or from decaying or excreted organic matter, or from
the products of certain fungi and microbes at its roots, the nitrogen in
one or other combination is brought into a plant by the upward current; and
by some unknown process (not dependent on light, since it goes on equally
well if not better in darkness) the protoplasm dissociates and appropriates
this combined nitrogen and unites it with a carbo-hydrate to form one or
other proteid--albumen, gluten, or some isomer; appropriating at the same
time from certain of the earth-salts the requisite amount of sulphur and in
some cases phosphorus. The ultimate step, as we must suppose, is the
formation of living protoplasm out of these non-living proteids. A cardinal
fact is that proteids admit of multitudinous transformations; and it seems
not improbable that in protoplasm various isomeric proteids are mingled. If
so, we must conclude that protoplasm admits of almost infinite variations
in nature. Of course _pari passu_ with this dual process--augmentation of
protoplasm and accompanying production of carbo-hydrates--there goes
extension of plant-structure and plant-life.
To these essential metabolic processes have to be added certain ancillary
and non-essential ones, ending in the formation of colouring matters,
odours, essential oils, acrid secretions, bitter compounds and poisons:
some serving to attract animals and others to repel them. Sundry of these
appear to be excretions--useless matters cast out, and are doubtless
katabolic.
The relation of these facts here sketched in rude outline to the doctrine
of Evolution at large should be observed. Already we have seen how (§ 8a),
in the course of terrestrial evolution, there has been an increasingly
heterogeneous assemblage of increasing heterogeneous compounds, preparing
the way for organic life. And here we may see that during the development
of plant-life from its lowest algoid and fungoid forms up to those forms
which constitute the chief vegetal world, there has been an increasing
number of complex organic compounds formed; displayed at once in the
diversity of them contained in the same plant and in the still greater
diversity displayed in the vast aggregate of species, genera, orders, and
classes of plants.
§ 23d. On passing to the metabolism characterizing animal life, which, as
already indicated, is in the main a process of decomposition undoing the
process of composition characterizing vegetal life, we may fitly note at
the outset that it must have wide limits of variation, alike in different
classes of animals and even in the same animal.
If we take, on the one hand, a carnivore living on muscular tissue (for
wild carnivores preying upon herbivores which can rarely become fat obtain
scarcely any carbo-hydrates) and observe that its food is almost
exclusively nitrogenous; and if, on the other hand, we take a graminivorous
animal the food of which (save when it eats seeds) contains comparatively
little nitrogenous matter; we seem obliged to suppose that the parts played
in the organic processes by the proteids and the carbo-hydrates can in
considerable measures replace one another. It is true that the quantity of
food and the required alimentary system in the last case, are very much
greater than in the first case. But this difference is mainly due to the
circumstance that the food of the graminivorous animal consists chiefly of
waste-matter--ligneous fibre, cellulose, chlorophyll--and that could the
starch, sugar, and protoplasm be obtained without the waste-matter, the
required bulks of the two kinds of food would be by no means so strongly
contrasted. This becomes manifest on comparing flesh-eating and
grain-eating birds--say a hawk and a pigeon. In powers of flight these do
not greatly differ, nor is the size of the alimentary system conspicuously
greater in the last than in the first; though probably the amount of food
consumed is greater. Still it seems clear that the supply of energy
obtained by a pigeon from carbo-hydrates with a moderate proportion of
proteids is not widely unlike that obtained by a hawk from proteids alone.
Even from the traits of men differently fed a like inference may be drawn.
On the one hand we have the Masai who, during their warrior-days, eat flesh
exclusively; and on the other hand we have the Hindus, feeding almost
wholly on vegetable food. Doubtless the quantities required in these cases
differ much; but the difference between the rations of the flesh-eater and
the grain-eater is not so immense as it would be were there no substitution
in the physiological uses of the materials.
Concerning the special aspects of animal-metabolism, we have first to note
those various minor transformations that are auxiliary to the general
transformation by which force is obtained from food. For many of the vital
activities merely subserve the elaboration of materials for activity at
large, and the getting rid of waste products. From blood passing through
the salivary glands is prepared in large quantity a secretion containing
among other matters a nitrogenous ferment, ptyaline, which, mixed with food
during mastication, furthers the change of its starch into sugar. Then in
the stomach come the more or less varying secretions known in combination
as gastric juice. Besides certain salts and hydrochloric acid, this
contains another nitrogenous ferment, pepsin, which is instrumental in
dissolving the proteids swallowed. To these two metabolic products aiding
solution of the various ingested solids, is presently added that product of
metabolism in the pancreas which, added to the chyme, effects certain other
molecular changes--notably that of such amylaceous matters as are yet
unaltered, into saccharine matters to be presently absorbed. And let us
note the significant fact that the preparation of food-materials in the
alimentary canal, again shows us that unstable nitrogenous compounds are
the agents which, while themselves changing, set up changes in the
carbo-hydrates and proteids around: the nitrogen plays the same part here
as elsewhere. It does the like in yet another viscus. Blood which passes
through the spleen on its way to the liver, is exposed to the action of "a
special proteid of the nature of alkali-albumin, holding iron in some way
peculiarly associated with it." Lastly we come to that all-important organ
the liver, at once a factory and a storehouse. Here several metabolisms are
simultaneously carried on. There is that which until recent years was
supposed to be the sole hepatic process--the formation of bile. In some
liver-cells are masses of oil-globules, which seem to imply a carbo-hydrate
metamorphosis. And then, of leading importance, comes the extensive
production of that animal-starch known as glycogen--a substance which, in
each of the cells generating it, is contained in a plexus of protoplasmic
threads: again a nitrogenous body diffused through a mass which is now
formed out of sugar and is now dissolved again into sugar. For it appears
that this soluble form of carbo-hydrate, taken into the liver from the
intestine, is there, when not immediately needed, stored up in the form of
glycogen, ready to be re-dissolved and carried into the system either for
immediate use or for re-deposit as glycogen at the places where it is
presently to be consumed: the great deposit in the liver and the minor
deposits in the muscles being, to use the simile of Prof. Michael Foster,
analogous in their functions to a central bank and branch banks.
An instructive parallelism may be noted between these processes carried on
in the animal organism and those carried on in the vegetal organism. For
the carbo-hydrates named, easily made to assume the soluble or the
insoluble form by the addition or subtraction of a molecule of water, and
thus fitted sometimes for distribution and sometimes for accumulation, are
similarly dealt with in the two cases. As the animal-starch, glycogen, is
now stored up in the liver or elsewhere and now changed into glucose to be
transferred, perhaps for consumption and perhaps for re-deposit; so the
vegetal starch, made to alternate between soluble and insoluble states, is
now carried to growing parts where by metabolic change it becomes cellulose
or other component of tissue and now carried to some place where, changed
back into starch, it is laid aside for future use; as it is in the turgid
inside leaves of a cabbage, the root of a turnip, or the swollen
underground stem we know as a potato: the matter which in the animal is
used up in generating movement and heat, being in the plant used up in
generating structures. Nor is the parallelism even now exhausted; for, as
by a plant starch is stored up in each seed for the subsequent use of the
embryo, so in an embryo-animal glycogen is stored up in the developing
muscles for subsequent use in the completion of their structures.
§ 23e. We come now to the supreme and all-pervading metabolism which has
for its effects the conspicuous manifestations of life--the nervous and
muscular activities. Here comes up afresh a question discussed in the
edition of 1864--a question to be reconsidered in the light of recent
knowledge--the question what particular metabolic changes are they by which
in muscle the energy existing under the form of molecular motion is
transformed into the energy manifested as molar motion?
There are two views respecting the nature of this transformation. One is
that the carbo-hydrate present in muscle must, by further metabolism, be
raised into the form of a nitrogenous compound or compounds before it can
be made to undergo that sudden decomposition which initiates muscular
contraction. The other is the view set forth in § 15, and there reinforced
by further illustrations which have occurred to me while preparing this
revised edition--the view that the carbo-hydrate in muscle, everywhere in
contact with unstable nitrogenous substance, is, by the shock of a small
molecular change in this, made to undergo an extensive molecular change,
resulting in the oxidation of its carbon and consequent liberation of much
molecular motion. Both of these are at present only hypotheses, in support
of which respectively the probabilities have to be weighed. Let us compare
them and observe on which side the evidence preponderates.
We are obliged to conclude that in carnivorous animals the katabolic
process is congruous with the first of these views, in so far that the
evolution of energy must in some way result solely from the fall of complex
nitrogenous compounds into those simpler matters which make their
appearance as waste; for, practically, the carnivorous animal has no
carbo-hydrates out of which otherwise to evolve force. To this admission,
however, it should be added that possibly out of the exclusively
nitrogenous food, glycogen or sugar has to be obtained by partial
decomposition before muscular action can take place. But when we pass to
animals having food consisting mainly of carbo-hydrates, several
difficulties stand in the way of the hypothesis that, by further
compounding, proteids must be formed from the carbo-hydrates before
muscular energy can be evolved. In the first place the anabolic change
through which, by the addition of nitrogen, &c., a proteid is formed from a
carbo-hydrate, must absorb an energy equal to a moiety of that which is
given out in the subsequent katabolic change. There can be no dynamic
profit on such part of the transaction as effects the composition and
subsequent decomposition of the proteid, but only on such part of the
transaction as effects the decomposition of the carbo-hydrate. In the
second place there arises the question--whence comes the nitrogen required
for the compounding of the carbo-hydrates into proteids? There is none save
that contained in the serum-albumen or other proteid which the blood
brings; and there can be no gain in robbing this proteid of nitrogen for
the purpose of forming another proteid. Hence the nitrogenizing of the
surplus carbo-hydrates is not accounted for. One more difficulty remains.
If the energy given out by a muscle results from the katabolic consumption
of its proteids, then the quantity of nitrogenous waste matters formed
should be proportionate to the quantity of work done. But experiments have
proved that this is not the case. Long ago it was shown that the amount of
urea excreted does not increase in anything like proportion to the amount
of muscular energy expended; and recently this has been again shown.
On this statement a criticism has been made to the following
effect:--Considering that muscle will contract when deprived of oxygen and
blood and must therefore contain matter from which the energy is derived;
and considering that since carbonic acid is given out the required carbon
and oxygen must be derived from some component of muscle; it results that
the energy must be obtained by decomposition of a nitrogenous body. To this
reasoning it may be objected, in the first place, that the conditions
specified are abnormal, and that it is dangerous to assume that what takes
place under abnormal conditions takes place also under normal ones. In
presence of blood and oxygen the process may possibly, or even probably, be
unlike that which arises in their absence: the muscular substance may begin
consuming itself when it has not the usual materials to consume. Then, in
the second place, and chiefly, it may be replied that the difficulty raised
in the foregoing argument is not escaped but merely obscured. If, as is
alleged, the carbon and oxygen from which carbonic acid is produced, form,
under the conditions stated, parts of a complex nitrogenous substance
contained in muscle, then the abstraction of the carbon and oxygen must
cause decomposition of this nitrogenous substance; and in that case the
excretion of nitrogenous waste must be proportionate to the amount of work
done, which it is not. This difficulty is evaded by supposing that the
"stored complex explosive substance must be, in living muscle, of such
nature" that after explosion it leaves a "nitrogenous residue available for
re-combination with fresh portions of carbon and oxygen derived from the
blood and thereby the re-constitution of the explosive substance." This
implies that a molecule of the explosive substance consists of a complex
nitrogenous molecule united with a molecule of carbo-hydrate, and that time
after time it suddenly decomposes this carbo-hydrate molecule and thereupon
takes up another such from the blood. That the carbon is abstracted from
the carbo-hydrate molecule can scarcely be said, since the feebler
affinities of the nitrogenous molecule can hardly be supposed to overcome
the stronger affinities of the carbo-hydrate molecule. The carbo-hydrate
molecule must therefore be incorporated bodily. What is the implication?
The carbo-hydrate part of the compound is relatively stable, while the
nitrogenous part is relatively unstable. Hence the hypothesis implies that,
time after time, the unstable nitrogenous part overthrows the stable
carbo-hydrate part, without being itself overthrown. This conclusion, to
say the least of it, does not appear very probable.
The alternative hypothesis, indirectly supported as we saw by proofs that
outside the body small amounts of change in nitrogenous compounds initiate
large amounts of change in carbonaceous compounds, may in the first place
be here supported by some further indirect evidences of kindred natures. A
haystack prematurely put together supplies one. Enough water having been
left in the hay to permit chemical action, the decomposing proteids forming
the dead protoplasm in each cell, set up decomposition of the
carbo-hydrates with accompanying oxidation of the carbon and genesis of
heat; even to the extent of producing fire. Again, as shown above, this
relation between these two classes of compounds is exemplified in the
alimentary canal; where, alike in the saliva and in the pancreatic
secretion, minute quantities of unstable nitrogenous bodies transform great
quantities of stable carbo-hydrates. Thus we find indirect reinforcements
of the belief that the katabolic change generating muscular energy is one
in which a large decomposition of a carbo-hydrate is set up by a small
decomposition of a proteid.[12]
§ 23f. A certain general trait of animal organization may fitly be named
because its relevance, though still more indirect, is very significant.
Under one of its aspects an animal is an apparatus for the multiplication
of energies--a set of appliances by means of which a minute amount of
motion initiates a larger amount of motion, and this again a still larger
amount. There are structures which do this mechanically and others which do
it chemically.
Associated with the peripheral ends of the nerves of touch are certain
small bodies--_corpuscula tactus_--each of which, when disturbed by
something in contact with the skin, presses on the adjacent fibre more
strongly than soft tissue would do, and thus multiplies the force producing
sensation. While serving the further purpose of touching at a distance, the
_vibrissæ_ or whiskers of a feline animal achieve a like end in a more
effectual way. The external portion of each bristle acts as the long arm of
a lever, and the internal portion as the short arm. The result is that a
slight touch at the outer end of the bristle produces a considerable
pressure of the inner end on the nerve-terminal: so intensifying the
impression. In the hearing organs of various inferior types of animals, the
otolites in contact with the auditory nerves, when they are struck by
sound-waves, give to the nerves much stronger impressions than these would
have were they simply immersed in loose tissue; and in the ears of
developed creatures there exist more elaborate appliances for augmenting
the effects of aerial vibrations. From this multiplication of molar actions
let us pass to the multiplication of molecular actions. The retina is made
up of minute rods and cones, so packed together side by side that they can
be separately affected by the separate parts of the images of objects. As
each of them is but 1/10,000th of an inch in diameter, the ethereal
undulations falling upon it can produce an amount of change almost
infinitesimal--an amount probably incapable of exciting a nerve-centre, or
indeed of overcoming the molecular inertia of the nerve leading to it. But
in close proximity are layers of granules into which the rods and cones
send fibres, and beyond these, about 1/100th of an inch from the retinal
layer, lie ganglion-cells, in each of which a minute disturbance may
readily evolve a larger disturbance; so that by multiplication, single or
perhaps double, there is produced a force sufficient to excite the fibre
connected with the centre of vision. Such, at least, judging from the
requirement and the structure, seems to me the probable interpretation of
the visual process; though whether it is the accepted one I do not know.
But now, carrying with us the conception made clear by the first cases and
suggested by the last, we shall appreciate the extent to which this general
physiological method, as we may call it, is employed. The convulsive action
caused by tickling shows it conspicuously. An extremely small amount of
molecular change in the nerve-endings produces an immense amount of
molecular change, and resulting molar motion, in the muscles. Especially is
this seen in one whose spinal cord has been so injured that it no longer
conveys sensations from the lower limbs to the brain; and in whom,
nevertheless, tickling of the feet produces convulsive actions of the legs
more violent even than result when sensation exists: clearly proving that
since the minute molecular change produced by the tickling in the
nerve-terminals cannot be equivalent in quantity to the amount implied by
the muscular contraction, there must be a multiplication of it in those
parts of the spinal cord whence issue the reflex stimuli to the muscles.
Returning now to the question of metabolism, we may see that the processes
of multiplication above supposed to take place in muscle, are analogous in
their general nature to various other physiological processes. Carrying
somewhat further the simile used in § 15 and going back to the days when
detonators, though used for small arms, were not used for artillery, we may
compare the metabolic process in muscle to that which would take place if a
pistol were fired against the touch-hole of a loaded cannon: the cap
exploding the pistol and the pistol the cannon. For in the case of the
muscle, the implication is that a nervous discharge works in certain
unstable proteids through which the nerve-endings are distributed, a small
amount of molecular change; that the shock of this causes a much larger
amount of molecular change in the inter-diffused carbo-hydrate, with
accompanying oxidation of its carbon; and that the heat liberated sets up a
transformation, probably isomeric, in the contractile substance of the
muscular fibre: an interpretation supported by cases in which small rises
and falls of temperature cause alternating isomeric changes; as instance
Mensel's salt.
Ending here this exposition, somewhat too speculative and running into
details inappropriate to a work of this kind, it suffices to note the most
general facts concerning metabolism. Regarded as a whole it includes, in
the first place, those anabolic or building-up processes specially
characterizing plants, during which the impacts of ethereal undulations are
stored up in compound molecules of unstable kinds; and it includes, in the
second place, those katabolic or tumbling-down changes specially
characterizing animals, during which this accumulated molecular motion
(contained in the food directly or indirectly supplied by plants), is in
large measure changed into those molar motions constituting animal
activities. There are multitudinous metabolic changes of minor kinds which
are ancillary to these--many katabolic changes in plants and many anabolic
changes in animals--but these are the essential ones.[13]
CHAPTER IV.[14]
PROXIMATE CONCEPTION OF LIFE.
§ 24. To those who accept the general doctrine of Evolution, it need
scarcely be pointed out that classifications are subjective conceptions,
which have no absolute demarcations in Nature corresponding to them. They
are appliances by which we limit and arrange the matters under
investigation; and so facilitate our thinking. Consequently, when we
attempt to define anything complex, or make a generalization of facts other
than the most simple, we can scarcely ever avoid including more than we
intended, or leaving out something which should be taken in. Thus it
happens that on seeking a definite idea of Life, we have great difficulty
in finding one that is neither more nor less than sufficient. Let us look
at a few of the most tenable definitions that have been given. While
recognizing the respects in which they are defective, we shall see what
requirements a more satisfactory one must fulfil.
Schelling said that Life is the tendency to individuation. This formula,
until studied, conveys little meaning. But we need only consider it as
illustrated by the facts of development, or by the contrast between lower
and higher forms of life, to recognize its significance; especially in
respect of comprehensiveness. As before shown, however (_First Principles_,
§ 56), it is objectionable; partly on the ground that it refers not so much
to the functional changes constituting Life, as to the structural changes
of those aggregates of matter which manifest Life; and partly on the ground
that it includes under the idea Life, much that we usually exclude from it:
for instance--crystallization.
The definition of Richerand,--"Life is a collection of phenomena which
succeed each other during a limited time in an organized body,"--is liable
to the fatal criticism, that it equally applies to the decay which goes on
after death. For this, too, is "a collection of phenomena which succeed
each other during a limited time in an organized body."
"Life," according to De Blainville, "is the two-fold internal movement of
composition and decomposition, at once general and continuous." This
conception is in some respects too narrow, and in other respects too wide.
On the one hand, while it expresses what physiologists distinguish as
vegetative life, it does not indicate those nervous and muscular functions
which form the most conspicuous and distinctive classes of vital phenomena.
On the other hand, it describes not only the integrating and disintegrating
process going on in a living body, but it equally well describes those
going on in a galvanic battery; which also exhibits a "two-fold internal
movement of composition and decomposition, at once general and continuous."
Elsewhere, I have myself proposed to define Life as "the co-ordination of
actions."[15] This definition has some advantages. It includes all organic
changes, alike of the viscera, the limbs, and the brain. It excludes the
great mass of inorganic changes; which display little or no co-ordination.
By making co-ordination the specific character of vitality, it involves the
truths, that an arrest of co-ordination is death, and that imperfect
co-ordination is disease. Moreover, it harmonizes with our ordinary ideas
of life in its different grades; seeing that the organisms which we rank as
low in their degrees of life, are those which display but little
co-ordination of actions; and seeing that from these up to man, the
recognized increase in degree of life corresponds with an increase in the
extent and complexity of co-ordinations. But, like the others, this
definition includes too much. It may be said of the Solar System, with its
regularly-recurring movements and its self-balancing perturbations, that
it, also, exhibits co-ordination of actions. And however plausibly it may
be argued that, in the abstract, the motions of the planets and satellites
are as properly comprehended in the idea of life as the changes going on in
a motionless, unsensitive seed: yet, it must be admitted that they are
foreign to that idea as commonly received, and as here to be formulated.
It remains to add the definition since suggested by Mr. G. H. Lewes--"Life
is a series of definite and successive changes, both of structure and
composition, which take place within an individual without destroying its
identity." The last fact which this statement brings into view--the
persistence of a living organism as a whole, in spite of the continuous
removal and replacement of its parts--is important. But otherwise it may be
argued that, since changes of structure and composition, though
concomitants of muscular and nervous actions, are not the muscular and
nervous actions themselves, the definite excludes the more visible
movements with which our idea of life is most associated; and further that,
in describing vital changes as _a series_, it scarcely includes the fact
that many of them, as Nutrition, Circulation, Respiration, and Secretion,
in their many subdivisions, go on simultaneously.
Thus, however well each of these definitions expresses the phenomena of
life under some of its aspects, no one of them is more than approximately
true. It may turn out that to find a formula which will bear every test is
impossible. Meanwhile, it is possible to frame a more adequate formula than
any of the foregoing. As we shall presently find, these all omit an
essential peculiarity of vital changes in general--a peculiarity which,
perhaps more than any other, distinguishes them from non-vital changes.
Before specifying this peculiarity, however, it will be well to trace our
way, step by step, to as complete an idea of Life as may be reached from
our present stand-point; by doing which we shall both see the necessity for
each limitation as it is made, and ultimately be led to feel the need for a
further limitation.
And here, as the best mode of determining what are the traits which
distinguish vitality from non-vitality, we shall do well to compare the two
most unlike kinds of vitality, and see in what they agree. Manifestly, that
which is essential to Life must be that which is common to Life of all
orders. And manifestly, that which is common to all forms of Life, will
most readily be seen on contrasting those forms of Life which have the
least in common, or are the most unlike.[16]
§ 25. Choosing assimilation, then, for our example of bodily life, and
reasoning for our example of that life known as intelligence; it is first
to be observed, that they are both processes of change. Without change,
food cannot be taken into the blood nor transformed into tissue; without
change, there can be no getting from premisses to conclusion. And it is
this conspicuous display of changes which forms the substratum of our idea
of Life in general. Doubtless we see innumerable changes to which no notion
of vitality attaches. Inorganic bodies are ever undergoing changes of
temperature, changes of colour, changes of aggregation; and decaying
organic bodies also. But it will be admitted that the great majority of the
phenomena displayed by inanimate bodies, are statical and not dynamical;
that the modifications of inanimate bodies are mostly slow and unobtrusive;
that on the one hand, when we see sudden movements in inanimate bodies, we
are apt to assume living agency, and on the other hand, when we see no
movements in living bodies, we are apt to assume death. Manifestly then, be
the requisite qualifications what they may, a true idea of Life must be an
idea of some kind of change or changes.
On further comparing assimilation and reasoning, with a view of seeing in
what respect the changes displayed in both differs from non-vital changes,
we find that they differ in being not simple changes; in each case there
are _successive_ changes. The transformation of food into tissue involves
mastication, deglutition, chymification, chylification, absorption, and
those various actions gone through after the lacteal ducts have poured
their contents into the blood. Carrying on an argument necessitates a long
chain of states of consciousness; each implying a change of the preceding
state. Inorganic changes, however, do not in any considerable degree
exhibit this peculiarity. It is true that from meteorologic causes,
inanimate objects are daily, sometimes hourly, undergoing modifications of
temperature, of bulk, of hygrometric and electric condition. Not only,
however, do these modifications lack that conspicuousness and that rapidity
of succession which vital ones possess, but vital ones form an _additional_
series. Living as well as not-living bodies are affected by atmospheric
influences; and beyond the changes which these produce, living bodies
exhibit other changes, more numerous and more marked. So that though
organic change is not rigorously distinguished from inorganic change by
presenting successive phases; yet vital change so greatly exceeds other
change in this respect, that we may consider it as a distinctive character.
Life, then, as thus roughly differentiated, may be regarded as change
presenting successive phases; or otherwise, as a series of changes. And it
should be observed, as a fact in harmony with this conception, that the
higher the life the more conspicuous the variations. On comparing inferior
with superior organisms, these last will be seen to display more rapid
changes, or a more lengthened series of them, or both.
On contemplating afresh our two typical phenomena, we may see that vital
change is further distinguished from non-vital change, by being made up of
many _simultaneous_ changes. Nutrition is not simply a series of actions,
but includes many actions going on together. During mastication the stomach
is busy with food already swallowed, on which it is pouring out solvent
fluids and expending muscular efforts. While the stomach is still active,
the intestines are performing their secretive, contractile, and absorbent
functions; and at the same time that one meal is being digested, the
nutriment obtained from a previous meal is undergoing transformation into
tissue. So too is it, in a certain sense, with mental changes. Though the
states of consciousness which make up an argument occur in series, yet, as
each of them is complex, a number of simultaneous changes have taken place
in establishing it. Here as before, however, it must be admitted that the
distinction between animate and inanimate is not precise. No mass of dead
matter can have its temperature altered, without at the same time
undergoing an alteration in bulk, and sometimes also in hygrometric state.
An inorganic body cannot be compressed, without being at the same time
changed in form, atomic arrangement, temperature, and electric condition.
And in a vast and mobile aggregate like the sea, the simultaneous as well
as the successive changes outnumber those going on in an animal.
Nevertheless, speaking generally, a living thing is distinguished from a
dead thing by the multiplicity of the changes at any moment taking place in
it. Moreover, by this peculiarity, as by the previous one, not only is the
vital more or less clearly marked off from the non-vital; but creatures
possessing high vitality are marked off from those possessing low vitality.
It needs but to contrast the many organs cooperating in a mammal, with the
few in a polype, to see that the actions which are progressing together in
the body of the first, as much exceed in number the actions progressing
together in the body of the last, as these do those in a stone. As at
present conceived, then, Life consists of simultaneous and successive
changes.
Continuance of the comparison shows that vital changes, both visceral and
cerebral, differ from other changes in their _heterogeneity_. Neither the
simultaneous acts nor the serial acts, which together constitute the
process of digestion, are alike. The states of consciousness comprised in
any ratiocination are not repetitions one of another, either in composition
or in modes of dependence. Inorganic processes, on the other hand, even
when like organic ones in the number of the simultaneous and successive
changes they involve, are unlike them in the relative homogeneity of these
changes. In the case of the sea, just referred to, it is observable that
countless as are the actions at any moment going on, they are mostly
mechanical actions that are to a great degree similar; and in this respect
differ widely from the actions at any moment taking place in an organism.
Even where life is nearly simulated, as by the working of a steam-engine,
we see that considerable as is the number of simultaneous changes, and
rapid as are the successive ones, the regularity with which they soon recur
in the same order and degree, renders them unlike those varied changes
exhibited by a living creature. Still, this peculiarity, like the
foregoing ones, does not divide the two classes of changes with precision;
since there are inanimate things presenting considerable heterogeneity of
change: for instance, a cloud. The variations of state which this
undergoes, both simultaneous and successive, are many and quick; and they
differ widely from one another both in quality and quantity. At the same
instant there may occur change of position, change of form, change of size,
change of density, change of colour, change of temperature, change of
electric state; and these several kinds of change are continuously
displayed in different degrees and combinations. Yet when we observe that
very few inorganic objects manifest heterogeneity of change comparable to
that manifested by organic objects, and further, that in ascending from low
to high forms of life, we meet with an increasing variety in the kinds of
changes displayed; we see that there is here a further leading distinction
between vital and non-vital actions. According to this modified conception,
then, Life is made up of heterogeneous changes both simultaneous and
successive.
If, now, we look for some trait common to the nutritive and logical
processes, by which they are distinguished from those inorganic processes
that are most like them in the heterogeneity of the simultaneous and
successive changes they comprise, we discover that they are distinguished
by the _combination_ among their constituent changes. The acts which make
up digestion are mutually dependent. Those composing a train of reasoning
are in close connection. And, generally, it is to be remarked of vital
changes, that each is made possible by all, and all are affected by each.
Respiration, circulation, absorption, secretion, in their many
sub-divisions, are bound up together. Muscular contraction involves
chemical change, change of temperature, and change in the excretions.
Active thought influences the operations of the stomach, of the heart, of
the kidneys. But we miss this union among non-vital activities. Life-like
as may seem the action of a volcano in respect of the heterogeneity of its
many simultaneous and successive changes, it is not life-like in respect of
their combination. Though the chemical, mechanical, thermal, and electric
phenomena exhibited have some inter-dependence, yet the emissions of
stones, mud, lava, flame, ashes, smoke, steam, take place irregularly in
quantity, order, intervals, and mode of conjunction. Even here, however,
it cannot be said that inanimate things present no parallels to animate
ones. A glacier may be instanced as showing nearly as much combination in
its change as a plant of the lowest organization. It is ever growing and
ever decaying; and the rates of its composition and decomposition preserve
a tolerably constant ratio. It moves; and its motion is in immediate
dependence on its thawing. It emits a torrent of water, which, in common
with its motion, undergoes annual variations as plants do. During part of
the year the surface melts and freezes alternately; and on these changes
depend the variations in movement, and in efflux of water. Thus we have
growth, decay, changes of temperature, changes of consistence, changes of
velocity, changes of excretion, all going on in connexion; and it may be as
truly said of a glacier as of an animal, that by ceaseless integration and
disintegration it gradually undergoes an entire change of substance without
losing its individuality. This exceptional instance, however, will scarcely
be held to obscure that broad distinction from inorganic processes which
organic processes derive from the combination among their constituent
changes. And the reality of this distinction becomes yet more manifest when
we find that, in common with previous ones, it not only marks off the
living from the not-living, but also things which live little from things
which live much. For while the changes going on in a plant or a zoophyte
are so imperfectly combined that they can continue after it has been
divided into two or more pieces, the combination among the changes going on
in a mammal is so close that no part cut off from the rest can live, and
any considerable disturbance of one chief function causes a cessation of
the others. Hence, as we now regard it, Life is a combination of
heterogeneous changes, both simultaneous and successive.
When we once more look for a character common to these two kinds of vital
action, we perceive that the combinations of heterogeneous changes which
constitute them, differ from the few combinations which they otherwise
resemble, in respect of _definiteness_. The associated changes going on in
a glacier, admit of indefinite variation. Under a conceivable alteration of
climate, its thawing and its progression may be stopped for a million
years, without disabling it from again displaying these phenomena under
appropriate conditions. By a geological convulsion, its motion may be
arrested without an arrest of its thawing; or by an increase in the
inclination of the surface it slides over, its motion may be accelerated
without accelerating its rate of dissolution. Other things remaining the
same, a more rapid deposit of snow may cause great increase of bulk; or,
conversely, the accretion may entirely cease, and yet all the other actions
continue until the mass disappears. Here, then, the combination has none of
that definiteness which, in a plant, marks the mutual dependence of
respiration, assimilation, and circulation; much less has it that
definiteness seen in the mutual dependence of the chief animal functions;
no one of which can be varied without varying the rest; no one of which can
go on unless the rest go on. Moreover, this definiteness of combination
distinguishes the changes occurring in a living body from those occurring
in a dead one. Decomposition exhibits both simultaneous and successive
changes, which are to some extent heterogeneous, and in a sense combined;
but they are not combined in a definite manner. They vary according as the
surrounding medium is air, water, or earth. They alter in nature with the
temperature. If the local conditions are unlike, they progress differently
in different parts of the mass, without mutual influence. They may end in
producing gases, or adipocire, or the dry substance of which mummies
consist. They may occupy a few days or thousands of years. Thus, neither in
their simultaneous nor in their successive changes, do dead bodies display
that definiteness of combination which characterizes living ones. It is
true that in some inferior creatures the cycle of successive changes admits
of a certain indefiniteness--that it may be suspended for a long period by
desiccation or freezing, and may afterwards go on as though there had been
no breach in its continuity. But the circumstance that only a low order of
life can have its changes thus modified, serves but to suggest that, like
the previous characteristics, this characteristic of definiteness in its
combined changes, distinguishes high vitality from low vitality, as it
distinguishes low vitality from inorganic processes. Hence, our formula as
further amended reads thus:--Life is a definite combination of heterogenous
changes, both simultaneous and successive.
Finally, we shall still better express the facts if, instead of saying _a_
definite combination of heterogeneous changes, we say _the_ definite
combination of heterogeneous changes. As it at present stands, the
definition is defective both in allowing that there may be _other_ definite
combinations of heterogeneous changes, and in directing attention to the
heterogeneous changes rather than to the definiteness of their combination.
Just as it is not so much its chemical elements which constitute an
organism, as it is the arrangement of them into special tissues and organs;
so it is not so much its heterogeneous changes which constitute Life, as it
is the co-ordination of them. Observe what it is that ceases when life
ceases. In a dead body there are going on heterogeneous changes, both
simultaneous and successive. What then has disappeared? The definite
combination has disappeared. Mark, too, that however heterogeneous the
simultaneous and successive changes exhibited by such an inorganic object
as a volcano, we much less tend to think of it as living than we do a watch
or a steam-engine, which, though displaying changes that, serially
contemplated, are largely homogeneous, displays them definitely combined.
So dominant an element is this in our idea of Life, that even when an
object is motionless, yet, if its parts be definitely combined, we conclude
either that it has had life, or has been made by something having life.
Thus, then, we conclude that Life is--_the_ definite combination of
heterogeneous changes, both simultaneous and successive.
§ 26. Such is the conception at which we arrive without changing our
stand-point. It is, however, an incomplete conception. This ultimate
formula (which is to a considerable extent identical with one above
given--"the co-ordination of actions;" seeing that "definite combination"
is synonymous with "co-ordination," and "changes both simultaneous and
successive" are comprehended under the term "actions;" but which differs
from it in specifying the fact, that the actions or changes are
"heterogeneous")--this ultimate formula, I say, is after all but a rude
approximation. It is true that it does not fail by including the growth of
a crystal; for the successive changes this implies cannot be called
heterogeneous. It is true that the action of a galvanic battery is not
comprised in it; since here, too, heterogeneity is not exhibited by the
successive changes. It is true that by this same qualification the motions
of the Solar System are excluded, as are also those of a watch and a
steam-engine. It is true, moreover, that while, in virtue of their
heterogeneity, the actions going on in a cloud, in a volcano, in a glacier,
fulfil the definition; they fall short of it in lacking definiteness of
combination. It is further true that this definiteness of combination
distinguishes the changes taking place in an organism during life from
those which commence at death. And beyond all this it is true that, as well
as serving to mark off, more or less clearly, organic actions from
inorganic actions, each member of the definition serves to mark off the
actions constituting high vitality from those constituting low vitality;
seeing that life is high in proportion to the number of successive changes
occurring between birth and death; in proportion to the number of
simultaneous changes; in proportion to the heterogeneity of the changes; in
proportion to the combination subsisting among the changes; and in
proportion to the definiteness of their combination. Nevertheless,
answering though it does to so many requirements, this definition is
essentially defective. _The definite combination of heterogeneous changes,
both simultaneous and successive_, is a formula which fails to call up an
adequate conception. And it fails from omitting the most distinctive
peculiarity--the peculiarity of which we have the most familiar experience,
and with which our notion of Life is, more than with any other, associated.
It remains now to supplement the conception by the addition of this
peculiarity.
CHAPTER V.
THE CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES.
§ 27. We habitually distinguish between a live object and a dead one, by
observing whether a change which we make in the surrounding conditions, or
one which Nature makes in them, is or is not followed by some perceptible
change in the object. By discovering that certain things shrink when
touched, or fly away when approached, or start when a noise is made, the
child first roughly discriminates between the living and the not-living;
and the man when in doubt whether an animal he is looking at is dead or
not, stirs it with his stick; or if it be at a distance, shouts, or throws
a stone at it. Vegetal and animal life are alike primarily recognized by
this process. The tree that puts out leaves when the spring brings increase
of temperature, the flower which opens and closes with the rising and
setting of the sun, the plant that droops when the soil is dry and
re-erects itself when watered, are considered alive because of these
induced changes; in common with the acorn-shell which contracts when a
shadow suddenly falls on it, the worm that comes to the surface when the
ground is continuously shaken, and the hedgehog that rolls itself up when
attacked.
Not only, however, do we look for some response when an external stimulus
is applied to a living organism, but we expect a fitness in the response.
Dead as well as living things display changes under certain changes of
condition: instance, a lump of carbonate of soda that effervesces when
dropped into sulphuric acid; a cord that contracts when wetted; a piece of
bread that turns brown when held near the fire. But in these cases, we do
not see a connexion between the changes undergone and the preservation of
the things that undergo them; or, to avoid any teleological
implication--the changes have no apparent relations to future events which
are sure or likely to take place. In vital changes, however, such relations
are manifest. Light being necessary to vegetal life, we see in the action
of a plant which, when much shaded, grows towards the unshaded side, an
appropriateness which we should not see did it grow otherwise. Evidently
the proceedings of a spider which rushes out when its web is gently shaken
and stays within when the shaking is violent, conduce better to the
obtainment of food and the avoidance of danger than were they reversed. The
fact that we feel surprise when, as in the case of a bird fascinated by a
snake, the conduct tends towards self-destruction, at once shows how
generally we have observed an adaptation of living changes to changes in
surrounding circumstances.
A kindred truth, rendered so familiar by infinite repetition that we forget
its significance, must be named. There is invariably, and necessarily, a
conformity between the vital functions of any organism and the conditions
in which it is placed--between the processes going on inside of it and the
processes going on outside of it. We know that a fish cannot live long in
air, or a man under water. An oak growing in the ocean and a seaweed on the
top of a hill, are incredible combinations of ideas. We find that each kind
of animal is limited to a certain range of climate; each kind of plant to
certain zones of latitude and elevation. Of the marine flora and fauna,
each species is found only between such and such depths. Some blind
creatures flourish in dark caves; the limpet where it is alternately
covered and uncovered by the tide; the red-snow alga rarely elsewhere than
in the arctic regions or among alpine peaks.
Grouping together the cases first named, in which a particular change in
the circumstances of an organism is followed by a particular change in it,
and the cases last named, in which the constant actions occurring within an
organism imply some constant actions occurring without it; we see that in
both, the changes or processes displayed by a living body are specially
related to the changes or processes in its environment. And here we have
the needful supplement to our conception of Life. Adding this all-important
characteristic, our conception of Life becomes--The definite combination of
heterogeneous changes, both simultaneous and successive, _in correspondence
with external co-existences and sequences_. That the full significance of
this addition may be seen, it will be necessary to glance at the
correspondence under some of its leading aspects.[17]
§ 28. Neglecting minor requirements, the actions going on in a plant
pre-suppose a surrounding medium containing at least carbonic acid and
water, together with a due supply of light and a certain temperature.
Within the leaves carbon is being appropriated and oxygen given off;
without them, is the gas from which the carbon is taken, and the
imponderable agents that aid the abstraction. Be the nature of the process
what it may, it is clear that there are external elements prone to undergo
special re-arrangements under special conditions. It is clear that the
plant in sunshine presents these conditions and so effects these
re-arrangements. And thus it is clear that the changes which primarily
constitute the plant's life, are in correspondence with co-existences in
its environment.
If, again, we ask respecting the lowest protozoon how it lives; the answer
is, that while on the one hand its substance is undergoing disintegration,
it is on the other hand absorbing nutriment; and that it may continue to
exist, the one process must keep pace with, or exceed, the other. If
further we ask under what circumstances these combined changes are
possible, there is the reply that the medium in which the protozoon is
placed, must contain oxygen and food--oxygen in such quantity as to produce
some disintegration; food in such quantity as to permit that disintegration
to be made good. In other words--the two antagonistic processes taking
place internally, imply the presence externally of materials having
affinities that can give rise to them.
Leaving those lowest animal forms which simply take in through their
surfaces the nutriment and oxygenated fluids coming in contact with them,
we pass to those somewhat higher forms which have their tissues slightly
specialized. In these we see a correspondence between certain actions in
the digestive sac, and the properties of certain surrounding bodies. That a
creature of this order may continue to live, it is necessary not only that
there be masses of substance in the environment capable of transformation
into its own tissue, but also that the introduction of these masses into
its stomach, shall be followed by the secretion of a solvent fluid which
will reduce them to a fit state for absorption. Special outer properties
must be met by special inner properties.
When, from the process by which food is digested, we turn to the process by
which it is seized, the same general truth faces us. The stinging and
contractile power of a polype's tentacle, correspond to the sensitiveness
and strength of the creatures serving it for prey. Unless that external
change which brings one of these creatures in contact with the tentacle,
were quickly followed by those internal changes which result in the coiling
and drawing up of the tentacle, the polype would die of inanition. The
fundamental processes of integration and disintegration within it, would
get out of correspondence with the agencies and processes without it, and
the life would cease.
Similarly, when the creature becomes so large that its tissue cannot be
efficiently supplied with nutriment by mere absorption through its lining
membrane, or duly oxygenated by contact with the fluid bathing its surface,
there arises a need for a distributing system by which nutriment and oxygen
may be carried throughout the mass; and the functions of this system, being
subsidiary to the two primary functions, form links in the correspondence
between internal and external actions. The like is obviously true of all
those subordinate functions, secretory and excretory, that facilitate
oxidation and assimilation.
Ascending from visceral actions to muscular and nervous actions, we find
the correspondence displayed in a manner still more obvious. Every act of
locomotion implies the expenditure of certain internal forces, adapted in
amounts and directions to balance or out-balance certain external forces.
The recognition of an object is impossible without a harmony between the
changes constituting perception, and particular properties co-existing in
the environment. Escape from enemies implies motions within the organism,
related in kind and rapidity to motions without it. Destruction of prey
requires a special combination of subjective actions, fitted in degree and
succession to overcome a group of objective ones. And so with those
countless automatic processes constituting instincts.
In the highest order of vital changes the same fact is equally manifest.
The empirical generalization that guides the farmer in his rotation of
crops, serves to bring his actions into concord with certain of the actions
going on in plants and soil. The rational deductions of the educated
navigator who calculates his position at sea, form a series of mental acts
by which his proceedings are conformed to surrounding circumstances. Alike
in the simplest inferences of the child and the most complex ones of the
man of science, we find a correspondence between simultaneous and
successive changes in the organism, and co-existences and sequences in its
environment.
§ 29. This general formula which thus includes the lowest vegetal processes
along with the highest manifestations of human intelligence, will perhaps
call forth some criticisms which it is desirable here to meet.
It may be thought that there are still a few inorganic actions included in
the definition; as, for example, that displayed by the mis-named
storm-glass. The feathery crystallization which, on a certain change of
temperature, takes place in its contained solution, and which afterwards
dissolves to reappear in new forms under new conditions, may be held to
present simultaneous and successive changes that are to some extent
heterogeneous, that occur with some definiteness of combination, and, above
all, occur in apparent correspondence with external changes. In this case
vegetal life is simulated to a considerable extent; but it is _merely_
simulated. The relation between the phenomena occurring in the storm-glass
and in the atmosphere respectively, is not a correspondence at all, in the
proper sense of the word. Outside there is a thermal change; inside there
is a change of atomic arrangement. Outside there is another thermal change;
inside there is another change of atomic arrangement. But subtle as is the
dependence of each internal upon each external change, the connexion
between them does not, in the abstract, differ from the connexion between
the motion of a straw and the motion of the wind that disturbs it. In
either case a change produces a change, and there it ends. The alteration
wrought by some environing agency on this or any other inanimate object,
does not tend to induce in it a secondary alteration which anticipates some
secondary alteration in the environment. But in every living body there is
a tendency towards secondary alterations of this nature; and it is in their
production that the correspondence consists. The difference may be best
expressed by symbols. Let A be a change in the environment, and B some
resulting change in an inorganic mass. Then A having produced B, the action
ceases. Though the change A in the environment is followed by some
consequent change _a_ in it; no parallel sequence in the inorganic mass
simultaneously generates in it some change _b_ that has reference to the
change _a_. But if we take a living body of the requisite organization, and
let the change A impress on it some change C; then, while in the
environment A is occasioning _a_, in the living body C will be occasioning
_c_; of which _a_ and _c_ will show a certain concord in time, place, or
intensity. And while it is _in_ the continuous production of such concords
or correspondences that Life consists, it is _by_ the continuous production
of them that Life is maintained.
The further criticism to be expected concerns certain verbal imperfections
in the definition, which it seems impossible to avoid. It may fairly be
urged that the word _correspondence_ will not include, without straining,
the various relations to be expressed by it. It may be asked:--How can the
continuous _processes_ of assimilation and respiration correspond with the
_co-existence_ of food and oxygen in the environment? or again:--How can
the act of secreting some defensive fluid correspond with some external
danger which may never occur? or again:--How can the _dynamical_ phenomena
constituting perception correspond with the _statical_ phenomena of the
solid body perceived? The only reply is, that we have no word sufficiently
general to comprehend all forms of this relation between the organism and
its medium, and yet sufficiently specific to convey an adequate idea of the
relation; and that the word _correspondence_ seems the least objectionable.
The fact to be expressed in all cases is that certain changes, continuous
or discontinuous, in the organism, are connected after such a manner that
in their amounts, or variations, or periods of occurrence, or modes of
succession, they have a reference to external actions, constant or serial,
actual or potential--a reference such that a definite relation among any
members of the one group, implies a definite relation among certain members
of the other group.
§ 30. The presentation of the phenomena under this general form, suggests
that our conception of Life may be reduced to its most abstract shape by
regarding its elements as relations only. If a creature's rate of
assimilation is increased in consequence of a decrease of temperature in
the environment, it is that the relation between the food consumed and the
heat produced, is so re-adjusted by multiplying both its members, that the
altered relation in the environment between the quantity of heat absorbed
from, and radiated to, bodies of a given temperature, is counterbalanced.
If a sound or a scent wafted to it on the breeze prompts the stag to dart
away from the deer-stalker, it is that there exists in its neighbourhood a
relation between a certain sensible property and certain actions dangerous
to the stag, while in its body there exists an adapted relation between the
impression this sensible property produces, and the actions by which danger
may be escaped. If inquiry has led the chemist to a law, enabling him to
tell how much of any one element will combine with so much of another, it
is that there has been established in him specific mental relations, which
accord with specific chemical relations in the things around. Seeing, then,
that in all cases we may consider the external phenomena as simply in
relation, and the internal phenomena also as simply in relation; our
conception of Life under its most abstract aspect will be--_The continuous
adjustment of internal relations to external relations_.[18]
While it is simpler, this formula has the further advantage of being
somewhat more comprehensive. To say that it includes not only those
definite combinations of simultaneous and successive changes in an
organism, which correspond to co-existences and sequences in the
environment, but also those structural arrangements which _enable_ the
organism to adapt its actions to actions in the environment, is going too
far; for though these structural arrangements present internal relations
adjusted to external relations, yet the _continuous adjustment_ of
relations cannot be held to include a _fixed adjustment_ already made.
Life, which is made up of _dynamical_ phenomena, cannot be described in
terms that shall at the same time describe the apparatus manifesting it,
which presents only _statical_ phenomena. But while this antithesis serves
to remind us that the distinction between the organism and its actions is
as wide as that between Matter and Motion, it at the same time draws
attention to the fact that, if the structural arrangements of the adult are
not properly included in the definition, yet the developmental processes by
which those arrangements were established, are included. For that process
of evolution during which the organs of the embryo are fitted to their
prospective functions, is the gradual or continuous adjustment of internal
relations to external relations. Moreover, those structural modifications
of the adult organism which, under change of climate, change of occupation,
change of food, bring about some re-arrangement in the organic balance, may
similarly be regarded as progressive or continuous adjustments of internal
relations to external relations. So that not only does the definition, as
thus expressed, comprehend all those activities, bodily and mental, which
constitute our ordinary idea of Life; but it also comprehends both those
processes of development by which the organism is brought into general
fitness for such activities, and those after-processes of adaptation by
which it is specially fitted to its special activities.
Nevertheless, so abstract a formula as this is scarcely fitted for our
present purpose. Reserving it for use where specially appropriate, it will
be best commonly to employ its more concrete equivalent--to consider the
internal relations as "definite combinations of simultaneous and successive
changes;" the external relations as "co-existences and sequences;" and the
connexion between them as a "correspondence."
CHAPTER VI.
THE DEGREE OF LIFE VARIES AS THE DEGREE OF CORRESPONDENCE.
§ 31. Already it has been shown respecting each other component of the
foregoing definition, that the life is high in proportion as that component
is conspicuous; and it is now to be remarked, that the same thing is
especially true respecting this last component--the correspondence between
internal and external relations. It is manifest, _a priori_, that since
changes in the physical state of the environment, as also of those
mechanical actions and those variations of available food which occur in
it, are liable to stop the processes going on in the organism; and since
the adaptive changes in the organism have the effects of directly or
indirectly counter-balancing these changes in the environment; it follows
that the life of the organism will be short or long, low or high, according
to the extent to which changes in the environment are met by corresponding
changes in the organism. Allowing a margin for perturbations, the life will
continue only while the correspondence continues; the completeness of the
life will be proportionate to the completeness of the correspondence; and
the life will be perfect only when the correspondence is perfect. Not to
dwell in general statements, however, let us contemplate this truth under
its concrete aspects.
§ 32. In life of the lowest order we find that only the most prevalent
co-existences and sequences in the environment, have any simultaneous and
successive changes answering to them in the organism. A plant's vital
processes display adjustment solely to the continuous co-existence of
certain elements and forces surrounding its roots and leaves; and vary only
with the variations produced in these elements and forces by the Sun--are
unaffected by the countless mechanical movements and contacts occurring
around; save when accidentally arrested by these. The life of a worm is
made up of actions referring to little else than the tangible properties of
adjacent things. All those visible and audible changes which happen near
it, and are connected with other changes that may presently destroy it,
pass unrecognized--produce in it no adapted changes: its only adjustment of
internal relations to external relations of this order, being seen when it
escapes to the surface on feeling the vibrations produced by an approaching
mole. Adjusted as are the proceedings of a bird to a far greater number of
co-existences and sequences in the environment, cognizable by sight,
hearing, scent, and their combinations: and numerous as are the dangers it
shuns and the needs it fulfils in virtue of this extensive correspondence;
it exhibits no such actions as those by which a human being counterbalances
variations in temperature and supply of food, consequent on the seasons.
And when we see the plant eaten, the worm trodden on, the bird dead from
starvation; we see alike that the death is an arrest of such correspondence
as existed, that it occurred when there was some change in the environment
to which the organism made no answering change, and that thus, both in
shortness and simplicity, the life was incomplete in proportion as the
correspondence was incomplete. Progress towards more prolonged and higher
life, evidently implies ability to respond to less general co-existences
and sequences. Each step upwards must consist in adding to the
previously-adjusted relations of actions or structures which the organism
exhibits, some further relation parallel to a further relation in the
environment. And the greater correspondence thus established, must, other
things equal, show itself both in greater complexity of life, and greater
length of life: a truth which will be fully perceived on remembering the
enormous mortality which prevails among lowly-organized creatures, and the
gradual increase of longevity and diminution of fertility which we meet
with on ascending to creatures of higher and higher developments.
It must be remarked, however, that while length and complexity of life are,
to a great extent, associated--while a more extended correspondence in the
successive changes commonly implies increased correspondence in the
simultaneous changes; yet it is not uniformly so. Between the two great
divisions of life--animal and vegetal--this contrast by no means holds. A
tree may live a thousand years, though the simultaneous changes going on in
it answer only to the few chemical affinities in the air and the earth, and
though its serial changes answer only to those of day and night, of the
weather and the seasons. A tortoise, which exhibits in a given time nothing
like the number of internal actions adjusted to external ones that are
exhibited by a dog, yet lives far longer. The tree by its massive trunk and
the tortoise by its hard carapace, are saved the necessity of responding to
those many surrounding mechanical actions which organisms not thus
protected must respond to or die; or rather--the tree and the tortoise
display in their structures, certain simple statical relations adapted to
meet countless dynamical relations external to them. But notwithstanding
the qualifications suggested by such cases, it needs but to compare a
microscopic fungus with an oak, an animalcule with a shark, a mouse with a
man, to recognize the fact that this increasing correspondence of its
changes with those of the environment which characterizes progressing life,
habitually shows itself at the same time in continuity and in complication.
Even were not the connexion between length of life and complexity of life
thus conspicuous, it would still be true that the life is great in
proportion as the correspondence is great. For if the lengthened existence
of a tree be looked upon as tantamount to a considerable amount of life;
then it must be admitted that its lengthened display of correspondence is
tantamount to a considerable amount of correspondence. If, otherwise, it be
held that notwithstanding its much shorter existence, a dog must rank above
a tortoise in degree of life because of its superior activity; then it is
implied that its life is higher because its simultaneous and successive
changes are more complex and more rapid--because the correspondence is
greater. And since we regard as the highest life that which, like our own,
shows great complexity in the correspondences, great rapidity in the
succession of them, and great length in the series of them; the equivalence
between degree of life and degree of correspondence is unquestionable.
§ 33. In further elucidation of this general truth, and especially in
explanation of the irregularities just referred to, it must be pointed out
that as the life becomes higher the environment itself becomes more
complex. Though, literally, the environment means all surrounding space
with the co-existences and sequences contained in it: yet, practically, it
often means but a small part of this. The environment of an entozoon can
scarcely be said to extend beyond the body of the animal in which the
entozoon lives. That of a freshwater alga is virtually limited to the ditch
inhabited by the alga. And, understanding the term in this restricted
sense, we shall see that the superior organisms inhabit the more
complicated environments.
Thus, contrasted with the life found on land, the lower life is that found
in the sea; and it has the simpler environment. Marine creatures are
affected by fewer co-existences and sequences than terrestrial ones. Being
very nearly of the same specific gravity as the surrounding medium, they
have to contend with less various mechanical actions. The sea-anemone fixed
to a stone, and the acalephe borne along in the current, need to undergo no
internal changes such as those by which the caterpillar meets the varying
effects of gravitation, while creeping over and under the leaves. Again,
the sea is liable to none of those extreme and rapid alterations of
temperature which the air suffers. Night and day produce no appreciable
modifications in it; and it is comparatively little affected by the
seasons. Thus its contained fauna show no marked correspondences similar to
those by which air-breathing creatures counterbalance thermal changes.
Further, in respect to the supply of nutriment, the conditions are more
simple. The lower tribes of animals inhabiting the water, like the plants
inhabiting the air, have their food brought to them. The same current which
brings oxygen to the oyster, also brings it the microscopic organisms on
which it lives: the disintegrating matter and the matter to be integrated,
co-exist under the simplest relation. It is otherwise with land animals.
The oxygen is everywhere, but the sustenance is not everywhere: it has to
be sought; and the conditions under which it is to be obtained are more or
less complex. So too with that liquid by the agency of which the vital
processes are carried on. To marine creatures water is ever present, and by
the lowest is passively absorbed; but to most creatures living on the earth
and in the air, it is made available only through those nervous changes
constituting perception, and those muscular ones by which drinking is
effected. Similarly, after tracing upwards from the _Amphibia_ the widening
extent and complexity which the environment, as practically considered,
assumes--after observing further how increasing heterogeneity in the flora
and fauna of the globe, itself progressively complicates the environment of
each species of organism--it might finally be shown that the same general
truth is displayed in the history of mankind, who, in the course of their
progress, have been adding to their physical environment a social
environment that has been growing ever more involved. Thus, speaking
generally, it is clear that those relations in the environment to which
relations in the organism must correspond, themselves increase in number
and intricacy as the life assumes a higher form.
§ 34. To make yet more manifest the fact that the degree of life varies as
the degree of correspondence, let me here point out, that those other
distinctions successively noted when contrasting vital changes with
non-vital changes, are all implied in this last distinction--their
correspondence with external co-existences and sequences; and further, that
the increasing fulfilment of those other distinctions which we found to
accompany increasing life, is involved in the increasing fulfilment of this
last distinction. We saw that living organisms are characterized by
successive changes, and that as the life becomes higher, the successive
changes become more numerous. Well, the environment is full of successive
changes, and the greater the correspondence, the greater must be the number
of successive changes in the organism. We saw that life presents
simultaneous changes, and that the more elevated it is, the more marked the
multiplicity of them. Well, besides countless co-existences in the
environment, there are often many changes occurring in it at the same
moment; and hence increased correspondence with it implies in the organism
an increased display of simultaneous changes. Similarly with the
heterogeneity of the changes. In the environment the relations are very
varied in their kinds, and hence, as the organic actions come more and more
into correspondence with them, they too must become very varied in their
kinds. So again is it even with definiteness of combination. As the most
important surrounding changes with which each animal has to deal, are the
definitely-combined changes exhibited by other animals, whether prey or
enemies, it results that definiteness of combination must be a general
characteristic of the internal ones which have to correspond with them. So
that throughout, the correspondence of the internal relations with the
external ones is the essential thing; and all the special characteristics
of the internal relations, are but the collateral results of this
correspondence.
§§ 35, 36. Before closing the chapter, it will be useful to compare the
definition of Life here set forth, with the definition of Evolution set
forth in _First Principles_. Living bodies being bodies which display in
the highest degree the structural changes constituting Evolution; and Life
being made up of the functional changes accompanying these structural
changes; we ought to find a certain harmony between the definitions of
Evolution and of Life. Such a harmony is not wanting.
The first distinction we noted between the kind of change shown in Life,
and other kinds of change, was its serial character. We saw that vital
change is substantially unlike non-vital change, in being made up of
_successive_ changes. Now since organic bodies display so much more than
inorganic bodies those continuous differentiations and integrations which
constitute Evolution; and since the re-distributions of matter thus carried
so far in a comparatively short period, imply concomitant re-distributions
of motion; it is clear that in a given time, organic bodies must undergo
changes so comparatively numerous as to render the successiveness of their
changes a marked characteristic. And it will follow _a priori_, as we found
it to do _a posteriori_, that the organisms exhibiting Evolution in the
highest degree, exhibit the longest or the most rapid successions of
changes, or both. Again, it was shown that vital change is distinguished
from non-vital change by being made up of many _simultaneous_ changes; and
also that creatures possessing high vitality are marked off from those
possessing low vitality, by the far greater number of their simultaneous
changes. Here, too, there is entire congruity. In _First Principles_,
§ 156, we reached the conclusion that a force falling on any aggregate is
divided into several forces; that when the aggregate consists of parts that
are unlike, each part becomes a centre of unlike differentiations of the
incident force; and that thus the multiplicity of such differentiations
must increase with the multiplicity of the unlike parts. Consequently
organic aggregates, which as a class are distinguished from inorganic
aggregates by the greater number of their unlike parts, must be also
distinguished from them by the greater number of simultaneous changes they
display; and, further, that the higher organic aggregates, having more
numerous unlike parts than the lower, must undergo more numerous
simultaneous changes. We next found that the changes occurring in living
bodies are contrasted with those occurring in other bodies, as being much
more _heterogeneous_; and that the changes occurring in the superior living
bodies are similarly contrasted with those occurring in inferior ones.
Well, heterogeneity of function is the correlate of heterogeneity of
structure; and heterogeneity of structure is the leading distinction
between organic and inorganic aggregates, as well as between the more
highly organized and the more lowly organized. By reaction, an incident
force must be rendered multiform in proportion to the multiformity of the
aggregate on which it falls; and hence those most multi-form aggregates
which display in the highest degree the phenomena of Evolution structurally
considered, must also display in the highest degree the multiform actions
which constitute Evolution functionally considered. These heterogeneous
changes, exhibited simultaneously and in succession by a living organism,
prove, on further inquiry, to be distinguished by their _combination_ from
certain non-vital changes which simulate them. Here, too, the parallelism
is maintained. It was shown in _First Principles_, Chap. XIV, that an
essential characteristic of Evolution is the integration of parts, which
accompanies their differentiation--an integration shown both in the
consolidation of each part, and in the union of all the parts into a whole.
Hence, animate bodies having greater co-ordination of parts than inanimate
ones must exhibit greater co-ordination of changes; and this greater
co-ordination of their changes must not only distinguish organic from
inorganic aggregates, but must, for the same reason, distinguish higher
organisms from lower ones, as we found that it did. Once more, it was
pointed out that the changes constituting Life differ from other changes in
the _definiteness_ of their combination, and that a distinction like in
kind though less in degree, holds between the vital changes of superior
creatures and those of inferior creatures. These, also, are contrasts in
harmony with the contrasts disclosed by the analysis of Evolution. We saw
(_First Principles_, §§ 129-137) that during Evolution there is an increase
of definiteness as well as an increase of heterogeneity. We saw that the
integration accompanying differentiation has necessarily the effect of
increasing the distinctness with which the parts are marked off from each
other, and that so, out of the incoherent and indefinite there arises the
coherent and definite. But a coherent whole made up of definite parts
definitely combined, must exhibit more definitely combined changes than a
whole made up of parts that are neither definite in themselves nor in their
combination. Hence, if living bodies display more than other bodies this
structural definiteness, then definiteness of combination must be a
characteristic of the changes constituting Life, and must also distinguish
the vital changes of higher organisms from those of lower organisms.
Finally, we discovered that all these peculiarities are subordinate to the
fundamental peculiarity, that vital changes take place in correspondence
with external co-existences and sequences, and that the highest Life is
reached, when there is some inner relation of actions fitted to meet every
outer relation of actions by which the organism can be affected. But this
conception of the highest Life, is in harmony with the conception, before
arrived at, of the limit of Evolution. When treating of equilibration as
exhibited in organisms (_First Principles_, §§ 173, 174), it was pointed
out that the tendency is towards the establishment of a balance between
inner and outer changes. It was shown that "the final structural
arrangements must be such as will meet all the forces acting on the
aggregate, by equivalent antagonistic forces," and that "the maintenance of
such a moving equilibrium" as an organism displays, "requires the habitual
genesis of internal forces corresponding in number, directions, and
amounts, to the external incident forces--as many inner functions, single
or combined, as there are single or combined outer actions to be met." It
was shown, too, that the relations among ideas are ever in progress towards
a better adjustment between mental actions and those actions in the
environment to which conduct must be adjusted. So that this continuous
correspondence between inner and outer relations which constitutes Life,
and the perfection of which is the perfection of Life, answers completely
to that state of organic moving equilibrium which we saw arises in the
course of Evolution and tends ever to become more complete.
CHAPTER VI^A.
THE DYNAMIC ELEMENT IN LIFE.
§ 36a. A critical comparison of the foregoing formula with the facts proves
it to be deficient in more ways than one. Let us first look at vital
phenomena which are not covered by it.
Some irritant left by an insect's ovipositor, sets up on a plant the morbid
growth named a gall. The processes in the gall do not correspond with any
external co-existences or sequences relevant to the plant's life--show no
internal relations adjusted to external relations. Yet we cannot deny that
the gall is alive. So, too, is it with a cancer in or upon an animal's
body. The actions going on in it have no reference, direct or indirect, to
actions in the environment. Nevertheless we are obliged to say that they
are vital; since it grows and after a time dies and decomposes.
A kindred lesson meets us when from pathological evidence we turn to
physiological evidence. The functions of some important organs may still be
carried on for a time apart from those of the body as a whole. An excised
liver, kept at a fit temperature and duly supplied with blood, secretes
bile. Still more striking is the independent action of the heart. If
belonging to a cold-blooded animal, as a frog, the heart, when detached,
continues to beat, even until its integuments have become so dry that they
crackle. Now though under such conditions its pulsations, which ordinarily
form an essential part of the linked processes by which the correspondence
between inner and outer actions is maintained, no longer form part of such
processes, we must admit that the continuance of them implies a vital
activity.
Embryological changes force the same truth upon us. What are we to say of
the repeated cell-fissions by which in some types a blastula, or
mulberry-mass, is formed, and in other types a blastoderm? Neither these
processes nor the structures immediately resulting from them, show any
correspondences with co-existences and sequences in the environment; though
they are first steps towards the organization which is to carry on such
correspondences. Even this extremely small fulfilment of the definition is
absent in the cases of rudimentary organs, and especially those rudimentary
organs which after being partly formed are absorbed. No adjustment can be
alleged between the inner relations which these present and any outer
relations. The outer relations they refer to ceased millions of years ago.
Yet unquestionably the changes which bring about the production and
absorption of these futile structures are vital changes.
Take another class of exceptions. What are we to say of a laugh? No
correspondence, or part of a correspondence, by which inner actions are
made to balance outer actions, can be seen in it. Or again, if, while
working, an artisan whistles, the making of the sounds and the
co-ordination of ideas controlling them, cannot be said to exhibit
adjustment between certain relations of thoughts, and certain relations of
things. Such kinds of vital activities lie wholly outside of the definition
given.
But perhaps the clearest and simplest proof is yielded by contrasting
voluntary and involuntary muscular actions. Here is a hawk adapting its
changing motions to the changing motions of a pigeon, so as eventually to
strike it: the adjustment of inner relations to outer relations is
manifest. Here is a boy in an epileptic fit. Between his struggles and the
co-existences and sequences around him there is no correspondence whatever.
Yet his movements betray vitality just as much as do the movements of the
hawk. Both exhibit that principle of _activity_ which constitutes the
essential element in our conception of life.
§ 36b. Evidently, then, the preceding chapters recognize only the _form_ of
our conception of life and ignore the _body_ of it. Partly sufficing as
does the definition reached to express the one, it fails entirely to
express the other. Life displays itself in ways which conform to the
definition; but it also displays itself in many other ways. We are obliged
to admit that the element which is common to the two groups of ways is the
essential element. The essential element, then, is that special kind of
energy seen alike in the usual classes of vital actions and in those
unusual classes instanced above.
Otherwise presenting the contrast, we may say that due attention has been
paid to the connexions among the manifestations, while no attention has
been paid to that which is manifested. When it is said that life is "the
definite correspondence of heterogeneous changes, both simultaneous and
successive, in correspondence with external co-existences and sequences,"
there arises the question--Changes of what? Within the body there go on
many changes, mechanical, chemical, thermal, no one of which is the kind of
change in question; and if we combine in thought so far as we can these
kinds of changes, in such wise that each maintains its character as
mechanical, chemical, or thermal, we cannot get out of them the idea of
Life. Still more clearly do we see this insufficiency when we take the more
abstract definition--"the continuous adjustment of internal relations to
external relations." Relations between what things? is the question then to
be asked. A relation of which the terms are unspecified does not connote a
thought but merely the blank form of a thought. Its value is comparable to
that of a cheque on which no amount is written. If it be said that the
terms cannot be specified because so many heterogeneous kinds of them have
to be included, then there comes the reply that under cover of this
inability to make a specification of terms that shall be adequately
comprehensive, there is concealed the inability to conceive the required
terms in any way.
Thus a critical testing of the definition brings us, in another way, to the
conclusion reached above, that that which gives the substance to our idea
of Life is a certain unspecified principle of activity. The dynamic element
in life is its essential element.
§ 36c. Under what form are we to conceive this dynamic element? Is this
principle of activity inherent in organic matter, or is it something
superadded? Of these alternative suppositions let us begin with the last.
As I have remarked, in another place, the worth of an hypothesis may be
judged from its genealogy; and so judged the hypothesis of an independent
vital principal does not commend itself. Its history carries us back to the
ghost-theory of the savage. Suggested by experiences of dreams, there
arises belief in a double--a second self which wanders away during sleep
and has adventures but comes back on waking; which deserts the body during
abnormal insensibility of one or other kind; and which is absent for a long
period at death, though even then is expected eventually to return. This
indwelling other-self, which can leave the body at will, is by-and-by
regarded as able to enter the bodies of fellow men or of animals; or again,
by implication, as liable to have its place usurped by the intruding
doubles of fellow men, living or dead, which cause fits or other ills.
Along with these developments its quality changes. At first thought of as
quite material it is gradually de-materialized, and in advanced times comes
to be regarded as spirit or breath; as we see in ancient religious books,
where "giving up the ghost" is shown by the emergence of a small floating
figure from the mouth of a dying man. This indwelling second self, more and
more conceived as the real self which uses the body for its purposes, is,
with the advance of intelligence, still further divested of its definite
characters; and, coming in mediæval days to be spoken of as "animal
spirits," ends in later days in being called a vital principle.
Entirely without assignable attributes, this something occurs in thought
not as an idea but as a pseud-idea (_First Principles_, Chap. II). It is
assumed to be representable while really unrepresentable. We need only
insist on answers to certain questions to see that it is simply a name for
an alleged existence which has not been conceived and cannot be conceived.
1. Is there one kind of vital principle for all kinds of organisms, or is
there a separate kind for each? To affirm the first alternative is to say
that there is the same vital principle for a microbe as for a whale, for a
tape-worm as for the person it inhabits, for a protococcus as for an oak;
nay more--is to assert community of vital principle in the thinking man and
the unthinking plant. Moreover, asserting unity of the vital principle for
all organisms, is reducing it to a force having the same unindividualized
character as one of the physical forces. If, on the other hand, different
kinds of organisms have different kinds of vital principles, these must be
in some way distinguished from one another. How distinguished? Manifestly
by attributes. Do they differ in extension? Evidently; since otherwise that
which animates the vast _Sequoia_ can be no larger than that which animates
a yeast-plant, and to carry on the life of an elephant requires a quantity
of vital principle no greater than that required for a microscopic monad.
Do they differ otherwise than in amount? Certainly; since otherwise we
revert to the preceding alternative, which implies that the same quality of
vital principle serves for all organisms, simple and complex: the vital
principle is a uniform force like heat or electricity. Hence, then, we have
to suppose that every species of animal and plant has a vital principle
peculiar to itself--a principle adapted to use the particular set of
structures in which it is contained. But dare anyone assert this
multiplication of vital principles, duplicating not only all existing
plants and animals but all past ones, and amounting in the aggregate to
some millions?
2. How are we to conceive that genesis of a vital principle which must go
along with the genesis of an organism? Here is a pollen-grain which,
through the pistil, sends its nucleus to unite with the nucleus of the
ovule; or here are the nuclei of spermatozoon and ovum, which, becoming
fused, initiate a new animal: in either case failure of union being
followed by decomposition of the proteid materials, while union is followed
by development. Whence comes that vital principle which determines the
organizing process? Is it created afresh for every plant and animal? or, if
not, where and how did it pre-exist? Take a simpler form of this problem. A
protophyte or protozoon, having grown to a certain size, undergoes a series
of complex changes ending in fission. In its undivided state it had a vital
principle. What of its divided state? The parts severally swim away, each
fully alive, each ready to grow and presently to subdivide, and so on and
so on, until millions are soon formed. That is to say, there is a
multiplication of vital principles as of the protozoa animated by them. A
vital principle, then, both divides and grows. But growth implies
incorporation of something. What does the vital principle incorporate? Is
it some other vital principle external to it, or some materials out of
which more vital principle is formed? And how, in either case, can the
vital principle be conceived as other than a material something, which in
its growth and multiplication behaves just as visible matter behaves?
3. Equally unanswerable is the question which arises in presence of life
that has become latent. Passing over the alleged case of the mummy wheat,
the validity of which is denied, there is experimental proof that seeds
may, under conditions unfavourable to germination, retain for ten, twenty,
and some even for thirty years, the power to germinate when due moisture
and warmth are supplied. (_Cf._ Kerner's _Nat. Hist. of Plants_, i, 51-2).
Under what form has the vital principle existed during these long
intervals? It is a principle of activity. In this case, then, the principle
of activity becomes inactive. But how can we conceive an inactive activity?
If it is a something which though inactive may be rendered active when
conditions favour, we are introduced to the idea of a vital principle of
which the vitality may become latent, which is absurd. What shall we say of
the desiccated rotifer which for years has seemed to be nothing more than a
particle of dust, but which now, when water is supplied, absorbs it, swells
up, and resumes those ciliary motions by which it draws in nutriment? Was
the vital principle elsewhere during these years of absolute quiescence? If
so, why did it come back at the right moment? Was it all along present in
the rotifer though asleep? How happened it then to awaken at the time when
the supply of water enabled the tissues to resume their functions? How
happened the physical agent to act not only on the material substance of
the rotifer, but also on this something which is not a material substance
but an immaterial source of activity? Evidently neither alternative is
thinkable.
Thus, the alleged vital principle exists in the minds of those who allege
it only as a verbal form, not as an idea; since it is impossible to bring
together in consciousness the terms required to constitute an idea. It is
not even "a figment of imagination," for that implies something imaginable,
but the supposed vital principle cannot even be imagined.
§ 36d. When, passing to the alternative, we propose to regard life as
inherent in the substances of the organisms displaying it, we meet with
difficulties different in kind but scarcely less in degree. The processes
which go on in living things are incomprehensible as results of any
physical actions known to us.
Consider one of the simplest--that presented by an ordinary vegetal cell
forming part of a leaf or other plant-structure. Its limiting membrane,
originally made polyhedral by pressure of adjacent cells, is gradually
moulded "into one of cylindrical, fibrous, or tabular shape, and
strengthening its walls with pilasters, borders, ridges, hooks, bands, and
panels of various kinds" (Kerner, i, 43): small openings into adjacent
cells being either left or subsequently made. Consisting of
non-nitrogenous, inactive matters, these structures are formed by the
inclosed protoplast. How formed? Is it by the agency of the nucleus? But
the nucleus, even had it characters conceivably adapting it to this
function, is irregularly placed; and that it should work the same effects
upon the cell-wall whether seated in the middle, at one end, or one side,
is incomprehensible. Is the protoplasm then the active agent? But this is
arranged into a network of strands and threads utterly irregular in
distribution and perpetually altering their shapes and connexions. Exercise
of fit directive action by the protoplasm is unimaginable.
Another instance:--Consider the reproductive changes exhibited by the
_Spirogyra_. The delicate threads which, in this low type of Alga, are
constituted of single elongated cells joined end to end, are here and there
adjacent to one another; and from a cell of one thread and a cell of
another at fit distance, grow out prominences which, meeting in the
interspace and forming a channel by the dissolution of their adjoined
cell-walls, empty through it the endochrome of the one cell into the other:
forming by fusion of the two a zygote or reproductive body. Under what
influence is this action initiated and guided? There is no conceivable
directive agency in either cell by which, when conditions are fit, a
papilla is so formed as to meet an opposite papilla.
Or again, contemplate the still more marvellous transformation occurring in
_Hydrodictyon utriculosum_. United with others to form a cylindrical
network, each sausage-shaped cell of this Alga contains, when fully
developed, a lining chromatophore made of nucleated protoplasm with
immersed chlorophyll-grains. This, when the cell is adult, divides into
multitudinous zoospores, which presently join their ends in such ways as to
form a network with meshes mostly hexagonal, minute in size, but like in
arrangement to the network of which the parent cell formed a part.
Eventually escaping from the mother-cell, this network grows and presently
becomes as large as the parent network. Under what play of forces do these
zoospores arrange themselves into this strange structure?
Kindred insoluble problems are presented by animal organisms of all grades.
Of microscopic types instance the Coccospheres and Rhabdospheres found in
the upper strata of sea-water. Each is a fragment of protoplasm less than
one-thousandth of an inch in diameter, shielded by the elaborate protective
structures it has formed. The elliptic coccoliths of the first, severally
having a definite pattern, unite to form by overlapping an imbricated
covering; and of the other the covering consists of numerous
trumpet-mouthed processes radiating on all sides. To the question--How does
this particle of granular protoplasm, without organs or definite structure,
make for itself this complicated calcareous armour? there is no conceivable
answer.
Like these _Protozoa_, the lowest _Metazoa_ do things which are quite
incomprehensible. Here is a sponge formed of classes of monads having among
them no internuncial appliances by which in higher types cooperation is
carried on--flagellate cells that produce the permeating currents of water,
flattened cells forming protective membranes, and amoeboid cells lying free
in the gelatinous mesoderm. These, without apparent concert, build up not
only the horny network constituting the chief mass of their habitation, but
also embodied spicules, having remarkable symmetrical forms. By what
combined influences the needful processes are effected, it is impossible to
imagine.
If we turn to higher types of _Metazoa_ in which, by the agency of a
nervous system, many cooperations of parts are achieved in ways that are
superficially comprehensible, we still meet with various actions of which
the causation cannot be represented in thought. Lacking other calcareous
matter, a hen picks up and swallows bits of broken egg-shells; and,
occasionally, a cow in calf may be seen mumbling a bone she has
found--evidently scraping off with her teeth some of its mass. These
proceedings have reference to constitutional needs; but how are they
prompted? What generates in the cow a desire to bite a substance so unlike
in character to her ordinary food? If it be replied that the blood has
become poor in certain calcareous salts and that hence arises the appetite
for things containing them, there remains the question--How does this
deficiency so act on the nervous system as to generate this vague desire
and cause the movements which satisfy it? By no effort can we figure to
ourselves the implied causal processes.
In brief, then, we are obliged to confess that Life in its essence cannot
be conceived in physico-chemical terms. The required principle of activity,
which we found cannot be represented as an independent vital principle, we
now find cannot be represented as a principle inherent in living matter.
If, by assuming its inherence, we think the facts are accounted for, we do
but cheat ourselves with pseud-ideas.
§ 36e. What then are we to say--what are we to think? Simply that in this
direction, as in all other directions, our explanations finally bring us
face to face with the inexplicable. The Ultimate Reality behind this
manifestation, as behind all other manifestations, transcends conception.
It needs but to observe how even simple forms of existence are in their
ultimate natures incomprehensible, to see that this most complex form of
existence is in a sense doubly incomprehensible.
For the actions of that which the ignorant contemptuously call brute
matter, cannot in the last resort be understood in their genesis. Were it
not that familiarity blinds us, the fall of a stone would afford matter for
wonder. Neither Newton nor anyone since his day has been able to conceive
how the molecules of matter in the stone are affected not only by the
molecules of matter in the adjacent part of the Earth but by those forming
parts of its mass 8,000 miles off which severally exercise their influence
without impediment from intervening molecules; and still less has there
been any conceivable interpretation of the mode in which every molecule of
matter in the Sun, 92 millions of miles away, has a share in controlling
the movements of the Earth. What goes on in the space between a magnet and
the piece of iron drawn towards it, or how on repeatedly passing a magnet
along a steel needle this, by some change of molecular state as we must
suppose, becomes itself a magnet and when balanced places its poles in
fixed directions, we do not know. And still less can we fathom the physical
process by which an ordered series of electric pulses sent through a
telegraph wire may be made to excite a corresponding series of pulses in a
parallel wire many miles off.
Turn to another class of cases. Consider the action of a surface of glass
struck by a cathode current and which thereupon generates an order of rays
able to pass through solid matters impermeable to light. Or contemplate the
power possessed by uranium and other metals of emitting rays imperceptible
by our eyes as light but which yet, in what appears to us absolute
darkness, will, if passed through a camera, produce photographs. Even the
actions of one kind of matter on another are sufficiently remarkable. Here
is a mass of gold which, after the addition of 1-500th part of bismuth, has
only 1-28th of the tensile strength it previously had; and here is a mass
of brass, ordinarily ductile and malleable, but which, on the addition of
1-10,000th part of antimony, loses its character. More remarkable still are
the influences of certain medicines. One-hundredth of a grain of
nitro-glycerine is a sufficient dose. Taking an average man's weight as 150
pounds, it results that his body is appreciably affected in its state by
the 115-millionth part of its weight of this nitrogenous compound.
In presence of such powers displayed by matter of simple kinds we shall see
how impossible it is even to imagine those processes going on in organic
matter out of which emerges the dynamic element in Life. As no separate
form of proteid possesses vitality, we seem obliged to assume that the
molecule of protoplasm contains many molecules of proteids, probably in
various isomeric states, all capable of ready change and therefore
producing great instability of the aggregate they form. As before pointed
out (§ 4), a proteid-molecule includes more than 220 equivalents of several
so-called elements. Each of these undecomposed substances is now recognized
by chemists as almost certainly consisting of several kinds of components.
Hence the implication is that a proteid-molecule contains thousands of
units, of which the different classes have their respective rates of
inconceivably rapid oscillation, while each unit, receiving and emitting
ethereal undulations, affects others of its kind in its own and adjacent
molecules: an immensely complex structure having immensely complex
activities. And this complexity, material and dynamic, in the
proteid-molecule we must regard as raised to a far higher degree in the
unit of protoplasm. Here as elsewhere alternative impossibilities of
thought present themselves. We find it impossible to think of Life as
imported into the unit of protoplasm from without; and yet we find it
impossible to conceive it as emerging from the cooperation of the
components.
§ 36f. But now, having confessed that Life as a principle of activity is
unknown and unknowable--that while its phenomena are accessible to thought
the implied noumenon is inaccessible--that only the manifestations come
within the range of our intelligence while that which is manifested lies
beyond it; we may resume the conclusions reached in the preceding chapters.
Our surface knowledge continues to be a knowledge valid of its kind, after
recognizing the truth that it is only a surface knowledge.
For the conclusions we lately reached and the definition emerging from
them, concern the _order_ existing among the actions which living things
exhibit; and this order remains the same whether we know or do not know the
nature of that from which the actions originate. We found a distinguishing
trait of Life to be that its changes display a correspondence with
co-existences and sequences in the environment; and this remains a
distinguishing trait, though the thing which changes remains inscrutable.
The statement that the continuous adjustment of internal relations to
external relations constitutes Life as cognizable by us, is not invalidated
by the admission that the reality in which these relations inhere is
incognizable.
Hence, then, after duly recognizing the fact that, as pointed out above,
Life, even phenomenally considered, is not entirely covered by the
definition, since there are various abnormal manifestations of life which
it does not include, we may safely accept it as covering the normal
manifestations--those manifestations which here concern us. Carrying with
us the definition, therefore we may hereafter use it for guidance through
all those regions of inquiry upon which we now enter.
CHAPTER VII.
THE SCOPE OF BIOLOGY.
§ 37. As ordinarily conceived, the science of Biology falls into two great
divisions, the one dealing with animal life, called Zoology, and the other
dealing with vegetal life, called Botany, or more properly to be called
Phytology. But convenient as is this division, it is not that which arises
if we follow the scientific method of including in one group all the
phenomena of fundamentally the same order and putting separately in another
group all the phenomena of a fundamentally different order. For animals and
plants are alike in having structures; and animals and plants are alike in
having functions performed by these structures; and the distinction between
structures and functions transcends the difference between any one
structure and any other or between any one function and any other--is,
indeed, an absolute distinction, like that between Matter and Motion.
Recognizing, then, the logic of the division thus indicated, we must group
the parts of Biology thus:--
1. An account of the structural phenomena presented by organisms. This
subdivides into:--
_a._ The established structural phenomena presented by individual
organisms.
_b._ The changing structural phenomena presented by successions of
organisms.
2. An account of the functional phenomena which organisms present. This,
too, admits of subdivision into:--
_a._ The established functional phenomena of individual organisms.
_b._ The changing functional phenomena of successions of organisms.
3. An account of the actions of Structures on Functions and the re-actions
of Functions on Structures. Like the others, this is divisible into:--
_a._ The actions and re-actions as exhibited in individual organisms.
_b._ The actions and re-actions as exhibited in successions of
organisms.
4. An account of the phenomena attending the production of successions of
organisms: in other words--the phenomena of Genesis.
Of course, for purposes of exploration and teaching, the division into
Zoology and Botany, founded on contrasts so marked and numerous, must
always be retained. But here recognizing this familiar distinction only as
much as convenience obliges us to do, let us now pass on to consider, more
in detail, the classification of biologic phenomena above set down in its
leading outlines.
§ 38. The facts of structure shown in an individual organism, are of two
chief kinds. In order of conspicuousness, though not in order of time,
there come first those arrangements of parts which characterize the mature
organism; an account of which, originally called Anatomy, is now called
Morphology. Then come those successive modifications through which the
organism passes in its progress from the germ to the developed form; an
account of which is called Embryology.
The structural changes which any series of individual organisms exhibits,
admit of similar classification. On the one hand, we have those inner and
outer differences of shape, that arise between the adult members of
successive generations descended from a common stock--differences which,
though usually not marked between adjacent generations, become great in
course of multitudinous generations. On the other hand, we have those
developmental modifications, seen in the embryos, through which such
modifications of the descended forms are reached.
Interpretation of the structures of individual organisms and successions of
organisms, is aided by two subsidiary divisions of biologic inquiry, named
Comparative Anatomy (properly Comparative Morphology) and Comparative
Embryology. These cannot be regarded as in themselves parts of Biology;
since the facts embraced under them are not substantive phenomena, but are
simply incidental to substantive phenomena. All the truths of structural
Biology are comprehended under the two foregoing subdivisions; and the
comparison of these truths as presented in different classes of organisms,
is simply a _method_ of interpreting them.
Nevertheless, though Comparative Morphology and Comparative Embryology do
not disclose additional concrete facts, they lead to the establishment of
certain abstract facts. By them it is made manifest that underneath the
superficial differences of groups and classes and types of organisms, there
are hidden fundamental similarities; and that the courses of development in
such groups and classes and types, though in many respects divergent, are
in some essential respects, coincident. The wide truths thus disclosed,
come under the heads of General Morphology and General Embryology.
By contrasting organisms there is also achieved that grouping of the like
and separation of the unlike, called Classification. First by observation
of external characters; second by observation of internal characters; and
third by observation of the phases of development; it is ascertained what
organisms are most similar in all respects; what organisms otherwise unlike
are like in important traits; what organisms though apparently unallied
have common primordial characters. Whence there results such an
arrangement of organisms, that if certain structural attributes of any one
be given, its other structural attributes may be _empirically_ predicted;
and which prepares the way for that interpretation of their relations and
genesis, which forms an important part of _rational_ Biology.
§ 39. The second main division of Biology, above described as embracing the
functional phenomena of organisms, is that which is in part signified by
Physiology: the remainder being distinguishable as Objective Psychology.
Both of these fall into subdivisions that may best be treated separately.
That part of Physiology which is concerned with the molecular changes going
on in organisms, is known as Organic Chemistry. An account of the modes in
which the force generated in organisms by chemical change, is transformed
into other forces, and made to work the various organs that carry on the
functions of Life, comes under the head of Organic Physics. Psychology,
which is mainly concerned with the adjustment of vital actions to actions
in the environment (in contrast with Physiology, which is mainly concerned
with vital actions apart from actions in the environment) consists of two
quite distinct portions. Objective Psychology deals with those functions of
the nervo-muscular apparatus by which such organisms as possess it are
enabled to adjust inner to outer relations; and includes also the study of
the same functions as externally manifested in conduct. Subjective
Psychology deals with the sensations, perceptions, ideas, emotions, and
volitions that are the direct or indirect concomitants of this visible
adjustment of inner to outer relations. Consciousness under its different
modes and forms, being a subject-matter radically distinct in nature from
the subject-matter of Biology in general; and the method of self-analysis,
by which alone the laws of dependence among changes of consciousness can be
found, being a method unparalleled by anything in the rest of Biology; we
are obliged to regard Subjective Psychology as a separate study. And since
it would be very inconvenient wholly to dissociate Objective Psychology
from Subjective Psychology, we are practically compelled to deal with the
two as forming an independent science.
Obviously, the functional phenomena presented in successions of organisms,
similarly divide into physiological and psychological. Under the
physiological come the modifications of bodily actions that arise in the
course of generations, as concomitants of structural modifications; and
these may be modifications, qualitative or quantitative, in the molecular
changes classed as chemical, or in the organic actions classed as physical,
or in both. Under the psychological come the qualitative and quantitative
modifications of instincts, feelings, conceptions, and mental processes in
general, which occur in creatures having more or less intelligence, when
certain of their conditions are changed. This, like the preceding
department of Psychology, has in the abstract two different aspects--the
objective and the subjective. Practically, however, the objective, which
deals with these mental modifications as exhibited in the changing habits
and abilities of successive generations of creatures, is the only one
admitting of investigation; since the corresponding alterations in
consciousness cannot be immediately known to any but the subjects of them.
Evidently, convenience requires us to join this part of Psychology along
with the other parts as components of a distinct sub-science.
Light is thrown on functions, as well as on structures, by comparing
organisms of different kinds. Comparative Physiology and Comparative
Psychology, are the names given to those collections of facts respecting
the homologies and analogies, bodily and mental, disclosed by this kind of
inquiry. These classified observations concerning likenesses and
differences of functions, are helpers to interpret functions in their
essential natures and relations. Hence Comparative Physiology and
Comparative Psychology are names of methods rather than names of true
subdivisions of Biology.
Here, however, as before, comparison of special truths, besides
facilitating their interpretation, brings to light certain general truths.
Contrasting functions bodily and mental as exhibited in various kinds of
organisms, shows that there exists, more or less extensively, a community
of processes and methods. Hence result two groups of propositions
constituting General Physiology and General Psychology.
§ 40. In these divisions and subdivisions of the first two great
departments of Biology, facts of Structure are considered separately from
facts of Function, so far as separate treatment of them is possible. The
third great department of Biology deals with them in their necessary
connexions. It comprehends the determination of functions by structures,
and the determination of structures by functions.
As displayed in individual organisms, the effects of structures on
functions are to be studied, not only in the broad fact that the general
kind of life an organism leads is necessitated by the main characters of
its organization, but in the more special and less conspicuous fact, that
between members of the same species, minor differences of structure lead to
minor differences of power to perform certain actions, and of tendencies to
perform such actions. Conversely, under the reactions of functions on
structures in individual organisms, come the facts showing that functions,
when fulfilled to their normal extents, maintain integrity of structure in
their respective organs; and that within certain limits increases of
functions are followed by such structural changes in their respective
organs, as enable them to discharge better their extra functions.
Inquiry into the influence of structure on function as seen in successions
of organisms, introduces us to such phenomena as Mr. Darwin's _Origin of
Species_ deals with. In this category come all proofs of the general truth,
that when an individual is enabled by a certain structural peculiarity to
perform better than others of its species some advantageous action; and
when it bequeaths more or less of its structural peculiarity to
descendants, among whom those which have it most markedly are best able to
thrive and propagate; there arises a visibly modified type of structure,
having a more or less distinct function. In the correlative class of facts
(by some asserted and by others denied), which come under the category of
reactions of function on structure as exhibited in successions of
organisms, are to be placed all those modifications of structure which
arise in races, when changes of conditions entail changes in the balance of
their functions--when altered function externally necessitated, produces
altered structure, and continues doing this through successive generations.
§ 41. The fourth great division of Biology, comprehending the phenomena of
Genesis, may be conveniently separated into three subdivisions.
Under the first, comes a description of all the special modes whereby the
multiplication of organisms is carried on; which modes range themselves
under the two chief heads of sexual and asexual. An account of Sexual
Multiplication includes the various processes by which germs and ova are
fertilized, and by which, after fertilization, they are furnished with the
materials, and maintained in the conditions, needful for their development.
An account of Asexual Multiplication includes the various processes by
which, from the same fertilized germ or ovum, there are produced many
organisms partially or totally independent of one another.
The second of these subdivisions deals with the phenomena of Genesis in the
abstract. It takes for its subject-matter such general questions as--What
is the end subserved by the union of sperm-cell and germ-cell? Why cannot
all multiplication be carried on after the asexual method? What are the
laws of hereditary transmission? What are the causes of variation?
The third subdivision is devoted to still more abstract aspects of the
subject. Recognizing the general facts of multiplication, without reference
to their modes or immediate causes, it concerns itself simply with the
different rates of multiplication in different kinds of organisms and
different individuals of the same kind. Generalizing the numerous contrasts
and variations of fertility, it seeks a rationale of them in their
relations to other organic phenomena.
§ 42. Such appears to be the natural arrangement of divisions and
subdivisions which Biology presents. It is, however, a classification of
the parts of the science when fully developed; rather than a classification
of them as they now stand. Some of the subdivisions above named have no
recognized existence, and some of the others are in quite rudimentary
states. It is impossible now to fill in, even in the roughest way, more
than a part of the outlines here sketched.
Our course of inquiry being thus in great measure determined by the present
state of knowledge, we are compelled to follow an order widely different
from this ideal one. It will be necessary first to give an account of those
empirical generalizations which naturalists and physiologists have
established: appending to those which admit of it, such deductive
interpretations as _First Principles_ furnishes us with. Having done this,
we shall be the better prepared for dealing with the leading truths of
Biology in connexion with the doctrine of Evolution.
PART II.
THE INDUCTIONS OF BIOLOGY.
CHAPTER I.
GROWTH.
§ 43. Perhaps the widest and most familiar induction of Biology, is that
organisms grow. While, however, this is a characteristic so uniformly and
markedly displayed by plants and animals, as to be carelessly thought
peculiar to them, it is really not so. Under appropriate conditions,
increase of size takes place in inorganic aggregates, as well as in organic
aggregates. Crystals grow; and often far more rapidly than living bodies.
Where the requisite materials are supplied in the requisite forms, growth
may be witnessed in non-crystalline masses: instance the fungous-like
accumulation of carbon that takes place on the wick of an unsnuffed candle.
On an immensely larger scale, we have growth in geologic formations: the
slow accumulation of deposited sediment into a stratum, is not
distinguishable from growth in its widest acceptation. And if we go back to
the genesis of celestial bodies, assuming them to have arisen by Evolution,
these, too, must have gradually passed into their concrete shapes through
processes of growth. Growth is, indeed, as being an integration of matter,
the primary trait of Evolution; and if Evolution of one kind or other is
universal, growth is universal--universal, that is, in the sense that all
aggregates display it in some way at some period.
The essential community of nature between organic growth and inorganic
growth, is, however, most clearly seen on observing that they both result
in the same way. The segregation of different kinds of detritus from each
other, as well as from the water carrying them, and their aggregation into
distinct strata, is but an instance of a universal tendency towards the
union of like units and the parting of unlike units (_First Principles_,
§ 163). The deposit of a crystal from a solution is a differentiation of
the previously mixed molecules; and an integration of one class of
molecules into a solid body, and the other class into a liquid solvent. Is
not the growth of an organism an essentially similar process? Around a
plant there exist certain elements like the elements which form its
substance; and its increase of size is effected by continually integrating
these surrounding like elements with itself. Nor does the animal
fundamentally differ in this respect from the plant or the crystal. Its
food is a portion of the environing matter that contains some compound
atoms like some of the compound atoms constituting its tissues; and either
through simple imbibition or through digestion, the animal eventually
integrates with itself, units like those of which it is built up, and
leaves behind the unlike units. To prevent misconception, it may be well to
point out that growth, as here defined, must be distinguished from certain
apparent and real augmentations of bulk which simulate it. Thus, the long,
white potato-shoots thrown out in the dark, are produced at the expense of
the substances which the tuber contains: they illustrate not the
accumulation of organic matter, but simply its re-composition and
re-arrangement. Certain animal-embryos, again, during their early stages,
increase considerably in size without assimilating any solids from the
environment; and they do this by absorbing the surrounding water. Even in
the highest organisms, as in children, there appears sometimes to occur a
rapid gain in dimensions which does not truly measure the added quantity of
organic matter; but is in part due to changes analogous to those just
named. Alterations of this kind must not be confounded with that growth,
properly so called, of which we have here to treat.
The next general fact to be noted respecting organic growth, is, that it
has limits. Here there appears to be a distinction between organic and
inorganic growth; but this distinction is by no means definite. Though that
aggregation of inanimate matter which simple attraction produces, may go on
without end; yet there appears to be an end to that more definite kind of
aggregation which results from polar attraction. Different elements and
compounds habitually form crystals more or less unlike in their sizes; and
each seems to have a size that is not usually exceeded without a tendency
arising to form new crystals rather than to increase the old. On looking
at the organic kingdom as a whole, we see that the limits between which
growth ranges are very wide apart. At the one extreme we have monads so
minute as to be rendered but imperfectly visible by microscopes of the
highest power; and at the other extreme we have trees of 400 to 500 feet
high and animals of 100 feet long. It is true that though in one sense this
contrast may be legitimately drawn, yet in another sense it may not; since
these largest organisms arise by the combination of units which are
individually like the smallest. A single plant of the genus _Protococcus_,
is of the same essential structure as one of the many cells united to form
the thallus of some higher Alga, or the leaf of a phænogam. Each separate
shoot of a phænogam is usually the bearer of many leaves. And a tree is an
assemblage of numerous united shoots. One of these great teleophytes is
thus an aggregate of aggregates of aggregates of units, which severally
resemble protophytes in their sizes and structures; and a like building up
is traceable throughout a considerable part of the animal kingdom. Even,
however, when we bear in mind this qualification, and make our comparisons
between organisms of the same degree of composition, we still find the
limit of growth to have a great range. The smallest branched flowering
plant is extremely insignificant by the side of a forest tree; and there is
an enormous difference in bulk between the least and the greatest mammal.
But on comparing members of the same species, we discover the limit of
growth to be much less variable. Among the _Protozoa_ and _Protophyta_,
each kind has a tolerably constant adult size; and among the most complex
organisms the differences between those of the same kind which have reached
maturity, are usually not very great. The compound plants do, indeed,
sometimes present marked contrasts between stunted and well-grown
individuals; but the higher animals diverge but inconsiderably from the
average standards of their species.
On surveying the facts with a view of empirically generalizing the causes
of these differences, we are soon made aware that by variously combining
and conflicting with one another, these causes produce great irregularities
of result. It becomes manifest that no one of them can be traced to its
consequences, unqualified by the rest. Hence the several statements
contained in the following paragraphs must be taken as subject to mutual
modification.
Let us consider first the connexion between degree of growth and complexity
of structure. This connexion, being involved with many others, becomes
apparent only on so averaging the comparisons as to eliminate differences
among the rest. Nor does it hold at all where the conditions are radically
dissimilar, as between plants and animals. But bearing in mind these
qualifications, we shall see that organization has a determining influence
on increase of mass. Of plants the lowest, classed as Thallophytes, usually
attain no considerable size. Algæ, Fungi, and the Lichens formed by
association of them count among their numbers but few bulky species: the
largest, such as certain Algæ found in antarctic seas, not serving greatly
to raise the average; and these gigantic seaweeds possess a considerable
complexity of histological organization very markedly exceeding that of
their smaller allies. Though among Bryophytes and Pteridophytes there are
some, as the Tree-ferns, which attain a considerable height, the majority
are but of humble growth. The Monocotyledons, including at one extreme
small grasses and at the other tall palms, show us an average and a maximum
greater than that reached by the Pteridophytes. And the Monocotyledons are
exceeded by the Dicotyledons; among which are found the monarchs of the
vegetal kingdom. Passing to animals, we meet the fact that the size
attained by _Vertebrata_ is usually much greater than the size attained by
_Invertebrata_. Of invertebrate animals the smallest, classed as
_Protozoa_, are also the simplest; and the largest, belonging to the
_Annulosa_ and _Mollusca_, are among the most complex of their respective
types. Of vertebrate animals we see that the greatest are Mammals, and that
though, in past epochs, there were Reptiles of vast bulks, their bulks did
not equal that of the whale: the great Dinosaurs, though as long, being
nothing like as massive. Between reptiles and birds, and between
land-vertebrates and water-vertebrates, the relation does not hold: the
conditions of existence being in these cases widely different. But among
fishes as a class, and among reptiles as a class, it is observable that,
speaking generally, the larger species are framed on the higher types. The
critical reader, who has mentally checked these statements in passing them,
has doubtless already seen that this relation is not a dependence of
organization on growth but a dependence of growth on organization. The
majority of Dicotyledons are smaller than some Monocotyledons; many
Monocotyledons are exceeded in size by certain Pteridophytes; and even
among Thallophytes, the least developed among compound plants, there are
kinds of a size which many plants of the highest order do not reach.
Similarly among animals. There are plenty of Crustaceans less than
_Actiniæ_; numerous reptiles are smaller than some fish; the majority of
mammals are inferior in bulk to the largest reptiles; and in the contrast
between a mouse and a well-grown _Medusa_, we see a creature that is
elevated in type of structure exceeded in mass by one that is extremely
low. Clearly then, it cannot be held that high organization is habitually
accompanied by great size. The proposition here illustrated is the converse
one, that great size is habitually accompanied by high organization. The
conspicuous facts that the largest species of both animals and vegetals
belong to the highest classes, and that throughout their various
sub-classes the higher usually contain the more bulky forms, show this
connexion as clearly as we can expect it to be shown, amid so many
modifying causes and conditions.
The relation between growth and supply of available nutriment, is too
familiar a relation to need proving. There are, however, some aspects of it
that must be contemplated before its implications can be fully appreciated.
Among plants, which are all constantly in contact with the gaseous, liquid,
and solid matters to be incorporated with their tissues, and which, in the
same locality, receive not very unlike amounts of light and heat,
differences in the supplies of available nutriment have but a subordinate
connexion with differences of growth. Though in a cluster of herbs
springing up from the seeds let fall by a parent, the greater sizes of some
than of others is doubtless due to better nutrition, consequent on
accidental advantages; yet no such interpretation can be given of the
contrast in size between these herbs and an adjacent tree. Other conditions
here come into play: one of the most important being, an absence in the one
case, and presence in the other, of an ability to secrete such a quantity
of ligneous fibre as will produce a stem capable of supporting a large
growth. Among animals, however, which (excepting some _Entozoa_) differ
from plants in this, that instead of bathing their surfaces the matters
they subsist on are dispersed, and have to be obtained, the relation
between available food and growth is shown with more regularity. The
_Protozoa_, living on microscopic fragments of organic matter contained in
the surrounding water, are unable, during their brief lives, to accumulate
any considerable quantity of nutriment. _Polyzoa_, having for food these
scarcely visible members of the animal kingdom, are, though large compared
with their prey, small as measured by other standards; even when aggregated
into groups of many individuals, which severally catch food for the common
weal, they are often so inconspicuous as readily to be passed over by the
unobservant. And if from this point upwards we survey the successive grades
of animals, it becomes manifest that, in proportion as the size is great,
the masses of nutriment are either large, or, what is practically the same
thing, are so abundant and so grouped that large quantities may be readily
taken in. Though, for example, the greatest of mammals, the arctic whale,
feeds on such comparatively small creatures as the acalephes and molluscs
floating in the seas it inhabits, its method of gulping in whole shoals of
them and filtering away the accompanying water, enables it to secure great
quantities of food. We may then with safety say that, other things equal,
the growth of an animal depends on the abundance and sizes of the masses of
nutriment which its powers enable it to appropriate. Perhaps it may be
needful to add that, in interpreting this statement, the proportion of
competitors must be taken into account. Clearly, not the absolute, but the
relative, abundance of fit food is the point; and this relative abundance
very much depends on the number of individuals competing for the food. Thus
all who have had experience in fishing in Highland lochs, know that where
the trout are numerous they are small, and that where they are
comparatively large they are comparatively few.
What is the relation between growth and expenditure of energy? is a
question which next presents itself. Though there is reason to believe such
a relation exists, it is not very readily traced: involved as it is with so
many other relations. Some contrasts, however, may be pointed out that
appear to give evidence of it. Passing over the vegetal kingdom, throughout
which the expenditure of force is too small to allow of such a relation
being visible, let us seek in the animal kingdom, some case where classes
otherwise allied, are contrasted in their locomotive activities. Let us
compare birds on the one hand, with reptiles and mammals on the other. It
is an accepted doctrine that birds are organized on a type closely allied
to the reptilian type, but superior to it; and though in some respects the
organization of birds is inferior to that of mammals, yet in other
respects, as in the greater heterogeneity and integration of the skeleton,
the more complex development of the respiratory system, and the higher
temperature of the blood, it may be held that birds stand above mammals.
Hence were growth dependent only on organization, we might infer that the
limit of growth among birds should not be much short of that among mammals;
and that the bird-type should admit of a larger growth than the
reptile-type. Again, we see no manifest disadvantages under which birds
labour in obtaining food, but from which reptiles and mammals are free. On
the contrary, birds are able to get at food that is fixed beyond the reach
of reptiles and mammals; and can catch food that is too swift of movement
to be ordinarily caught by reptiles and mammals. Nevertheless, the limit of
growth in birds falls far below that reached by reptiles and mammals. With
what other contrast between these classes, is this contrast connected? May
we not suspect that it is connected (partially though not wholly) with the
contrast between their amounts of locomotive exertion? Whereas mammals
(excepting bats, which are small), are during all their movements supported
by solid surfaces or dense liquids; and whereas reptiles (excepting the
ancient pterodactyles, which were not very large), are similarly restricted
in their spheres of movement; the majority of birds move more or less
habitually through a rare medium, in which they cannot support themselves
without relatively great efforts. And this general fact may be joined with
the special fact, that those members of the class _Aves_, as the _Dinornis_
and _Epiornis_, which approached in size to the larger _Mammalia_ and
_Reptilia_, were creatures incapable of flight--creatures which did not
expend this excess of force in locomotion. But as implied above, and as
will presently be shown, another factor of importance comes into play; so
that perhaps the safest evidence that there is an antagonism between the
increase of bulk and the quantity of motion evolved is that supplied by the
general experience, that human beings and domestic animals, when overworked
while growing, are prevented from attaining the ordinary dimensions.
One other general truth concerning degrees of growth, must be set down. It
is a rule, having exceptions of no great importance, that large organisms
commence their separate existences as masses of organic matter more or less
considerable in size, and commonly with organizations more or less
advanced; and that throughout each organic sub-kingdom, there is a certain
general, though irregular, relation between the initial and the final
bulks. Vegetals exhibit this relation less manifestly than animals. Yet
though, among the plants that begin life as minute spores, there are some
which, by the aid of an intermediate form, grow to large sizes, the immense
majority of them remain small. While, conversely, the great Monocotyledons
and Dicotyledons, when thrown off from their parents, have already the
formed organs of young plants, to which are attached stores of highly
nutritive matter. That is to say, where the young plant consists merely of
a centre of development, the ultimate growth is commonly insignificant; but
where the growth is to become great, there exists to start with, a
developed embryo and a stock of assimilable matter. Throughout the animal
kingdom this relation is tolerably manifest though by no means uniform.
Save among classes that escape the ordinary requirements of animal life,
small germs or eggs do not in most cases give rise to bulky creatures.
Where great bulk is to be reached, the young proceeds from an egg of
considerable bulk, or is born of considerable bulk ready-organized and
partially active. In the class Fishes, or in such of them as are subject to
similar conditions of life, some proportion usually obtains between the
sizes of the ova and the sizes of the adult individuals; though in the
cases of the sturgeon and the tunny there are exceptions, probably
determined by the circumstances of oviposition and those of juvenile life.
Reptiles have eggs that are smaller in number, and relatively greater in
mass, than those of fishes; and throughout this class, too, there is a
general congruity between the bulk of the egg and the bulk of the adult
creature. As a group, birds show us further limitations in the numbers of
their eggs as well as farther increase in their relative sizes; and from
the minute eggs of the humming-bird up to the immense ones of the
_Epiornis_, holding several quarts, we see that, speaking generally, the
greater the eggs the greater the birds., Finally, among mammals (omitting
the marsupials) the young are born, not only of comparatively large sizes,
but with advanced organizations; and throughout this sub-division of the
_Vertebrata_, as throughout the others, there is a manifest connexion
between the sizes at birth and the sizes at maturity. As having a kindred
meaning, there must finally be noted the fact that the young of these
highest animals, besides starting in life with bodies of considerable
sizes, almost fully organized, are, during subsequent periods of greater or
less length, supplied with nutriment--in birds by feeding and in mammals by
suckling and afterwards by feeding. So that beyond the mass and
organization directly bequeathed, a bird or mammal obtains a further large
mass at but little cost to itself.
Were exhaustive treatment of the topic intended, it would be needful to
give a paragraph to each of the incidental circumstances by which growth
may be aided or restricted:--such facts as that an entozoon is limited by
the size of the creature, or even the organ, in which it thrives; that an
epizoon, though getting abundant nutriment without appreciable exertion, is
restricted to that small bulk at which it escapes ready detection by the
animal it infests; that sometimes, as in the weazel, smallness is a
condition to successful pursuit of the animals preyed upon; and that in
some cases, the advantage of resembling certain other creatures, and so
deceiving enemies or prey, becomes an indirect cause of restricted size.
But the present purpose is simply to set down those most general relations
between growth and other organic traits, which induction leads us to.
Having done this, let us go on to inquire whether these general relations
can be deductively established.
§ 44. That there must exist a certain dependence of growth on organization,
may be shown _a priori_. When we consider the phenomena of Life, either by
themselves or in their relations to surrounding phenomena, we see that,
other things equal, the larger the aggregate the greater is the needful
complexity of structure.
In plants, even of the highest type, there is a comparatively small mutual
dependence of parts: a gathered flower-bud will unfold and flourish for
days if its stem be immersed in water; and a shoot cut off from its
parent-tree and stuck in the ground will grow. The respective parts having
vital activities that are not widely unlike, it is possible for great bulk
to be reached without that structural complexity required for combining the
actions of parts. Even here, however, we see that for the attainment of
great bulk there requires such a degree of organization as shall
co-ordinate the functions of roots and branches--we see that such a size as
is reached by trees, is not possible without a vascular system enabling the
remote organs to utilize each other's products. And we see that such a
co-existence of large growth with comparatively low organization as occurs
in some of the marine _Algæ_, occurs where the conditions of existence do
not necessitate any considerable mutual dependence of parts--where the near
approach of the plant to its medium in specific gravity precludes the need
of a well-developed stem, and where all the materials of growth being
derived from the water by each portion of the thallus, there requires no
apparatus for transferring the crude food materials from part to part.
Among animals which, with but few exceptions, are, by the conditions of
their existence, required to absorb nutriment through one specialized part
of the body, it is clear that there must be a means whereby other parts of
the body, to be supported by this nutriment, must have it conveyed to them.
It is clear that for an equally efficient maintenance of their nutrition,
the parts of a large mass must have a more elaborate propelling and
conducting apparatus; and that in proportion as these parts undergo greater
waste, a yet higher development of the vascular system is necessitated.
Similarly with the prerequisites to those mechanical motions which animals
are required to perform. The parts of a mass cannot be made to move, and
have their movements so co-ordinated as to produce locomotive and other
actions, without certain structural arrangements; and, other things equal,
a given amount of such activity requires more involved structural
arrangements in a large mass than in a small one. There must at least be a
co-ordinating apparatus presenting greater contrasts in its central and
peripheral parts.
The qualified dependence of growth on organization, is equally implied when
we study it in connexion with that adjustment of inner to outer relations
which constitutes Life as phenomenally known to us. In plants this is less
striking than in animals, because the adjustment of inner to outer
relations does not involve conspicuous motions. Still, it is visible in the
fact that the condition on which alone a plant can grow to a great size,
is, that it shall, by the development of a massive trunk, present inner
relations of forces fitted to counterbalance those outer relations of
forces which tend continually, and others which tend occasionally, to
overthrow it; and this formation of a core of regularly-arranged woody
fibres is an advance in organization. Throughout the animal kingdom this
connexion of phenomena is manifest. To obtain materials for growth; to
avoid injuries which interfere with growth; and to escape those enemies
which bring growth to a sudden end; implies in the organism the means of
fitting its movements to meet numerous external co-existences and
sequences--implies such various structural arrangements as shall make
possible these variously-adapted actions. It cannot be questioned that,
everything else remaining constant, a more complex animal, capable of
adjusting its conduct to a greater number of surrounding contingencies,
will be the better able to secure food and evade damage, and so to increase
bulk. And evidently, without any qualification, we may say that a large
animal, living under such complex conditions of existence as everywhere
obtain, is not possible without comparatively high organization.
While, then, this relation is traversed and obscured by sundry other
relations, it cannot but exist. Deductively we see that it must be
modified, as inductively we saw that it is modified, by the circumstances
amid which each kind of organism is placed, but that it is always a factor
in determining the result.
§ 45. That growth is, _cæteris paribus_, dependent on the supply of
assimilable matter, is a proposition so continually illustrated by special
experience, as well as so obvious from general experience, that it would
scarcely need stating, were it not requisite to notice the qualifications
with which it must be taken.
The materials which each organism requires for building itself up, are not
of one kind but of several kinds. As a vehicle for transferring matter
through their structures, all organisms require water as well as solid
constituents; and however abundant the solid constituents there can be no
growth in the absence of water. Among the solids supplied, there must be a
proportion ranging within certain limits. A plant round which carbonic
acid, water, and ammonia exist in the right quantities, may yet be arrested
in its growth by a deficiency of potassium. The total absence of lime from
its food may stop the formation of a mammal's skeleton: thus dwarfing, if
not eventually destroying, the mammal; and this no matter what quantities
of other needful colloids and crystalloids are furnished.
Again, the truth that, other things equal, growth varies according to the
supply of nutriment, has to be qualified by the condition that the supply
shall not exceed the ability to appropriate it. In the vegetal kingdom, the
assimilating surface being external and admitting of rapid expansion by the
formation of new roots, shoots, and leaves, the effect of this limitation
is not conspicuous. By artificially supplying plants with those materials
which they have usually the most difficulty in obtaining, we can greatly
facilitate their growth; and so can produce striking differences of size in
the same species. Even here, however, the effect is confined within the
limits of the ability to appropriate; since in the absence of that solar
light and heat by the help of which the chief appropriation is carried on,
the additional materials for growth are useless. In the animal kingdom this
restriction is rigorous. The absorbent surface being, in the great majority
of cases, internal; having a comparatively small area, which cannot be
greatly enlarged without reconstruction of the whole body; and being in
connexion with a vascular system which also must be re-constructed before
any considerable increase of nutriment can be made available; it is clear
that beyond a certain point, very soon reached, increase of nutriment will
not cause increase of growth. On the contrary, if the quantity of food
taken in is greatly beyond the digestive and absorbent power, the excess,
becoming an obstacle to the regular working of the organism, may retard
growth rather than advance it.
While then it is certain, _a priori_, that there cannot be growth in the
absence of such substances as those of which an organism consists; and
while it is equally certain that the amount of growth must primarily be
governed by the supply of these substances; it is not less certain that
extra supply will not produce extra growth, beyond a point very soon
reached. Deduction shows to be necessary, as induction makes familiar, the
truths that the value of food for purposes of growth depends not on the
quantity of the various organizable materials it contains, but on the
quantity of the material most needed; that given a right proportion of
materials, the pre-existing structure of the organism limits their
availability; and that the higher the structure, the sooner is this limit
reached.
§ 46. But why should the growth of every organism be finally arrested?
Though the rate of increase may, in each case, be necessarily restricted
within a narrow range of variation--though the increment that is possible
in a given time, cannot exceed a certain amount; yet why should the
increments decrease and finally become insensible? Why should not all
organisms, when supplied with sufficient materials, continue to grow as
long as they live? To find an answer to this question we must revert to the
nature and functions of organic matter.
In the first three chapters of Part I, it was shown that plants and animals
mainly consist of substances in states of unstable equilibrium--substances
which have been raised to this unstable equilibrium by the expenditure of
the forces we know as solar radiations, and which give out these forces in
other forms on falling into states of stable equilibrium. Leaving out the
water, which serves as a vehicle for these materials and a medium for their
changes; and excluding those mineral matters that play either passive or
subsidiary parts; organisms are built up of compounds which are stores of
force. Thus complex colloids and crystalloids which, as united together,
form organized bodies, are the same colloids and crystalloids which give
out, on their decomposition, the forces expended by organized bodies. Thus
these nitrogenous and carbonaceous substances, being at once the materials
for organic growth and the sources of organic energy, it results that as
much of them as is used up for the genesis of energy is taken away from the
means of growth, and as much as is economized by diminishing the genesis of
energy, is available for growth. Given that limited quantity of nutritive
matter which the pre-existing structure of an organism enables it to
absorb; and it is a necessary corollary from the persistence of force, that
the matter accumulated as growth cannot exceed that surplus which remains
undecomposed after the production of the required amounts of sensible and
insensible motion. This, which would be rigorously true under all
conditions if exactly the same substances were used in exactly the same
proportions for the production of force and for the formation of tissue,
requires, however, to be taken with the qualification that some of the
force-evolving substances are not constituents of tissue; and that thus
there may be a genesis of force which is not at the expense of potential
growth. But since organisms (or at least animal organisms, with which we
are here chiefly concerned) have a certain power of selective absorption,
which, partially in an individual and more completely in a race, adapts the
proportions of the substances absorbed to the needs of the system; then if
a certain habitual expenditure of force leads to a certain habitual
absorption of force-evolving matters that are not available for growth; and
if, were there less need for such matters, the ability to absorb matters
available for growth would be increased to an equivalent extent; it follows
that the antagonism described does, in the long run, hold even without this
qualification. Hence, growth is substantially equivalent to the absorbed
nutriment, minus the nutriment used up in action.
This, however, is no answer to the question--why has individual growth a
limit?--why do the increments of growth bear decreasing ratios to the mass
and finally come to an end? The question is involved. There are more causes
than one why the excess of absorbed nutriment over expended nutriment must,
other things equal, become less as the size of the animal becomes greater.
In similarly-shaped bodies the masses, and therefore the weights, vary as
the cubes of the dimensions; whereas the powers of bearing the stresses
imposed by the weights vary as the squares of the dimensions. Suppose a
creature which a year ago was one foot high, has now become two feet high,
while it is unchanged in proportions and structure; what are the necessary
concomitant changes? It is eight times as heavy; that is to say, it has to
resist eight times the strain which gravitation puts upon certain of its
parts; and when there occurs sudden arrest of motion or sudden genesis of
motion, eight times the strain is put upon the muscles employed. Meanwhile
the muscles and bones have severally increased their abilities to bear
strains in proportion to the areas of their transverse sections, and hence
have severally only four times the tenacity they had. This relative
decrease in the power of bearing stress does not imply a relative decrease
in the power of generating energy and moving the body; for in the case
supposed the muscles have not only increased four times in their transverse
sections but have become twice as long, and will therefore generate an
amount of energy proportionate to their bulk. The implication is simply
that each muscle has only half the power to withstand those shocks and
strains which the creature's movements entail; and that consequently the
creature must be either less able to bear these, or must have muscles and
bones having relatively greater transverse dimensions: the result being
that greater cost of nutrition is inevitably caused and therefore a
correlative tendency to limit growth. This necessity will be seen still
more clearly if we leave out the motor apparatus, and consider only the
forces required and the means of supplying them. For since, in similar
bodies, the areas vary as the squares of the dimensions, and the masses
vary as the cubes; it follows that the absorbing surface has become four
times as great, while the weight to be moved by the matter absorbed has
become eight times as great. If then, a year ago, the absorbing surface
could take up twice as much nutriment as was needed for expenditure, thus
leaving one-half for growth, it is now able only just to meet expenditure,
and can provide nothing for growth. However great the excess of
assimilation over waste may be during the early life of an active organism,
we see that because a series of numbers increasing as the cubes, overtakes
a series increasing as the squares, even though starting from a much
smaller number, there must be reached, if the organism lives long enough, a
point at which the surplus assimilation is brought down to nothing--a point
at which expenditure balances nutrition--a state of moving equilibrium. The
only way in which the difficulty can be met is by gradual re-organization
of the alimentary system; and, in the first place, this entails direct cost
upon the organism, and, in the second place, indirect cost from the
carrying of greater weight: both tending towards limitation. There are two
other varying relations between degrees of growth and amounts of expended
force; one of which conspires with the last, while the other conflicts with
it. Consider, in the first place, the cost at which nutriment is
distributed through the body and effete matters removed from it. Each
increment of growth being added at the periphery of the organism, the force
expended in the transfer of matter must increase in a rapid progression--a
progression more rapid than that of the mass. But as the dynamic expense of
distribution is small compared with the dynamic value of the materials
distributed, this item in the calculation is unimportant. Now consider, in
the second place, the changing proportion between production and loss of
heat. In similar organisms the quantities of heat generated by similar
actions going on throughout their substance, must increase as the masses,
or as the cubes of the dimensions. Meanwhile, the surfaces from which loss
of heat takes place, increase only as the squares of the dimensions. Though
the loss of heat does not therefore increase only as the squares of the
dimensions, it certainly increases at a smaller rate than the cubes. And to
the extent that augmentation of mass results in a greater retention of
heat, it effects an economization of force. This advantage is not, however,
so important as at first appears. Organic heat is a concomitant of organic
action, and is so abundantly produced during action that the loss of it is
then usually of no consequence: indeed the loss is often not rapid enough
to keep the supply from rising to an inconvenient excess. It is chiefly in
respect of that maintenance of heat which is needful during quiescence,
that large organisms have an advantage over small ones in this relatively
diminished loss. Thus these two subsidiary relations between degrees of
growth and amounts of expended force, being in antagonism, we may conclude
that their differential result does not greatly modify the result of the
chief relation.
Comparisons of these deductions with the facts appear in some cases to
verify them and in other cases not to do so. Throughout the vegetal
kingdom, there are no distinct limits to growth except those which death
entails. Passing over a large proportion of plants which never exceed a
comparatively small size, because they wholly or partially die down at the
end of the year, and looking only at trees that annually send forth new
shoots, even when their trunks are hollowed by decay; we may ask--How does
growth happen here to be unlimited? The answer is, that plants are only
accumulators: they are in no very appreciable degree expenders. As they do
not undergo waste there is no reason why their growth should be arrested by
the equilibration of assimilation and waste. Again, among animals there
are sufficient reasons why the correspondence cannot be more than
approximate. Besides the fact above noted, that there are other varying
relations which complicate the chief one. We must bear in mind that the
bodies compared are not truly similar: the proportions of trunk to limbs
and trunk to head, vary considerably. The comparison is still more
seriously vitiated by the inconstant ratio between the constituents of
which the body is composed. In the flesh of adult mammalia, water forms
from 68 to 71 per cent., organic substance from 24 to 28 per cent., and
inorganic substance from 3 to 5 per cent.; whereas in the foetal state, the
water amounts to 87 per cent., and the solid organic constituents to only
11 per cent. Clearly this change from a state in which the force-evolving
matter forms one-tenth of the whole, to a state in which it forms two and a
half tenths, must greatly interfere with the parallelism between the actual
and the theoretical progression. Yet another difficulty may come under
notice. The crocodile is said to grow as long as it lives; and there
appears reason to think that some predaceous fishes, such as the pike, do
the same. That these animals of comparatively high organization have no
definite limits of growth, is, however, an exceptional fact due to the
exceptional non-fulfilment of those conditions which entail limitation.
What kind of life does a crocodile lead? It is a cold-blooded, or almost
cold-blooded, creature; that is, it expends very little for the maintenance
of heat. It is habitually inert: not usually chasing prey but lying in wait
for it; and undergoes considerable exertion only during its occasional
brief contests with prey. Such other exertion as is, at intervals, needful
for moving from place to place, is rendered small by the small difference
between the animal's specific gravity and that of water. Thus the crocodile
expends in muscular action an amount of force that is insignificant
compared with the force commonly expended by land-animals. Hence its
habitual assimilation is diminished much less than usual by habitual waste;
and beginning with an excessive disproportion between the two, it is quite
possible for the one never quite to lose its advance over the other while
life continues. On looking closer into such cases as this and that of the
pike, which is similarly cold-blooded, similarly lies in wait, and is
similarly able to obtain larger and larger kinds of prey as it increases in
size; we discover a further reason for this absence of a definite limit. To
overcome gravitative force the creature has not to expend a muscular power
that is large at the outset, and increases as the cubes of its dimensions:
its dense medium supports it. The exceptional continuance of growth
observed in creatures so circumstanced, is therefore perfectly explicable.
§ 46a. If we go back upon the conclusions set forth in the preceding
section, we find that from some of them may be drawn instructive
corollaries respecting the limiting sizes of creatures inhabiting different
media. More especially I refer to those varying proportions between mass
and stress from which, as we have seen, there results, along with
increasing size, a diminishing power of mechanical self-support: a relation
illustrated in its simplest form by the contrast between a dew-drop, which
can retain its spheroidal form, and the spread-out mass of water which
results when many dew-drops run together. The largest bird that flies (the
argument excludes birds which do not fly) is the Condor, which reaches a
weight of from 30 to 40 lbs. Why does there not exist a bird of the size of
an elephant? Supposing its habits to be carnivorous, it would have many
advantages in obtaining prey: mammals would be at its mercy. Evidently the
reason is one which has been pointed out--the reason that while the weight
to be raised and kept in the air by a bird increases as the cubes of its
dimensions, the ability of its bones and muscles to resist the strains
which flight necessitates, increases only as the squares of the dimensions.
Though, could the muscles withstand any tensile strain they were subject
to, the power like the weight might increase with the cubes, yet since the
texture of muscle is such that beyond a certain strain it tears, it results
that there is soon reached a size at which flight becomes impossible: the
structures must give way. In a preceding paragraph the limit to the size of
flying creatures was ascribed to the greater physiological cost of the
energy required; but it seems probable that the mechanical obstacle here
pointed out has a larger share in determining the limit.
In a kindred manner there results a limitation of growth in a land-animal,
which does not exist for an animal living in the water. If, after comparing
the agile movements of a dog with those of a cow, the great weight of which
obviously prevents agility; or if, after observing the swaying flesh of an
elephant as it walks along, we consider what would happen could there be
formed a land-animal equal in mass to the whale (the long Dinosaurs were
not proportionately massive) it needs no argument to show that such a
creature could not stand, much less move about. But in the water the strain
put upon its structures by the weights of its various parts is almost if
not quite taken away. Probably limitation in the quantity of food
obtainable becomes now the chief, if not the sole, restraint.
And here we may note, before leaving the topic, something like a converse
influence which comes into play among creatures inhabiting the water. Up to
the point at which muscles tear from over-strain, larger and smaller
creatures otherwise alike, remain upon a par in respect of the relative
amounts of energy they can evolve. Had they to encounter no resistance from
their medium, the implication would be that neither would have an advantage
over the other in respect of speed. But resistance of the medium comes into
play; and this, other things equal, gives to the larger creature an
advantage. It has been found, experimentally, that the forces to be
overcome by vessels moving through the water, built as they are with
immersed hinder parts which taper as fish taper, are mainly due to what is
called "skin-friction." Now in two fish unlike in size but otherwise
similar skin-friction bears to the energy that can be generated, a smaller
proportion in the larger than in the smaller; and the larger can therefore
acquire a greater velocity. Hence the reason why large fish, such as the
shark, become possible. In a habitat where there is no ambush (save in
exceptional cases like that of the _Lophius_ or Angler) everything depends
on speed; and if, other things equal, a larger fish had no mechanical
advantage over a smaller, a larger fish could not exist--could not catch
the requisite amount of prey.
§ 47. Obviously this antagonism between accumulation and expenditure, must
be a leading cause of the contrasts in size between allied organisms that
are in many respects similarly conditioned. The life followed by each kind
of animal is one involving a certain average amount of exertion for the
obtainment of a given amount of nutriment--an exertion, part of which goes
to the gathering or catching of food, part to the tearing and mastication
of it, and part to the after-processes requisite for separating the
nutritive molecules--an exertion which therefore varies according as the
food is abundant or scarce, fixed or moving, according as it is
mechanically easy or difficult to deal with when secured, and according as
it is, or is not, readily soluble. Hence, while among animals of the same
species having the same mode of life, there will be a tolerably constant
ratio between accumulation and expenditure, and therefore a tolerably
constant limit of growth, there is every reason to expect that different
species, following different modes of life, will have unlike ratios between
accumulation and expenditure, and therefore unlike limits of growth.
Though the facts as inductively established, show a general harmony with
this deduction, we cannot usually trace it in any specific way; since the
conflicting and conspiring factors which affect growth are so numerous.
§ 48. One of the chief causes, if not the chief cause, of the differences
between the sizes of organisms, has yet to be considered. We are introduced
to it by pushing the above inquiry a little further. Small animals have
been shown to possess an advantage over large ones in the greater ratio
which, other things equal, assimilation bears to expenditure; and we have
seen that hence small animals in becoming large ones, gradually lose that
surplus of assimilative power which they had, and eventually cannot
assimilate more than is required to balance waste. But how come these
animals while young and small to have surplus assimilative powers? Have all
animals equal surpluses of assimilative powers? And if not, how far do
differences between the surpluses determine differences between the limits
of growth? We shall find, in the answers to these questions, the
interpretation of many marked contrasts in growth that are not due to any
of the causes above assigned. For example, an ox immensely exceeds a sheep
in mass. Yet the two live from generation to generation in the same fields,
eat the same grass, obtain these aliments with the same small expenditure
of energy, and differ scarcely at all in their degrees of organization.
Whence arises, then, their striking unlikeness of bulk?
We noted when studying the phenomena of growth inductively, that organisms
of the larger and higher types commence their separate existences as masses
of organic matter having tolerable magnitudes. Speaking generally, we saw
that throughout each organic sub-kingdom the acquirement of great bulk
occurs only where the incipient bulk and organization are considerable; and
that they are the more considerable in proportion to the complexity of the
life which the organism is to lead.
The deductive interpretation of this induction may best be commenced by an
analogy. A street orange-vendor makes but a trifling profit on each
transaction; and unless more than ordinarily fortunate, he is unable to
realize during the day a larger amount than will meet his wants; leaving
him to start on the morrow in the same condition as before. The trade of
the huxter in ounces of tea and half-pounds of sugar, is one similarly
entailing much labour for small returns. Beginning with a capital of a few
pounds, he cannot have a shop large enough, or goods sufficiently abundant
and various, to permit an extensive business. He must be content with the
half-pence and pence which he makes by little sales to poor people; and if,
avoiding bad debts, he is able by strict economy to accumulate anything, it
can be but a trifle. A large retail trader is obliged to lay out much money
in fitting up an adequate establishment; he must invest a still greater sum
in stock; and he must have a further floating capital to meet the charges
that fall due before his returns come in. Setting out, however, with means
enough for these purposes, he is able to make many and large sales; and so
to get greater and more numerous increments of profit. Similarly, to get
returns in thousands merchants and manufacturers must make their
investments in tens of thousands. In brief, the rate at which a man's
wealth accumulates is measured by the surplus of income over expenditure;
and this, save in exceptionably favourable cases, is determined by the
capital with which he begins business. Now applying the analogy, we may
trace in the transactions of an organism, the same three ultimate elements.
There is the expenditure required for the obtainment and digestion of food;
there is the gross return in the shape of nutriment assimilated or fit for
assimilation; and there is the difference between this gross return of
nutriment and the nutriment that was used up in the labour of securing
it--a difference which may be a profit or a loss. Clearly, however, a
surplus implies that the force expended is less than the force latent in
the assimilated food. Clearly, too, the increment of growth is limited to
the amount of this surplus of income over expenditure; so that large growth
implies both that the excess of nutrition over waste shall be relatively
considerable, and that the waste and nutrition shall be on extensive
scales. And clearly, the ability of an organism to expend largely and
assimilate largely, so as to make a large surplus, presupposes a large
physiological capital in the shape of organic matter more or less developed
in its structural arrangements.
Throughout the vegetal kingdom, the illustrations of this truth are not
conspicuous and regular: the obvious reason being that since plants are
accumulators and in so small a degree expenders, the premises of the above
argument are but very partially fulfilled. The food of plants (excepting
Fungi and certain parasites) being in great measure the same for all, and
bathing all so that it can be absorbed without effort, their vital
processes result almost entirely in profit. Once fairly rooted in a fit
place, a plant may thus from the outset add a very large proportion of its
entire returns to capital; and may soon be able to carry on its processes
on a large scale, though it does not at first do so. When, however, plants
are expenders, namely, during their germination and first stages of growth,
their degrees of growth _are_ determined by their amounts of vital capital.
It is because the young tree commences life with a ready-formed embryo and
store of food sufficient to last for some time, that it is enabled to
strike root and lift its head above the surrounding herbage. Throughout the
animal kingdom, however, the necessity of this relation is everywhere
obvious. The small carnivore preying on small herbivores, can increase in
size only by small increments: its organization unfitting it to digest
larger creatures, even if it can kill them, it cannot profit by amounts of
nutriment exceeding a narrow limit; and its possible increments of growth
being small to set out with, and rapidly decreasing, must come to an end
before any considerable size is attained. Manifestly the young lion, born
of tolerable bulk, suckled until much bigger, and fed until half-grown, is
enabled by the power and organization which he thus gets _gratis_, to catch
and kill animals big enough to give him the supply of nutriment needed to
meet his large expenditure and yet leave a large surplus for growth. Thus,
then, is explained the above-named contrast between the ox and the sheep. A
calf and a lamb commence their physiological transactions on widely
different scales; their first increments of growth are similarly contrasted
in their amounts; and the two diminishing series of such increments end at
similarly-contrasted limits.
§ 49. Such are the several conditions by which the phenomena of growth are
determined. Conspiring and conflicting in endless unlike ways and degrees,
they in every case qualify more or less differently each other's effects.
Hence it happens that we are obliged to state each generalization as true
on the average, or to make the proviso--other things equal.
Understood in this qualified form, our conclusions are these. First, that
growth being an integration with the organism of such environing matters as
are of like natures with the matters composing the organism, its growth is
dependent on the available supply of them. Second, that the available
supply of assimilable matter being the same, and other conditions not
dissimilar, the degree of growth varies according to the surplus of
nutrition over expenditure--a generalization which is illustrated in some
of the broader contrasts between different divisions of organisms. Third,
that in the same organism the surplus of nutrition over expenditure differs
at different stages; and that growth is unlimited or has a definite limit,
according as the surplus does or does not rapidly decrease. This
proposition we found exemplified by the almost unceasing growth of
organisms that expend relatively little energy; and by the definitely
limited growth of organisms that expend much energy. Fourth, that among
organisms which are large expenders of force, the size ultimately attained
is, other things equal, determined by the initial size: in proof of which
conclusion we have abundant facts, as well as the _a priori_ necessity that
the sum-totals of analogous diminishing series, must depend upon the
amounts of their initial terms. Fifth, that where the likeness of other
circumstances permits a comparison, the possible extent of growth depends
on the degree of organization; an inference testified to by the larger
forms among the various divisions and sub-divisions of organisms.
CHAPTER II.
DEVELOPMENT.[19]
§ 50. Certain general aspects of Development may be studied apart from any
examination of internal structures. These fundamental contrasts between the
modes of arrangement of parts, originating, as they do, the leading
external distinctions among the various forms of organization, will be best
dealt with at the outset. If all organisms have arisen by Evolution, it is
of course not to be expected that such several modes of development can be
absolutely demarcated: we are sure to find them united by transitional
modes. But premising that a classification of modes can but approximately
represent the facts, we shall find our general conceptions of Development
aided by one.
Development is primarily _central_. All organic forms of which the entire
history is known, set out with a symmetrical arrangement of parts round a
centre. In organisms of the lowest grade no other mode of arrangement is
ever definitely established; and in the highest organisms central
development, though subordinate to another mode of development, continues
to be habitually shown in the changes of minute structure. Let us glance at
these propositions in the concrete. Practically every plant and every
animal in its earliest stage is a portion of protoplasm, in the great
majority of cases approximately spherical but sometimes elongated,
containing a rounded body consisting of specially modified protoplasm,
which is called a nucleus; and the first changes that occur in the germ
thus constituted, are changes that take place in this nucleus, followed by
changes round the centres produced by division of this original centre.
From this type of structure, the simplest organisms do not depart; or
depart in no definite or conspicuous ways. Among plants, many of the
simplest _Algæ_ and _Fungi_ permanently maintain such a central
distribution; while among animals it is permanently maintained by creatures
like the _Gregarina_, and in a different manner by the _Amoeba_,
_Actinophrys_, and their allies: the irregularities which are many and
great do not destroy this general relation of parts. In larger organisms,
made up chiefly of units that are analogous to these simplest organisms,
the formation of units ever continues to take place round nuclei; though
usually the nuclei soon cease to be centrally placed.
Central development may be distinguished into _unicentral_ and
_multicentral_; according as the product of the original germ develops more
or less symmetrically round one centre, or develops without subordination
to one centre--develops, that is, in subordination to many centres.
Unicentral development, as displayed not in the formation of single cells
but in the formation of aggregates, is not common. The animal kingdom shows
it only in some of the small group of colonial _Radiolaria_. It is feebly
represented in the vegetal kingdom by a few members of the _Volvocineæ_. On
the other hand, multicentral development, or development round
insubordinate centres, is variously exemplified in both divisions of the
organic world. It is exemplified in two distinct ways, according as the
insubordination among the centres of development is partial or total. We
may most conveniently consider it under the heads hence arising.
Total insubordination among the centres of development, is shown where the
units or cells, as fast as they are severally formed, part company and lead
independent lives. This, in the vegetal kingdom, habitually occurs among
the _Protophyta_, and in the animal kingdom, among the _Protozoa_. Partial
insubordination is seen in those somewhat advanced organisms, that consist
of units which, though they have not separated, have so little mutual
dependence that the aggregate they form is irregular. Among plants, the
Thallophytes very generally exemplify this mode of development. Lichens,
spreading with flat or corrugated edges in this or that direction as the
conditions determine, have no manifest co-ordination of parts. In the
_Algæ_ the Nostocs and various other forms similarly show us an
unsymmetrical structure. Of _Fungi_ we may say that creeping kinds display
no further dependence of one part on another than is implied by their
cohesion. And even in such better-organized plants as the _Marchantia_, the
general arrangement shows no reference to a directive centre. Among animals
many of the Sponges in their adult forms may be cited as devoid of that
co-ordination implied by symmetry: the units composing them, though they
have some subordination to local centres, have no subordination to a
general centre. To distinguish that kind of development in which the whole
product of a germ coheres in one mass, from that kind of development in
which it does not, Professor Huxley has introduced the words "_continuous_"
and "_discontinuous_;" and these seem the best fitted for the purpose.
Multicentral development, then, is divisible into continuous and
discontinuous.
From central development we pass insensibly to that higher kind of
development for which _axial_ seems the most appropriate name. A tendency
towards this is vaguely manifested almost everywhere. The great majority
even of _Protophyta_ and _Protozoa_ have different longitudinal and
transverse dimensions--have an obscure if not a distinct axial structure.
The originally spheroidal and polyhedral units out of which higher
organisms are mainly built, usually pass into shapes that are subordinated
to lines rather than to points. And in the higher organisms, considered as
wholes, an arrangement of parts in relation to an axis is distinct and
nearly universal. We see it in the superior orders of Thallophytes; and in
all the cormophytic plants. With few exceptions the _Coelenterata_ clearly
exhibit it; it is traceable, though less conspicuously, throughout the
_Mollusca_; and the _Annelida_, _Arthropoda_, and _Vertebrata_ uniformly
show it with perfect definiteness.
This kind of development, like the first kind, is of two orders. The whole
germ-product may arrange itself round a single axis, or it may arrange
itself round many axes: the structure may be _uniaxial_ or _multiaxial_.
Each division of the organic kingdom furnishes examples of both these
orders. In such _Fungi_ as exhibit axial development at all, we commonly
see development round a single axis. Some of the _Algæ_, as the common
tangle, show us this arrangement. And of the higher plants, many
Monocotyledons and small Dicotyledons are uniaxial. Of animals, the
advanced are without exception in this category. There is no known
vertebrate in which the whole of the germ-product is not subordinated to a
single axis. In the _Arthropoda_, the like is universal; as it is also in
the superior orders of _Mollusca_. Multiaxial development occurs in most of
the plants we are familiar with--every branch of a shrub or tree being an
independent axis. But while in the vegetal kingdom multiaxial development
prevails among the highest types, in the animal kingdom it prevails only
among the lowest types. It is extremely general, if not universal, among
the _Coelenterata_; it is characteristic of the _Polyzoa_; the compound
Ascidians exhibit it; and it is seen, though under another form, in certain
of the inferior Annelids.
Development that is axial, like development that is central, may be either
continuous or discontinuous: the parts having different axes may continue
united, or they may separate. Instances of each alternative are supplied by
both plants and animals. Continuous multiaxial development is that which
plants usually display, and need not be illustrated further than by
reference to every garden. As cases of it in animals may be named all the
compound _Hydrozoa_ and _Actinozoa_; and such ascidian forms as the
_Botryllidæ_. Of multiaxial development that is discontinuous, a familiar
instance among plants exists in the common strawberry. This sends out over
the neighbouring surface, long slender shoots, bearing at their extremities
buds that presently strike roots and become new individuals; and these by
and by lose their connexions with the original axis. Other plants there are
that produce certain specialized buds called bulbils, which separating
themselves and falling to the ground, grow into independent plants. Among
animals the fresh-water polype very clearly shows this mode of development:
the young polypes, budding out from its surface, severally arrange their
parts around distinct axes, and eventually detaching themselves, lead
separate lives, and produce other polypes after the same fashion. By some
of the lower _Annelida_, this multiplication of axes from an original axis,
is carried on after a different manner: the string of segments
spontaneously divides; and after further growth, division recurs in one or
both of the halves. Moreover in the _Syllis ramosa_, there occurs lateral
branching also.
Grouping together its several modes as above delineated, we see that
{ Unicentral
{ Central { or { Continuous
{ { Multicentral { or
{ { Discontinuous
DEVELOPMENT is { or
{
{ { Uniaxial
{ Axial { or { Continuous
{ Multiaxial { or
{ Discontinuous
Any one well acquainted with the facts, may readily raise objections to
this arrangement. He may name forms which do not obviously come under any
of these heads. He may point to plants that are for a time multicentral but
afterwards develop axially. And from lower types of animals he may choose
many in which the continuous and discontinuous modes are both displayed.
But, as already hinted, an arrangement free from such anomalies must be
impossible, if the various kinds of organization have arisen by Evolution.
The one above sketched out is to be regarded as a rough grouping of the
facts, which helps us to a conception of them in their totality; and, so
regarded, it will be of service when we come to treat of Individuality and
Reproduction.
§ 51. From these most general external aspects of organic development, let
us now turn to its internal and more special aspects. When treating of
Evolution as a universal process of things, a rude outline of the course of
structural changes in organisms was given (_First Principles_, §§ 110, 119,
132). Here it will be proper to describe these changes more fully.
The bud of any common flowering plant in its earliest stage, consists of a
small hemispherical or sub-conical projection. While it increases most
rapidly at the apex, this presently develops on one side of its base, a
smaller projection of like general shape with itself. Here is the rudiment
of a leaf, which presently spreads more or less round the base of the
central hemisphere or main axis. At the same time that the central
hemisphere rises higher, this lateral prominence, also increasing, gives
rise to subordinate prominences or lobes. These are the rudiments of
stipules, where the leaves are stipulated. Meanwhile, towards the other
side of the main axis and somewhat higher up, another lateral prominence
arising marks the origin of a second leaf. By the time that the first leaf
has produced another pair of lobes, and the second leaf has produced its
primary pair, the central hemisphere, still increasing at its apex,
exhibits the rudiment of a third leaf. Similarly throughout. While the germ
of each succeeding leaf thus arises, the germs of the previous leaves, in
the order of their priority, are changing their rude nodulated shapes into
flattened-out expansions; which slowly put on those sharp outlines they
show when unfolded. Thus from that extremely indefinite figure, a rounded
lump, giving off from time to time lateral lumps, which severally becoming
symmetrically lobed gradually assume specific and involved forms, we pass
little by little to that comparatively complex thing--a leaf-bearing shoot.
Internally, a bud undergoes analogous changes; as witness this
account:--"The general mass of thin-walled parenchymatous cells which
occupies the apical region, and forms the _growing point_ of the shoot, is
covered by a single external layer of similar cells, which increase in
number by the formation of new walls in one direction only, perpendicular
to the surface of the shoot, and thus give rise only to the _epidermis_ or
single layer of cells covering the whole surface of the shoot. Meanwhile
the general mass below grows as a whole, its constituent cells dividing in
all directions. Of the new cells so formed, those removed by these
processes of growth and division from the actual apex, begin, at a greater
or less distance from it, to show signs of the differentiation which will
ultimately lead to the formation of the various tissues enclosed by the
epidermis of the shoot. First the pith, then the vascular bundles, and then
the cortex of the shoot, begin to take on their special characters."
Similarly with secondary structures, as the lateral buds whence leaves
arise. In the, at first, unorganized mass of cells constituting the
rudimentary leaf, there are formed vascular bundles which eventually become
the veins of the leaf; and _pari passu_ with these are formed the other
tissues of the leaf. Nor do we fail to find an essentially parallel set of
changes, when we trace the histories of the individual cells. While the
tissues they compose are separating, the cells are growing step by step
more unlike. Some become flat, some polyhedral, some cylindrical, some
prismatic, some spindle-shaped. These develop spiral thickenings in their
interiors; and those, reticulate thickenings. Here a number of cells unite
together to form a tube: and there they become almost solid by the internal
deposition of woody or other substance. Through such changes, too numerous
and involved to be here detailed, the originally uniform cells go on
diverging and rediverging until there are produced various forms that seem
to have very little in common.
The arm of a man makes its first appearance in as simple a way as does the
shoot of a plant. According to Bischoff, it buds-out from the side of the
embryo as a little tongue-shaped projection, presenting no differences of
parts; and it might serve for the rudiment of some one of the various other
organs that also arise as buds. Continuing to lengthen, it presently
becomes somewhat enlarged at its end; and is then described as a pedicle
bearing a flattened, round-edged lump. This lump is the representative of
the future hand, and the pedicle of the future arm. By and by, at the edges
of this flattened lump, there appear four clefts, dividing from each other
the buds of the future fingers; and the hand as a whole grows a little more
distinguishable from the arm. Up to this time the pedicle has remained one
continuous piece, but it now begins to show a bend at its centre, which
indicates the division into arm and forearm. The distinctions thus rudely
indicated gradually increase: the fingers elongate and become jointed, and
the proportions of all the parts, originally very unlike those of the
complete limb, slowly approximate to them. During its bud-like stage, the
rudimentary arm consists only of partially-differentiated tissues. By the
diverse changes these gradually undergo they are transformed into bones,
muscles, blood-vessels, and nerves. The extreme softness and delicacy of
these primary tissues, renders it difficult to trace the initial stages of
the differentiations. In consequence of the colour of their contents, the
blood-vessels are the first parts to become distinct. Afterwards the
cartilaginous parts, which are the bases of the future bones, become marked
out by the denser aggregation of their constituent cells, and by the
production between these of a hyaline substance which unites them into a
translucent mass. When first perceptible, the muscles are gelatinous, pale,
yellowish, transparent, and indistinguishable from their tendons. The
various other tissues of which the arm consists, beginning with very
faintly-marked differences, become day by day more definite in their
qualitative appearances. In like manner the units composing these tissues
severally assume increasingly-specific characters. The fibres of muscle, at
first made visible in the midst of their gelatinous matrix only by
immersion in alcohol, grow more numerous and distinct; and by and by they
begin to exhibit transverse stripes. The bone-cells put on by degrees their
curious structure of branching canals. And so in their respective ways with
the units of skin and the rest.
Thus in each of the organic sub-kingdoms, we see this change from an
incoherent, indefinite homogeneity to a coherent, definite heterogeneity,
illustrated in a quadruple way. The originally-like units called cells,
become unlike in various ways, and in ways more numerous and marked as the
development goes on. The several tissues which these several classes of
cells form by aggregation, grow little by little distinct from each other;
and little by little put on those structural complexities that arise from
differentiations among their component units. In the shoot, as in the limb,
the external form, originally very simple, and having much in common with
simple forms in general, gradually acquires an increasing complexity, and
an increasing unlikeness to other forms. Meanwhile, the remaining parts of
the organism to which the shoot or limb belongs, having been severally
assuming structures divergent from one another and from that of this
particular shoot or limb, there has arisen a greater heterogeneity in the
organism as a whole.
§ 52. One of the most remarkable inductions of embryology comes next in
order. And here we find illustrated the general truth that in mental
evolution as in bodily evolution the progress is from the indefinite and
inexact to the definite and exact. For the first statement of this
induction was but an adumbration of the correct statement.
As a result of his examinations von Baer alleged that in its earliest stage
every organism has the greatest number of characters in common with all
other organisms in their earliest stages; that at a stage somewhat later
its structure is like the structures displayed at corresponding phases by a
less extensive assemblage of organisms; that at each subsequent stage
traits are acquired which successively distinguish the developing embryo
from groups of embryos that it previously resembled--thus step by step
diminishing the group of embryos which it still resembles; and that thus
the class of similar forms is finally narrowed to the species of which it
is a member. This abstract proposition will perhaps not be fully
comprehended by the general reader. It will be best to re-state it in a
concrete shape. Supposing the germs of all kinds of organisms to be
simultaneously developing, we may say that all members of the vast
multitude take their first steps in the same direction; that at the second
step one-half of this vast multitude diverges from the other half, and
thereafter follows a different course of development; that the immense
assemblage contained in either of these divisions very soon again shows a
tendency to take two or more routes of development; that each of the two or
more minor assemblages thus resulting, shows for a time but small
divergences among its members, but presently again divides into groups
which separate ever more widely as they progress; and so on until each
organism, when nearly complete, is accompanied in its further modifications
only by organisms of the same species; and last of all, assumes the
peculiarities which distinguish it as an individual--diverges to a slight
extent to the organisms it is most like.
But, as above said, this statement is only an adumbration. The order of
Nature is habitually more complex than our generalizations represent it as
being--refuses to be fully expressed in simple formulæ; and we are obliged
to limit them by various qualifications. It is thus here. Since von Baer's
day the careful observations of numerous observers have shown his
allegation to be but approximately true. Hereafter, when discussing the
embryological evidence of Evolution, the causes of deviations will be
discussed. For the present it suffices to recognize as unquestionable the
fact that whereas the germs of organisms are extremely similar, they
gradually diverge widely, in modes now regular and now irregular, until in
place of a multitude of forms practically alike we finally have a multitude
of forms most of which are extremely unlike. Thus, in conformity with the
law of evolution, not only do the parts of each organism advance from
indefinite homogeneity to definite heterogeneity, but the assemblage of all
organisms does the same: a truth already indicated in _First Principles_.
§ 53. This comparison between the course of development, in any creature,
and the course of development in all other creatures--this arrival at the
conclusion that the course of development in each, at first the same as in
all others, becomes stage by stage differentiated from the courses in all
others, brings us within view of an allied conclusion. If we contemplate
the successive stages passed through by any higher organism, and observe
the relation between it and its environment at each of these stages; we
shall see that this relation is modified in a way analogous to that in
which the relation between the organism and its environment is modified, as
we advance from the lowest to the highest grades. Along with the
progressing differentiation of each organism from others, we find a
progressing differentiation of it from its environment; like that
progressing differentiation from the environment which we meet with in the
ascending forms of life. Let us first glance at the way in which the
ascending forms of life exhibit this progressing differentiation from the
environment.
In the first place, it is illustrated in _structure_. Advance from the
homogeneous to the heterogeneous, itself involves an increasing distinction
from the inorganic world. Passing over the _Protozoa_, of which the
simplest probably disappeared during the earliest stages of organic
evolution, and limiting our comparison to the _Metazoa_, we see that low
types of these, as the _Coelenterata_, are relatively simple in their
organization; and the ascent to organisms of greater and greater complexity
of structure, is an ascent to organisms which are in that respect more
strongly contrasted with the structureless environment. In _form_, again,
we see the same truth. An ordinary characteristic of inorganic matter is
its indefiniteness of form; and this is also a characteristic of the lower
organisms, as compared with the higher. Speaking generally, plants are less
definite than animals, both in shape and size--admit of greater
modifications from variations of position and nutrition. Among animals, the
simplest Rhizopods may almost be called amorphous: the form is never
specific, and is constantly changing. Of the organisms resulting from the
aggregation of such creatures, we see that while some, as the
_Foraminifera_, assume a certain definiteness of form, in their shells at
least, others, as the Sponges, are very irregular. The Zoophytes and the
_Polyzoa_ are compound organisms, most of which have a mode of growth not
more determinate than that of plants. But among the higher animals, we find
not only that the mature shape of each species is very definite, but that
the individuals of each species differ little in size. A parallel increase
of contrast is seen in _chemical composition_. With but few exceptions, and
those only partial ones, the lowest animal and vegetal forms are
inhabitants of the water; and water is almost their sole constituent.
Desiccated _Protophyta_ and _Protozoa_ shrink into mere dust; and among the
Acalephes we find but a few grains of solid matter to a pound of water. The
higher aquatic plants, in common with the higher aquatic animals,
possessing as they do increased tenacity of substance, also contain a
greater proportion of the organic elements; further they show us a greater
variety of composition in their different parts; and thus in both ways are
chemically more unlike their medium. And when we pass to the superior
classes of organisms--land-plants and land-animals--we see that, chemically
considered, they have little in common either with the earth on which they
stand or the air which surrounds them. In _specific gravity_ too, we may
note a like truth. The simplest forms, in common with the spores and
gemmules of higher ones, are as nearly as may be of the same specific
gravity as the water in which they float; and though it cannot be said that
among aquatic creatures, superior specific gravity is a standard of general
superiority, yet we may fairly say that the higher orders of them, when
divested of the appliances by which their specific gravity is regulated,
differ more from water in their relative weights than do the lowest. In
terrestrial organisms, the contrast becomes marked. Trees and plants, in
common with insects, reptiles, mammals, birds, are all of a specific
gravity considerably less than that of the earth and immensely greater than
that of the air. Yet further, we see the law fulfilled in respect of
_temperature_. Plants generate but extremely small quantities of heat,
which are to be detected only by delicate experiments; and practically they
may be considered as having the same temperature as their environment. The
temperature of aquatic animals is very little above that of the surrounding
water: that of the invertebrata being mostly less than a degree above it,
and that of fishes not exceeding it by more than two or three degrees; save
in the case of some large red-blooded fishes, as the tunny, which exceed it
in temperature by nearly ten degrees. Among insects the range is from two
to ten degrees above that of the air: the excess varying according to their
activity. The heat of reptiles is from four to fifteen degrees more than
the heat of their medium. While mammals and birds maintain a heat which
continues almost unaffected by external variations, and is often greater
than that of the air by seventy, eighty, ninety, and even a hundred
degrees. Once more, in greater _self-mobility_ a progressive
differentiation is traceable. The chief characteristic by which we
distinguish dead matter is its inertness: some form of independent motion
is our most familiar proof of life. Passing over the indefinite border-land
between the animal and vegetal kingdoms, we may roughly class plants as
organisms which, while they exhibit that kind of motion implied in growth,
are not only devoid of locomotive power, but with some unimportant
exceptions are devoid of the power of moving their parts in relation to
each other; and thus are less differentiated from the inorganic world than
animals. Though in those microscopic _Protophyta_ and _Protozoa_ inhabiting
the water we see locomotion produced by ciliary action; yet this
locomotion, while rapid relatively to the sizes of their bodies, is
absolutely slow. Of the _Coelenterata_ a great part are either permanently
rooted or habitually stationary; and so have scarcely any self-mobility but
that implied in the relative movements of parts; while the rest, of which
the common jelly-fish serves as a sample, have mostly but little ability to
move themselves through the water. Among the higher aquatic
_Invertebrata_,--cuttlefishes and lobsters, for instance,--there is a very
considerable power of locomotion; and the aquatic _Vertebrata_ are,
considered as a class, much more active in their movements than the other
inhabitants of the water. But it is only when we come to air-breathing
creatures that we find the vital characteristics of self-mobility
manifested in the highest degree. Flying insects, mammals, birds, travel
with velocities far exceeding those attained by any of the lower classes of
animals. Thus, on contemplating the various grades of organisms in their
ascending order, we find them more and more distinguished from their
inanimate media, in _structure_, in _form_, in _chemical composition_, in
_specific gravity_, in _temperature_, in _self-mobility_. It is true that
this generalization does not hold with complete regularity. Organisms which
are in some respects the most strongly contrasted with the environing
inorganic world, are in other respects less contrasted than inferior
organisms. As a class, mammals are higher than birds; and yet they are of
lower temperature and have smaller powers of locomotion. The stationary
oyster is of higher organization than the free-swimming medusa; and the
cold-blooded and less heterogeneous fish is quicker in its movements than
the warm-blooded and more heterogeneous sloth. But the admission that the
several aspects under which this increasing contrast shows itself, bear
variable ratios to each other, does not conflict with the general truth
that as we ascend in the hierarchy of organisms, we meet with not only an
increasing differentiation of parts but also an increasing differentiation
from the surrounding medium in sundry other physical attributes. It would
seem that this trait has some necessary connexion with superior vital
manifestations. One of those lowly gelatinous forms, so transparent and
colourless as to be with difficulty distinguished from the water it floats
in, is not more like its medium in chemical, mechanical, optical, thermal,
and other properties, than it is in the passivity with which it submits to
all the influences and actions brought to bear upon it; while the mammal
does not more widely differ from inanimate things in these properties, than
it does in the activity with which it meets surrounding changes by
compensating changes in itself. And between these extremes, these two kinds
of contrast vary together. So that in proportion as an organism is
physically like its environment it remains a passive partaker of the
changes going on in its environment; while in proportion as it is endowed
with powers of counteracting such changes, it exhibits greater unlikeness
to its environment.[20]
If now, from this same point of view, we consider the relation borne to its
environment by any superior organism in its successive stages, we find an
analogous series of contrasts. Of course in respect of degrees of
_structure_ the parallelism is complete. The difference, at first small,
between the little-structured germ and the little-structured inorganic
world, necessarily becomes greater, step by step, as the differentiations
of the germ become more numerous and definite. How of _form_ the like
holds is equally manifest. The sphere, which is the point of departure
common to all organisms, is the most generalized of figures; and one that
is, under various circumstances, assumed by inorganic matter. But as it
develops it loses all likeness to inorganic objects in the environment; and
eventually becomes distinct even from nearly all organic objects in its
environment. In _specific gravity_ the alteration, though not very marked,
is still in the same direction. Development being habitually accompanied by
a relative decrease in the quantity of water and an increase in the
quantity of constituents that are heavier than water, there results a small
augmentation of relative weight. In power of maintaining a _temperature_
above that of surrounding things, the differentiation from the environment
that accompanies development is marked. All ova are absolutely dependent
for their heat on external sources. The mammalian young one is, during its
uterine life, dependent on the maternal heat; and at birth has but a
partial power of making good the loss by radiation. But as it advances in
development it gains an ability to maintain a constant temperature above
that of surrounding things: so becoming markedly unlike them. Lastly, in
_self-mobility_ this increasing contrast is no less decided. Save in a few
aberrant tribes, chiefly parasitic, we find the general fact to be that the
locomotive power, totally absent or very small at the outset, increases
with the advance towards maturity. The more highly developed the organism
becomes, the stronger grows the contrast between its activity and the
inertness of the objects amid which it moves.
Thus we may say that the development of an individual organism, is at the
same time a differentiation of its parts from each other, and a
differentiation of the consolidated whole from the environment; and that in
the last as in the first respect, there is a general analogy between the
progression of an individual organism and the progression from the lowest
orders of organisms to the highest orders. It may be remarked that some
kinship seems to exist between these generalizations and the doctrine of
Schelling, that Life is the tendency to individuation. For evidently, in
becoming more distinct from one another and from their environment,
organisms acquire more marked individualities. As far as I can gather from
outlines of his philosophy, however, Schelling entertained this conception
in a general and transcendental sense, rather than in a special and
scientific one.
§ 54. Deductive interpretations of these general facts of development, in
so far as they are possible, must be postponed until we arrive at the
fourth and fifth divisions of this work. There are, however, one or two
general aspects of these inductions which may be here conveniently dealt
with deductively.
Grant that each organism is at the outset relatively homogeneous and that
when complete it is relatively heterogeneous, and it necessarily follows
that development is a change from the homogeneous to the heterogeneous--a
change during which there must be gone through all the gradations of
heterogeneity that lie between these extremes. If, again, there is at first
indefiniteness and at last definiteness, the transition cannot but be from
the one to the other of these through all intermediate degrees of
definiteness. Further, if the parts, originally incoherent or uncombined,
eventually become relatively coherent or combined, there must be a
continuous increase of coherence or combination. Hence the general truth
that development is a change from incoherent, indefinite homogeneity, to
coherent, definite heterogeneity, becomes a self-evident one when
observation has shown us the state in which organisms begin and the state
in which they end.
Just in the same way that the growth of an entire organism is carried on by
abstracting from the environment substances like those composing the
organism; so the production of each organ within the organism is carried on
by abstracting from the substances contained in the organism, those
required by this particular organ. Each organ at the expense of the
organism as a whole, integrates with itself certain kinds and proportions
of the matters circulating around it; in the same way that the organism as
a whole, integrates with itself certain kinds and proportions of matters at
the expense of the environment as a whole. So that the organs are
qualitatively differentiated from each other, in a way analogous to that by
which the entire organism is qualitatively differentiated from things
around it. Evidently this selective assimilation illustrates the general
truth, set forth and illustrated in _First Principles_, that like units
tend to segregate. It illustrates, moreover, the further aspect of this
general truth, that the pre-existence of a mass of certain units produces a
tendency for diffused units of the same kind to aggregate with this mass
rather than elsewhere. It has been shown of particular salts, A and B,
co-existing in a solution not sufficiently concentrated to crystallize,
that if a crystal of the salt A be put into the solution, it will increase
by uniting with itself the dissolved atoms of the salt A; and that
similarly, though there otherwise takes place no deposition of the salt B,
yet if a crystal of the salt B is placed in the solution, it will exercise
a coercive force on the diffused atoms of this salt, and grow at their
expense. Probably much organic assimilation occurs in the same way.
Particular parts of the organism are composed of special units or have the
function of secreting special units, which are ever present in them in
large quantities. The fluids circulating through the body contain special
units of this same order. And these diffused units are continually being
deposited along with the groups of like units that already exist. How
purely physical are the causes of this selective assimilation, is, indeed,
shown by the fact that abnormal constituents of the blood are segregated in
the same way. The chalky deposits of gout beginning at certain points,
collect more and more around those points. And similarly in numerous
pustular diseases. Where the component units of an organ, or some of them,
do not exist as such in the circulating fluids, but are formed out of
elements or compounds that exist separately in the circulating fluids, the
process of differential assimilation must be of a more complex kind. Still,
however, it seems not impossible that it is carried on in an analogous way.
If there be an aggregate of compound atoms, each of which contains the
constituents A, B, C; and if round this aggregate the constituents A and B
and C are diffused in uncombined states; it may be suspected that the
coercive force of these aggregated compound atoms A, B, C, may not only
bring into union with themselves adjacent compound atoms A, B, C, but may
cause the adjacent constituents A and B and C to unite into such compound
atoms, and then aggregate with the mass.
CHAPTER II^A.
STRUCTURE.[21]
§ 54a. As, in the course of evolution, we rise from the smallest to the
largest aggregates by a process of integration, so do we rise by a process
of differentiation from the simplest to the most complex aggregates. The
initial types of life are at once extremely small and almost structureless.
Passing over those which swarm in the air, the water, and the soil, and are
now some of them found to be causes of diseases, we may set out with those
ordinarily called _Protozoa_ and _Protophyta_: the lowest of which,
however, are either at once plants and animals, or are now one and now the
other.
That the first living things were minute portions of simple protoplasm is
implied by the general theory of Evolution; but we have no evidence that
such portions exist now. Even admitting that there are protoplasts (using
this word to include plant and animal types) which are without nuclei,
still they are not homogeneous--they are granular. Whether a nucleus is
always present is a question still undecided; but in any case the types
from which it is absent are extremely exceptional. Thus the most general
structural traits of protoplasts are--the possession of an internal part,
morphologically central though often not centrally situated, a general mass
of protoplasm surrounding it, and an inclosing differentiated portion in
contact with the environment. These essential elements are severally
subject to various complications.
In some simple types the limiting layer or cortical substance can scarcely
be said to exist as a separate element. The exoplasm, distinguished from
the endoplasm by absence or paucity of granules, is continually changing
places with it by the sending out of pseudopodia which are presently drawn
back into the general mass: the inner and outer, being unsettled in
position, are not permanently differentiated. Then we have types,
exemplified by _Lithamoeba_, constituted of protoplasm covered by a
distinct pellicle, which in sundry groups becomes an outer shell of various
structure: now jelly-like, now of cellulose, now siliceous or calcareous.
While here this envelope has a single opening, there it is perforated all
over--a fenestrated shell. In some cases an external layer is formed of
agglutinated sand-particles; in others of imbricated plates, as in
Coccospheres; and in many others radiating spicules stand out on all sides.
Throughout sundry classes the exoplasm develops cilia, by the wavings of
which the creatures are propelled through the water--cilia which may be
either general or local. And then this cortical layer, instead of being
spherical or spheroidal, may become plano-spiral, cyclical, crosier-shaped,
and often many-chambered; whence there is a transition to colonies.
Meanwhile the inclosed protoplasm, at first little more than a network or
foamwork containing granules and made irregular by objects drawn in as
nutriment, becomes variously complicated. In some low types its continuity
is broken by motionless, vacant spaces, but in higher types there are
contractile vacuoles slowly pulsing, and, as we may suppose, moving the
contained liquid hither and thither; while there are types having many
passive vacuoles along with a few active ones. In some varieties the
protruded parts, or pseudopodia, into which the protoplasm continually
shapes itself, are comparatively short and club-shaped; in others they are
long and fine filaments which anastomose, so forming a network running here
and there into little pools of protoplasm. Then there are kinds in which
the protoplasm streams up and down the protruding spicules: sometimes
inside of them, sometimes outside. Always, too, there is included in the
protoplasm a small body known as a centrosome.
Lastly, we have the innermost element, considered the essential
element--the nucleus. According to Prof. Lankester, it is absent from
_Archerina_, and there are types in which it is made visible only by the
aid of special reagents. Ordinarily it is marked off from the surrounding
protoplasm by a delicate membrane, just as the protoplasm itself is marked
off by the exoplasm from the environment. Most commonly there is a single
nucleus, but occasionally there are many, and sometimes there is a chief
one with minor ones. Moreover, within the nucleus itself there have of late
years been discovered remarkable structural elements which undergo
complicated changes.
These brief statements indicate only the most general traits of an immense
variety of structures--so immense a variety that Prof. Lankester, in
distinguishing the classes, sub-classes, orders, and genera in the briefest
way, occupies 37 quarto pages of small type. And to give a corresponding
account of _Protophyta_ would require probably something like equal space.
Thus these living things, so minute that unaided vision fails to disclose
them, constitute a world exhibiting varieties of structure which it
requires the devotion of a life to become fully acquainted with.
§ 54b. If higher forms of life have arisen from lower forms by evolution,
the implication is that there must once have existed, if there do not still
exist, transitional forms; and there follows the comment that there _do_
still exist transitional forms. Both in the plant-world and in the
animal-world there are types in which we see little more than simple
assemblages of _Protophyta_ or of _Protozoa_--types in which the units,
though coherent, are not differentiated but constitute a uniform mass. In
treating of structure we are not here concerned with these unstructured
types, but may pass on to those aggregates of protoplasts which show us
differentiated parts--_Metaphyta_ and _Metazoa_: economizing space by
limiting our attention chiefly to the last.
When, half a century ago, some currency was given to the statement that all
kinds of organisms, plant and animal, which our unaided eyes disclose, are
severally composed of myriads of living units, some of them partially, if
not completely, independent, and that thus a man is a vast nation of minute
individuals of which some are relatively passive and others relatively
active, the statement met, here with incredulity and there with a shudder.
But what was then thought a preposterous assertion has now come to be an
accepted truth.
Along with gradual establishment of this truth has gone gradual
modification in the form under which it was originally asserted. If some
inhabitant of another sphere were to describe one of our towns as composed
exclusively of houses, saying nothing of the contained beings who had built
them and lived in them, we should say that he had made a profound error in
recognizing only the inanimate elements of the town and disregarding the
animate elements. Early histologists made an analogous error. Plants and
animals were found to consist of minute members, each of which appeared to
be simply a wall inclosing a cavity--a cell. But further investigation
proved that the content of the cell, presently distinguished as protoplasm,
is its essential living part, and that the cell-wall, when present, is
produced by it. Thus the unit of composition is a protoplast, usually
enclosed, with its contained nucleus and centrosome.
§ 54c. As above implied, the individualities of the units are not wholly
lost in the individuality of the aggregate, but continue, some of them, to
be displayed in various degrees: the great majority of them losing their
individualities more and more as the type of the aggregate becomes higher.
In a slightly organized Metazoon like the sponge, the subordination is but
small. Only those members of the aggregate which, flattened and united
together, form the outer layer and those which become metamorphosed into
spicules, have entirely lost their original activities. Of the rest nearly
all, lining the channels which permeate the mass, and driving onwards the
contained sea-water by the motions of their whip-like appendages,
substantially retain their separate lives; and beyond these there exist in
the gelatinous substance lying between the inner and outer layers, which is
regarded as homologous with a mesoderm, amoeba-form protoplasts which move
about from place to place.
Relations between the aggregate and the units which are in this case
permanent, are in other cases temporary: characterizing early stages of
embryonic development. For example, drawings of Echinoderm larvæ at an
early stage, show us the potential independence of all the cells forming
the blastosphere; for in the course of further development some of these
resume the primitive amoeboid state, migrate through the internal space,
and presently unite to form certain parts of the growing structures. But
with the progress of organization independence of this kind diminishes.
Converse facts are presented after development has been completed; for with
the commencement of reproduction we everywhere see more or less resumption
of individual life among the units, or some of them. It is a trait of
transitional types between _Protozoa_ and _Metazoa_ to lead an aggregate
life as a plasmodium, and then for this to break up into its members, which
for a time lead individual lives as generative agents; and sundry low kinds
of plants possessing small amounts of structure, have generative
elements--zoospores and spermatozoids--which show us a return to unit life.
Nor, indeed, are we shown this only in the lowest plants; for it has
recently been found that in certain of the higher plants--even in
Phænogams--spermatozoids are produced. That is to say, the units resume
active lives at places where the controlling influence of the aggregate is
failing; for, as we shall hereafter see, places at which generation
commences answer to this description.
These different kinds of evidence jointly imply that the individual lives
of the units are subordinate to the general life in proportion as this is
high. Where the organism is very inferior in type the unit-life remains
permanently conspicuous. In some superior types there is a display of
unit-life during embryonic stages in which the co-ordinating action of the
aggregate is but incipient. With the advance of development the unit-life
diminishes; but still, in plants, recommences where the disintegrating
process which initiates generation shows the coercive power of the
organization to have become small.
Even in the highest types, however, and even when they are fully developed,
unit-life does not wholly disappear: it is clearly shown in ourselves. I do
not refer simply to the fact that, as throughout the animal kingdom at
large and a considerable part of the vegetal kingdom, the male generative
elements are units which have resumed the primitive independent life, but I
refer to a much more general fact. In that part of the organism which,
being fundamentally an aqueous medium, is in so far like the aqueous medium
in which ordinary protozoon life is carried on, we find an essentially
protozoon life. I refer of course to the blood. Whether the tendency of the
red corpuscles (which are originally developed from amoeba-like cells) to
aggregate into _rouleaux_ is to be taken as showing life in them, may be
left an open question. It suffices that the white corpuscles or leucocytes,
retaining the primitive amoeboid character, exhibit individual activities:
send out prolongations like pseudopodia, take in organic particles as food,
and are independently locomotive. Though far less numerous than the red
corpuscles, yet, as ten thousand are contained in a cubic millimetre of
blood--a mass less than a pin's head--it results that the human body is
pervaded throughout all its blood-vessels by billions of these separately
living units. In the lymph, too, which also fulfils the requirements of
liquidity, these amoeboid units are found. Then we have the curious
transitional stage in which units partially imbedded and partially free
display a partial unit-life. These are the ciliated epithelium-cells,
lining the air-passages and covering sundry of the mucous membranes which
have more remote connexions with the environment, and covering also the
lining membranes of certain main canals and chambers in the nervous system.
The inner parts of these unite with their fellows to form an epithelium,
and the outer parts of them, immersed either in liquid or semi-liquid
(mucus), bear cilia that are in constant motion and "produce a current of
fluid over the surface they cover:" thus simulating in their positions and
actions the cells lining the passages ramifying through a sponge. The
partially independent lives of these units is further seen in the fact that
after being detached they swim about in water for a time by the aid of
their cilia.
§ 54d. But in the _Metazoa_ and _Metaphyta_ at large, the associated units
are, with the exceptions just indicated, completely subordinated. The
unit-life is so far lost in the aggregate life that neither locomotion nor
the relative motion of parts remains; and neither in shape nor composition
is there resemblance to protozoa. Though in many cases the internal
protoplasm continues to carry on vital processes subserving the needs of
the aggregate, in others vital processes of an independent kind appear to
cease.
It will naturally be supposed that after recognizing this fundamental trait
common to all types of organisms above the _Protozoa_ and _Protophyta_, the
next step in an account of structure must be a description of their organs,
variously formed and combined--if not in detail yet in their general
characters. This, however, is an error. There are certain truths of
structure higher in generality than any which can be alleged of organs. We
shall see this if we compare organs with one another.
Here is a finger stiffened by its small bones and yet made flexible by the
uniting joints. There is a femur which helps its fellow to support the
weight of the body; and there again is a rib which, along with others,
forms a protective box for certain of the viscera. Dissection reveals a set
of muscles serving to straighten and bend the fingers, certain other
muscles that move the legs, and some inconspicuous muscles which,
contracting every two or three seconds, slightly raise the ribs and aid in
inflating the lungs. That is to say, fingers, legs, and chest possess
certain structures in common. There is in each case a dense substance
capable of resisting stress and a contractile substance capable of moving
the dense substance to which it is attached. Hence, then, we have first to
give an account of these and other chief elements which, variously joined
together, form the different organs: we have to observe the general
characters of _tissues_.
On going back to the time when the organism begins with a single cell, then
becomes a spherical cluster of cells, and then exhibits differences in the
modes of aggregation of these cells, the first conspicuous rise of
structure (limiting ourselves to animals) is the formation of three layers.
Of these the first is, at the outset and always, the superficial layer in
direct contact with the environment. The second, being originally a part of
the first, is also in primitive types in contact with the environment, but,
being presently introverted, forms the rudiment of the food-cavity; or,
otherwise arising in higher types, is in contact with the yelk or food
provided by the parent. And the third, presently formed between these two,
consists at the outset of cells derived from them imbedded in an
intercellular substance of jelly-like consistence. Hence originate the
great groups classed as epithelium-tissue, connective tissue (including
osseous tissue), muscular tissue, nervous tissue. These severally contain
sub-kinds, each of which is a complex of differentiated cells. Being brief,
and therefore fitted for the present purposes, the sub-classification given
by Prof. R. Hertwig may here be quoted;--
"The physiological character of epithelia is given in the fact that they
cover the surfaces of the body, their morphological character in that
they consist of closely compressed cells united only by a cementing
substance.
"According to their further functional character epithelia are divided
into glandular epithelia (unicellular and multicellular glands), sensory,
germinal, and pavement epithelia.
"According to the structure are distinguished one-layered (cubical,
cylindrical, pavement epithelia) and many-layered epithelia, ciliated and
flagellated epithelia, epithelia with or without cuticle.
"The physiological character of the connective tissues rests upon the
fact that they fill up spaces between other tissues in the interior of
the body.
"The morphological character depends upon the presence of the
intercellular substance.
"According to the quantity and the structure of the intercellular
substance the connective substances are divided into (1) cellular (with
little intercellular substance); (2) homogeneous; (3) fibrillar
connective tissue; (4) cartilage; (5) bone.
"The physiological character of muscular tissue is contained in the
increased capacity for contraction.
"The morphological character is found in the fact that the cells have
secreted muscle-substance.
"According to the nature of the muscle-substance are distinguished smooth
and cross-striated muscle-fibres.
"According to the character and derivation of the cells
(muscle-corpuscles) the musculature is divided into epithelial
(epithelial muscle-cells, primary bundles) and connective-tissue muscle
cells (contractile fibre-cells).
"The physiological character of nervous tissue rests upon the
transmission of sensory stimuli and voluntary impulses, and upon the
co-ordination of these into unified psychic activity.
"The conduction takes place by means of nerve-fibres (non-medullated and
medullated fibrils and bundles of fibrils); the co-ordination of stimuli
by means of ganglion-cells (bipolar, multipolar ganglion-cells)."
(_General Principles of Zoology_, pp. 117-8.)
But now concerning cells out of which, variously modified, obscured, and
sometimes obliterated, tissues are formed, we have to note a fact of much
significance. Along with the cell-doctrine as at first held, when attention
was given to the cell itself rather than to its contents, there went the
belief that each of these morphological units is structurally separate from
its neighbours. But since establishment of the modern view that the
essential element is the contained protoplasm, histologists have discovered
that there are protoplasmic connexions between the contents of adjacent
cells. Though cursorily observed at earlier dates, it was not until some
twenty years ago that in plant-tissues these were clearly shown to pass
through openings in the cell-walls. It is said that in some cases the
openings are made, and the junctions established, by a secondary process;
but the implication is that usually these living links are left between
multiplying protoplasts; so that from the outset the protoplasm pervading
the whole plant maintains its continuity. More recently sundry zoologists
have alleged that a like continuity exists in animals. Especially has this
been maintained by Mr. Adam Sedgwick. Numerous observations made on
developing ova of fishes have led him to assert that in no case do the
multiplying cells so-called--blastomeres and their progeny--become entirely
separate. Their fission is in all cases incomplete. A like continuity has
been found in the embryos of many Arthropods, and more recently in the
segmenting eggs and blastulæ of Echinoderms. The _syncytium_ thus formed is
held by Mr. Sedgwick to be maintained in adult life, and in this belief he
is in agreement with sundry others. Bridges of protoplasm have been seen
between epithelium-cells, and it is maintained that cartilage-cells,
connective tissue cells, the cells forming muscle-fibres, as well as
nerve-cells, have protoplasmic unions. Nay, some even assert that an ovum
preserves a protoplasmic connexion with the matrix in which it develops.
A corollary of great significance may here be drawn. It has been observed
that within a vegetal cell the strands of protoplasm stretched in this or
that direction contain moving granules, showing that the strands carry
currents. It has also been observed that when the fission of a protozoon is
so nearly complete that its two halves remain connected only by a thread,
currents of protoplasm move through this thread, now one way now the other.
The inference fairly to be drawn is that such currents pass also through
the strands which unite the protoplasts forming a tissue. What must happen?
So long as adjacent cells with their contents are subject to equal
pressures no tendency to redistribution of the protoplasm exists, and there
may then occur the action sometimes observed inside the strands within a
cell: currents with their contained granules moving in opposite directions.
But if the cells forming a portion of tissue are subject to greater
pressure than the cells around, their contained protoplasm must be forced
through the connecting threads into these surrounding cells. Every change
of pressure at every point must cause movements and counter-movements of
this kind. Now in the _Metazoa_ at large, or at least in all exhibiting
relative motions of parts, and especially in all which are capable of rapid
locomotion, such changes of pressure are everywhere and always taking
place. The contraction of a muscle, besides compressing its components,
compresses neighbouring tissues; and every instant contractions and
relaxations of muscles go on throughout the limbs and body during active
exertion. Moreover, each attitude--standing, sitting, lying down, turning
over--entails a different set of pressures, both of the parts on one
another and on the ground; and those partial arrests of motion which result
from sitting down the feet alternately when running, send jolts or waves of
varying pressure through the body. The vital actions, too, have kindred
effects. An inspiration alters the stress on the tissues throughout a
considerable part of the trunk, and a heart-beat propels, down to the
smallest arteries, waves which slightly strain the tissues at large. The
component cells, thus subject to mechanical disturbances, small and great,
perpetual and occasional, are ever having protoplasm forced into them and
forced out of them. There are gurgitations and regurgitations which, if
they do not constitute a circulation properly so called, at least imply an
unceasing redistribution. And the implication is that in the course of
days, weeks, months, years, each portion of protoplasm visits every part of
the body.
Without here stating specifically the bearings of these inferences upon the
problems of heredity, it will be manifest that certain difficulties they
present are in a considerable degree diminished.
§ 54e. Returning from this parenthetical discussion to the subject of
structure, we have to observe that besides facts presented by tissues and
facts presented by organs, there are certain facts, less general than the
one and more general than the other, which must now be noted. In the order
of decreasing generality an account of organs should be preceded by an
account of systems of organs. Some of these, as the muscular system and the
osseous system, are co-extensive with tissues, but others of them are not.
The nervous system, for example, contains more than one kind of tissue and
is constituted of many different structures: besides afferent and efferent
nerves there are the ganglia immediately controlling the viscera, and there
are the spinal and cerebral masses, the last of which is divisible into
numerous unlike parts. Then we have the vascular system made up of the
heart, arteries, veins, and capillaries. The lymphatic system, too, with
its scattered glands and ramifying channels has to be named. And then, not
forgetting the respiratory system with its ancillary appliances, we have
the highly heterogeneous alimentary system; including a great number of
variously-constructed organs which work together. On contemplating these
systems we see their common character to be that while as wholes they
cooperate for the carrying on of the total life, each of them consists of
cooperative parts: there is cooperation within cooperation.
There is another general aspect under which structures must be
contemplated. They are divisible into the universal and the
particular--those which are everywhere present and those which occupy
special places. The blood which a scratch brings out shows us that the
vascular system sends branches into each spot. The sensation accompanying a
scratch proves that the nervous system, too, has there some of its ultimate
fibrils. Unobtrusive, and yet to be found at every point, are the ducts of
the lymphatic system. And in all parts exists the connective tissue--an
inert tough substance which, running through interspaces, wraps up and
binds together the other tissues. As is implied by this description, these
structures stand in contrast with local structures. Here is a bone, there
is a muscle, in this place a gland, in that a sense-organ. Each has a
limited extent and a particular duty. But through every one of them ramify
branches of these universal structures. Every one of them has its arteries
and veins and capillaries, its nerves, its lymphatics, its connective
tissue.
Recognition of this truth introduces what little has here to be said
concerning organs; for of course in a work limited to principles no
detailed account of these can be entered upon. This remainder truth is
that, different as they may be in the rest of their structures, all organs
are alike in certain of their structures. All are furnished with these
appliances for nutrition, depuration and excitation: they have all to be
sustained, all to be stimulated, all to be kept clean. It has finally to be
remarked that the general structures which pervade all the special
structures at the same time pervade one another. The universal nervous
system has everywhere ramifying through it the universal vascular system
which feeds it; and the universal vascular system is followed throughout
all its ramifications by special nerves which control it. The lymphatics
forming a drainage-system run throughout the other systems; and in each of
these universal systems is present the connective tissue holding their
parts in position.
§ 54f. So vast and varied a subject as organic structure, even though the
treatment of it is limited to the enunciation of principles, cannot, of
course, be dealt with in the space here assigned. Next to nothing has been
said about plant-structures, and in setting forth the leading traits of
animal-structures the illustrations given have been mostly taken from
highly-developed creatures. In large measure adumbration rather than
exposition is the descriptive word to be applied.
Nevertheless the reader may carry away certain truths which, exemplified in
a few cases, are exemplified more or less fully in all cases. There is the
fundamental fact that the plants and animals with which we are
familiar--_Metaphyta_ and _Metazoa_--are formed by the aggregation of units
homologous with _Protozoa_. These units, often conspicuously showing their
homology in early embryonic stages, continue some of them to show it
throughout the lives of the highest type of _Metazoa_, which contain
billions of units carrying on a protozoon life. Of the protoplasts not thus
active the great mass, comparatively little transformed in low organisms,
become more and more transformed as the ascent to high organisms goes on;
so that, undergoing numerous kinds of metamorphoses, they lose all likeness
to their free homologues, both in shape and composition. The cell-contained
protoplasts thus variously changed are fused together into tissues in which
their individualities are practically lost; but they nevertheless remain
connected throughout by permeable strands of protoplasm. Arising by
complication of the outer and inner layers of the embryo and growing more
unlike as their units become more obscured, these tissues are formed into
systems, which develop into sets of organs. Some of the resulting
structures are localized and special but others are everywhere interfused.
While the first named of these facts are displayed in every _Metazoon_, and
while the last named are visible only in _Metazoa_ of considerably
developed structures, a gradual transition is shown in intermediate kinds
of _Metazoa_. Of this transition it remains to say that it is effected by
the progressive development of auxiliary appliances. For example, the
primitive foot-cavity is a sac with one opening only; then comes a second
opening through which the waste-matter of the food is expelled. The
alimentary canal between these openings is at first practically uniform;
afterwards in a certain part of its wall arise numerous bile-cells; these
accumulating form a hollow prominence; and this, enlarging, becomes in
higher types a liver, while the hollow becomes its duct. In other gradual
ways are formed other appended glands. Meanwhile the canal itself has its
parts differentiated: one being limited to swallowing, another to
triturating, another to adding various solvents, another to absorbing the
prepared nutriment, another to ejecting the residue. Take again the visual
organ. The earliest form of it is a mere pigment-speck below the surface.
From this (saying nothing here of multiple eyes) we rise by successive
complications to a retina formed of multitudinous sensory elements, lenses
for throwing images upon it, a curtain for shutting out more or less light,
muscles for moving the apparatus about, others for adjusting its focus;
and, finally, added to these, either a nictitating membrane or eyelids for
perpetually wiping its surface, and a set of eyelashes giving notice when a
foreign body is dangerously near. This process of elaborating organs so as
to meet additional requirements by additional parts, is the process pursued
throughout the body at large.
Of plant-structures, concerning which so little has been said, it may here
be remarked that their relative simplicity is due to the simplicity of
their relations to food. The food of plants is universally distributed,
while that of animals is dispersed. The immediate consequences are that in
the one case motion and locomotion are superfluous, while in the other case
they are necessary: the differences in the degrees of structure being
consequences. Recognizing the locomotive powers of minute _Algæ_ and the
motions of such other _Algæ_ as _Oscillatoria_, as well as those movements
of leaves and fructifying organs seen in some Phænogams, we may say,
generally, that plants are motionless; but that they can nevertheless carry
on their lives because they are bathed by the required nutriment in the air
and in the soil. Contrariwise, the nutriment animals require is distributed
through space in portions: in some cases near one another and in other
cases wide apart. Hence motion and locomotion are necessitated; and the
implication is that animals must have organs which render them possible. In
the first place there must be either limbs or such structures as those
which in fish, snakes, and worms move the body along. In the second place,
since action implies waste, there must be a set of channels to bring
repairing materials to the moving parts. In the third place there must be
an alimentary system for taking in and preparing these materials. In the
fourth place there must be organs for separating and excreting
waste-products. All these appliances must be more highly developed in
proportion as the required activity is greater. Then there must be an
apparatus for directing the motions and locomotions--a nervous system; and
as fast as these become rapid and complex the nervous system must be
largely developed, ending in great nervous centres--seats of intelligence
by which the activities at large are regulated. Lastly, underlying all the
structural contrasts between plants and animals thus originating, there is
the chemical contrast; since the necessity for that highly nitrogenous
matter of which animals are formed, is entailed by the necessity for
rapidly evolving the energy producing motion. So that, strange as it seems,
those chemical, physical, and mental characters of animals which so
profoundly distinguish them from plants, are all remote results of the
circumstance that their food is dispersed instead of being everywhere
present.
CHAPTER III.
FUNCTION.
§ 55. Does Structure originate Function, or does Function originate
Structure? is a question about which there has been disagreement. Using the
word Function in its widest signification, as the totality of all vital
actions, the question amounts to this--does Life produce Organization, or
does Organization produce Life?
To answer this question is not easy, since we habitually find the two so
associated that neither seems possible without the other; and they appear
uniformly to increase and decrease together. If it be said that the
arrangement of organic substances in particular forms, cannot be the
ultimate cause of vital changes, which must depend on the properties of
such substances; it may be replied that, in the absence of structural
arrangements, the forces evolved cannot be so directed and combined as to
secure that correspondence between inner and outer actions which
constitutes Life. Again, to the allegation that the vital activity of every
germ whence an organism arises, is obviously antecedent to the development
of its structures, there is the answer that such germ is not absolutely
structureless.
But in truth this question is not determinable by any evidence now
accessible to us. The very simplest forms of life known (even the
non-nucleated, if there are any) consist of granulated protoplasm; and
granulation implies structure. Moreover since each kind of protozoon, even
the lowest, has its specific mode of development and specific
activity--even down to bacteria, some kinds of which, otherwise
indistinguishable, are distinguishable by their different reactions on
their media--we are obliged to conclude that there must be constitutional
differences between the protoplasms they consist of, and this implies
structural differences. It seems that structure and function must have
advanced _pari passu_: some difference of function, primarily determined by
some difference of relation to the environment, initiating a slight
difference of structure, and this again leading to a more pronounced
difference of function; and so on through continuous actions and reactions.
§ 56. Function falls into divisions of several kinds according to our point
of view. Let us take these divisions in the order of their simplicity.
Under Function in its widest sense, are included both the statical and the
dynamical distributions of force which an organism opposes to the forces
brought to bear on it. In a tree the woody core of trunk and branches, and
in an animal the skeleton, internal or external, may be regarded as
passively resisting the gravity and momentum which tend habitually or
occasionally to derange the requisite relations between the organism and
its environment; and since they resist these forces simply by their
cohesion, their functions may be classed as _statical_. Conversely, the
leaves and sap-vessels in a tree, and those organs which in an animal
similarly carry on nutrition and circulation, as well as those which
generate and direct muscular motion, must be considered as _dynamical_ in
their actions. From another point of view Function is divisible into the
_accumulation of energy_ (latent in food); the _expenditure of energy_
(latent in the tissues and certain matters absorbed by them); and the
_transfer of energy_ (latent in the prepared nutriment or blood) from the
parts which accumulate to the parts which expend. In plants we see little
beyond the first of these: expenditure being comparatively slight, and
transfer required mainly to facilitate accumulation. In animals the
function of _accumulation_ comprehends those processes by which the
materials containing latent energy are taken in, digested, and separated
from other materials; the function of _transfer_ comprehends those
processes by which these materials, and such others as are needful to
liberate the energies they contain, are conveyed throughout the organism;
and the function of _expenditure_ comprehends those processes by which the
energy is liberated from these materials and transformed into properly
co-ordinated motions. Each of these three most general divisions includes
several more special divisions. The accumulation of energy may be separated
into _alimentation_ and _aeration_; of which the first is again separable
into the various acts gone through between prehension of food and the
transformation of part of it into blood. By the transfer of energy is to be
understood what we call _circulation_; if the meaning of circulation be
extended to embrace the duties of both the vascular system and the
lymphatics. Under the head of expenditure of energy come _nervous actions_
and _muscular actions_: though not absolutely co-extensive with expenditure
these are almost so. Lastly, there are the subsidiary functions which do
not properly fall within any of these general functions, but subserve them
by removing the obstacles to their performance: those, namely, of
_excretion_ and _exhalation_, whereby waste products are got rid of. Again,
disregarding their purposes and considering them analytically, the general
physiologist may consider functions in their widest sense as the
correlatives of tissues--the actions of epidermic tissue, cartilaginous
tissue, elastic tissue, connective tissue, osseous tissue, muscular tissue,
nervous tissue, glandular tissue. Once more, physiology in its concrete
interpretations recognizes special functions as the ends of special
organs--regards the teeth as having the office of mastication; the heart as
an apparatus to propel blood; this gland as fitted to produce one requisite
secretion and that to produce another; each muscle as the agent of a
particular motion; each nerve as the vehicle of a special sensation or a
special motor impulse.
It is clear that dealing with Biology only in its larger aspects,
specialities of function do not concern us; except in so far as they serve
to illustrate, or to qualify, its generalities.
§ 57. The first induction to be here set down is a familiar and obvious
one; the induction, namely, that complexity of function is the correlative
of complexity of structure. The leading aspects of this truth must be
briefly noted.
Where there are no distinctions of structure there are no distinctions of
function. A Rhizopod will serve as an illustration. From the outside of
this creature, which has not even a limiting membrane, there are protruded
numerous processes. Originating from any point of the surface, each of
these may contract again and disappear, or it may touch some fragment of
nutriment which it draws with it, when contracting, into the general
mass--thus serving as hand and mouth; or it may come in contact with its
fellow-processes at a distance from the body and become confluent with
them; or it may attach itself to an adjacent fixed object, and help by its
contraction to draw the body into a new position. In brief, this speck of
animated jelly is at once all stomach, all skin, all mouth, all limb, and
doubtless, too, all lung. In organisms having a fixed distribution of parts
there is a concomitant fixed distribution of actions. Among plants we see
that when, instead of a uniform tissue like that of many _Algæ_, everywhere
devoted to the same process of assimilation, there arise, as in the higher
plants, root and stem and leaves, there arise correspondingly unlike
processes. Still more conspicuously among animals do there result varieties
of function when the originally homogeneous mass is replaced by
heterogeneous organs; since, both singly and by their combinations,
modified parts generate modified changes. Up to the highest organic types
this dependence continues manifest; and it may be traced not only under
this most general form, but also under the more special form that in
animals having one set of functions developed to more than usual
heterogeneity there is a correspondingly heterogeneous apparatus devoted to
them. Thus among birds, which have more varied locomotive powers than
mammals, the limbs are more widely differentiated; while the higher
mammals, which rise to more numerous and more involved adjustments of inner
to outer relations than birds, have more complex nervous systems.
§ 58. It is a generalization almost equally obvious with the last, that
functions, like structures, arise by progressive differentiations. Just as
an organ is first an indefinite rudiment, having nothing but some most
general characteristic in common with the form it is ultimately to take; so
a function begins as a kind of action that is like the kind of action it
will eventually become, only in a very vague way. And in functional
development, as in structural development, the leading trait thus early
manifested is followed successively by traits of less and less importance.
This holds equally throughout the ascending grades of organisms and
throughout the stages of each organism. Let us look at cases: confining our
attention to animals, in which functional development is better displayed
than in plants.
The first differentiation established separates the two
fundamentally-opposed functions above named--the accumulation of energy and
the expenditure of energy. Passing over the _Protozoa_ (among which,
however, such tribes as present fixed distributions of parts show us
substantially the same thing), and commencing with the lowest
_Coelenterata_, where definite tissues make their appearance, we observe
that the only large functional distinction is between the endoderm, which
absorbs nutriment, and the ectoderm which, by its own contractions and
those of the tentacles it bears, produces motion: the contractility being
however to some extent shared by the endoderm. That the functions of
accumulation and expenditure are here very incompletely distinguished, may
be admitted without affecting the position that this is the first
specialization which begins to appear. These two most general and most
radically-opposed functions become in the _Polyzoa_, much more clearly
marked-off from each other: at the same time that each of them becomes
partially divided into subordinate functions. The endoderm and ectoderm are
no longer merely the inner and outer walls of the same simple sac into
which the food is drawn: but the endoderm forms a true alimentary canal,
separated from the ectoderm by a peri-visceral cavity, containing the
nutritive matters absorbed from the food. That is to say, the function of
accumulating force is exercised by a part distinctly divided from the part
mainly occupied in expending force: the structure between them, full of
absorbed nutriment, effecting in a vague way that transfer of force which,
at a higher stage of evolution, becomes a third leading function.
Meanwhile, the endoderm no longer discharges the accumulative function in
the same way throughout its whole extent; but its different portions,
oesophagus, stomach and intestine, perform different portions of this
function. And instead of a contractility uniformly diffused through the
ectoderm, there have arisen in the intermediate mesoderm some parts which
have the office of contracting (muscles), and some parts which have the
office of making them contract (nerves and ganglia). As we pass upwards,
the transfer of force, hitherto effected quite incidentally, comes to have
a special organ. In the ascidian, circulation is produced by a muscular
tube, open at both ends, which, by a wave of contraction passing along it,
sends out at one end the nutrient fluid drawn in at the other; and which,
having thus propelled the fluid for a time in one direction, reverses its
movement and propels it in the opposite direction. By such means does this
rudimentary heart generate alternating currents in the nutriment occupying
the peri-visceral cavity. How the function of transferring energy, thus
vaguely indicated in these inferior forms, comes afterwards to be the
definitely-separated office of a complicated apparatus made up of many
parts, each of which has a particular portion of the general duty, need not
be described. It is sufficiently manifest that this general function
becomes more clearly marked-off from the others, at the same time that it
becomes itself parted into subordinate functions.
In a developing embryo, the functions or more strictly the structures which
are to perform them, arise in the same general order. A like primary
distinction very early appears between the endoderm and the ectoderm--the
part which has the office of accumulating energy, and the part out of which
grow those organs that are the great expenders of energy. Between these two
there presently arises the mesoderm in which becomes visible the rudiment
of that vascular system, which has to fulfil the intermediate duty of
transferring energy. Of these three general functions, that of accumulating
energy is carried on from the outset: the endoderm, even while yet
incompletely differentiated from the ectoderm, absorbs nutritive matters
from the subjacent yelk. The transfer of energy is also to some extent
effected by the rudimentary vascular system, as soon as its central cavity
and attached vessels are sketched out. But the expenditure of energy (in
the higher animals at least) is not appreciably displayed by those
ectodermic and mesodermic structures that are afterwards to be mainly
devoted to it: there is no sphere for the actions of these parts. Similarly
with the chief subdivisions of these fundamental functions. The distinction
first established separates the office of transforming other energy into
mechanical motion, from the office of liberating the energy to be so
transformed. While in the layer between endoderm and ectoderm are arising
the rudiments of the muscular system, there is marked out in the ectoderm
the rudiment of the nervous system. This indication of structures which are
to share between them the general duty of expending energy, is soon
followed by changes that foreshadow further specializations of this general
duty. In the incipient nervous system there begins to arise that contrast
between the cerebral mass and the spinal cord, which, in the main, answers
to the division of nervous actions into directive and executive; and, at
the same time, the appearance of vertebral laminæ foreshadows the
separation of the osseous system, which has to resist the strains of
muscular action, from the muscular system, which, in generating motion,
entails these strains. Simultaneously there have been going on similar
actual and potential specializations in the functions of accumulating
energy and transferring energy. And throughout all subsequent phases the
method is substantially the same.
This progress from general, indefinite, and simple kinds of action to
special, definite, and complex kinds of action, has been aptly termed by
Milne-Edwards, "the physiological division of labour." Perhaps no metaphor
can more truly express the nature of this advance from vital activity in
its lowest forms to vital activity in its highest forms. And probably the
general reader cannot in any other way obtain so clear a conception of
functional development in organisms, as he can by tracing out functional
development in societies: noting how there first comes a distinction
between the governing class and the governed class; how while in the
governing class there slowly grow up such differences of duty as the civil,
military, and ecclesiastical, there arise in the governed class fundamental
industrial differences like those between agriculturists and artizans; and
how there is a continual multiplication of such specialized occupations and
specialized shares of each occupation.
§ 59. Fully to understand this change from homogeneity of function to
heterogeneity of function, which accompanies the change from homogeneity of
structure to heterogeneity of structure, it is needful to contemplate it
under a converse aspect. Standing alone, the above exposition conveys an
idea that is both inadequate and erroneous. The divisions and subdivisions
of function, becoming definite as they become multiplied, do not lead to a
more and more complete independence of functions; as they would do were the
process nothing beyond that just described; but by a simultaneous process
they are rendered more mutually dependent. While in one respect they are
separating from each other, they are in another respect combining with each
other. At the same time that they are being differentiated they are also
being integrated. Some illustrations will make this plain.
In animals which display little beyond the primary differentiation of
functions, the activity of that part which absorbs nutriment or accumulates
energy, is not immediately bound up with the activity of that part which,
in producing motion, expends energy. In the higher animals, however, the
performance of the alimentary functions depends on the performance of
various muscular and nervous functions. Mastication and swallowing are
nervo-muscular acts; the rhythmical contractions of the stomach and the
allied vermicular motions of the intestines, result from the reflex
stimulation of certain muscular coats caused by food; the secretion of the
several digestive fluids by their respective glands, is due to nervous
excitation of them; and digestion, besides requiring these special aids, is
not properly performed in the absence of a continuous discharge of energy
from the great nervous centres. Again, the function of transferring
nutriment or latent energy, from part to part, though at first not closely
connected with the other functions, eventually becomes so. The short
contractile tube which propels backwards and forwards the blood contained
in the peri-visceral cavity of an ascidian, is neither structurally nor
functionally much entangled with the creature's other organs. But on
passing upwards through higher types, in which this simple tube is replaced
by a system of branched tubes, that deliver their contents through their
open ends into the tissues at distant parts; and on coming to those
advanced types which have closed arterial and venous systems, ramifying
minutely in every corner of every organ; we find that the vascular
apparatus, while it has become structurally interwoven with the whole body,
has become unable properly to fulfil its office without the help of offices
that are quite separated from its own. The heart, though mainly automatic
in its actions, is controlled by the nervous system, which takes a share in
regulating the contractions both of the heart and the arteries. On the due
discharge of the respiratory function, too, the function of circulation is
directly dependent: if the aeration of the blood is impeded the vascular
activity is lowered; and arrest of the one very soon causes stoppage of the
other. Similarly with the duties of the nervo-muscular system. Animals of
low organization, in which the differentiation and integration of the vital
actions have not been carried far, will move about for a considerable time
after being eviscerated, or deprived of those appliances by which energy is
accumulated and transferred. But animals of high organization are instantly
killed by the removal of these appliances, and even by the injury of minor
parts of them: a dog's movements are suddenly brought to an end, by cutting
one of the main canals along which the materials that evolve movements are
conveyed. Thus while in well-developed creatures the distinction of
functions is very marked, the combination of functions is very close. From
instant to instant the aeration of blood implies that certain respiratory
muscles are being made to contract by nervous impulses passing along
certain nerves; and that the heart is duly propelling the blood to be
aerated. From instant to instant digestion proceeds only on condition that
there is a supply of aerated blood, and a due current of nervous energy
through the digestive organs. That the heart of a mammal may act, its
muscle substance must be continuously fed with an abundant supply of
arterial blood.
It is not easy to find an adequate expression for this double
re-distribution of functions. It is not easy to realize a transformation
through which the functions thus become in one sense separated and in
another sense combined, or even interfused. Here, however, as before, an
analogy drawn from social organization helps us. If we observe how the
increasing division of labour in societies is accompanied by a closer
co-operation; and how the agencies of different social actions, while
becoming in one respect more distinct, become in another respect more
minutely ramified through one another; we shall understand better the
increasing physiological co-operation that accompanies increasing
physiological division of labour. Note, for example, that while local
divisions and classes of the community have been growing unlike in their
several occupations, the carrying on of their several occupations has been
growing dependent on the due activity of that vast organization by which
sustenance is collected and diffused. During the early stages of social
development, every small group of people, and often every family, obtained
separately its own necessaries; but now, for each necessary, and for each
superfluity, there exists a combined body of wholesale and retail
distributors, which brings its branched channels of supply within reach of
all. While each citizen is pursuing a business that does not immediately
aim at the satisfaction of his personal wants, his personal wants are
satisfied by a general agency which brings from all places commodities for
him and his fellow-citizens--an agency which could not cease its special
duties for a few days, without bringing to an end his own special duties
and those of most others. Consider, again, how each of these differentiated
functions is everywhere pervaded by certain other differentiated functions.
Merchants, manufacturers, wholesale distributors of their several species,
together with lawyers, bankers, &c., all employ clerks. In clerks we have a
specialized class dispersed through various other classes; and having its
function fused with the different functions of these various other classes.
Similarly commercial travellers, though having in one sense a separate
occupation, have in another sense an occupation forming part of each of the
many occupations which it aids. As it is here with the sociological
division of labour, so is it with the physiological division of labour
above described. Just as we see in an advanced community, that while the
magisterial, the clerical, the medical, the legal, the manufacturing, and
the commercial activities, have grown distinct, they have yet their
agencies mingled together in every locality; so in a developed organism, we
see that while the general functions of circulation, secretion, absorption,
excretion, contraction, excitation, &c., have become differentiated, yet
through the ramifications of the systems apportioned to them, they are
closely combined with one another in every organ.
§ 60. The physiological division of labour is usually not carried so far as
wholly to destroy the primary physiological community of labour. As in
societies the adaptation of special classes to special duties, does not
entirely disable these classes from performing one another's duties on an
emergency; so in organisms, tissues and structures that have become fitted
to the particular offices they have ordinarily to discharge, often remain
partially able to discharge other offices. It has been pointed out by Dr.
Carpenter, that "in cases where the different functions are highly
specialized, the general structure retains, more or less, the primitive
community of function which originally characterized it." A few instances
will bring home this generalization.
The roots and leaves of plants are widely differentiated in their
functions: by the roots, water and mineral substances are absorbed; while
the leaves take in, and decompose, carbonic acid. Nevertheless, by many
botanists it is held that some leaves, or parts of them, can absorb water;
and in what are popularly called "air-plants," or at any rate in some kinds
of them, the absorption of water is mainly and in some cases wholly carried
on by them and by the stems. Conversely, the underground parts can
partially assume the functions of leaves. The exposed tuber of a potato
develops chlorophyll on its surface, and in other cases, as in that of the
turnip, roots, properly so called, do the like. In trees the trunks, which
have in great measure ceased to produce buds, recommence producing them if
the branches are cut off; sometimes aerial branches send down roots to the
earth; and under some circumstances the roots, though not in the habit of
developing leaf-bearing organs, send up numerous suckers. When the
excretion of bile is arrested, part goes to the skin and some to the
kidneys, which presently suffer under their new task. Various examples of
vicarious functions may be found among animals. The excretion of carbonic
acid and absorption of oxygen are mainly performed by the lungs, in
creatures which have lungs; but in such creatures there continues a certain
amount of cutaneous respiration, and in soft-skinned batrachians like the
frog, this cutaneous respiration is important. Again, when the kidneys are
not discharging their duties a notable quantity of urea is got rid of by
perspiration. Other instances are supplied by the higher functions. In man
the limbs, which among lower vertebrates are almost wholly organs of
locomotion, are specialized into organs of locomotion and organs of
manipulation. Nevertheless, the human arms and legs do, when needful,
fulfil, to some extent, each other's offices. Not only in childhood and old
age are the arms used for purposes of support, but on occasions of
emergency, as when mountaineering, they are used by men in full vigour. And
that legs are to a considerable degree capable of performing the duties of
arms, is proved by the great amount of manipulatory skill reached by them
when the arms are absent. Among the perceptions, too, there are examples of
partial substitution. The deaf Dr. Kitto described himself as having become
excessively sensitive to vibrations propagated through the body; and as so
having gained the power of perceiving, through his general sensations,
those neighbouring concussions of which the ears ordinarily give notice.
Blind people make hearing perform, in part, the office of vision. Instead
of identifying the positions and sizes of neighbouring objects by the
reflection of light from their surfaces, they do this in a rude way by the
reflection of sound from their surfaces.
We see, as we might expect to see, that this power of performing more
general functions, is great in proportion as the organs have been but
little adapted to their special functions. Those parts of plants which show
so considerable an ability to discharge each others' offices, are not
widely unlike in their minute structures. And the tissues which in animals
are to some extent mutually vicarious, are tissues in which the original
cellular composition is still conspicuous. But we do not find evidence that
the muscular, nervous, or osseous tissues are able in any degree to perform
those processes which the less differentiated tissues perform. Nor have we
any proof that nerve can partially fulfil the duty of muscle, or muscle
that of nerve. We must say, therefore, that the ability to resume the
primordial community of function, varies inversely as the established
specialization of function; and that it disappears when the specialization
of function becomes great.
§ 61. Something approaching to _a priori_ reasons may be given for the
conclusions thus reached _a posteriori_. They must be accepted for as much
as they seem worth.
It may be argued that on the hypothesis of Evolution, Life necessarily
comes before organization. On this hypothesis, organic matter in a state of
homogeneous aggregation must precede organic matter in a state of
heterogeneous aggregation. But since the passing from a structureless state
to a structured state, is itself a vital process, it follows that vital
activity must have existed while there was yet no structure: structure
could not else arise. That function takes precedence of structure, seems
also implied in the definition of Life. If Life is shown by inner actions
so adjusted as to balance outer actions--if the implied energy is the
_substance_ of Life while the adjustment of the actions constitutes its
_form_; then may we not say that the actions to be formed must come before
that which forms them--that the continuous change which is the basis of
function, must come before the structure which brings function into shape?
Or again, since in all phases of Life up to the highest, every advance is
the effecting of some better adjustment of inner to outer actions; and
since the accompanying new complexity of structure is simply a means of
making possible this better adjustment; it follows that the achievement of
function is, throughout, that for which structure arises. Not only is this
manifestly true where the modification of structure results by reaction
from modification of function; but it is also true where a modification of
structure otherwise produced, apparently initiates a modification of
function. For it is only when such so-called spontaneous modification of
structure subserves some advantageous action, that it is permanently
established. If it is a structural modification that happens to facilitate
the vital activities, "natural selection" retains and increases it; but if
not, it disappears.
The connexion which we noted between heterogeneity of structure and
heterogeneity of function--a connexion made so familiar by experience as to
appear scarcely worth specifying--is clearly a necessary one. It follows
from the general truth that in proportion to the heterogeneity of any
aggregate, is the heterogeneity it will produce in any incident force
(_First Principles_, § 156). The energy continually liberated in the
organism by decomposition, is here the incident force; the functions are
the variously modified forms produced in its divisions by the organs they
pass through; and the more multiform the organs the more multiform must be
the differentiations of the force passing through them.
It follows obviously from this, that if structure progresses from the
homogeneous, indefinite, and incoherent, to the heterogeneous, definite,
and coherent, so too must function. If the number of different parts in an
aggregate must determine the number of differentiations produced in the
energies passing through it--if the distinctness of these parts from one
another, must involve distinctness in their reactions, and therefore
distinctness between the divisions of the differentiated energy; there
cannot but be a complete parallelism between the development of structure
and the development of function. If structure advances from the simple and
general to the complex and special, function must do the same.
CHAPTER IV.
WASTE AND REPAIR.
§ 62. Throughout the vegetal kingdom, the processes of Waste and Repair are
comparatively insignificant in their amounts. Though all parts of plants
save the leaves, or other parts which are green, give out carbonic acid;
yet this carbonic acid, assuming it to indicate consumption of tissue, or
rather of the protoplasm contained in the tissue, indicates but a small
consumption. Of course if there is little waste there can be but little
repair--that is, little of the interstitial repair which restores the
integrity of parts worn by functional activity. Nor, indeed, is there
displayed by plants in any considerable degree, if at all, that other
species of repair which consists in the restoration of lost or injured
organs. Torn leaves and the shoots that are shortened by the pruner, do not
reproduce their missing parts; and though when the branch of a tree is cut
off close to the trunk, the place is in course of years covered over, it is
not by any reparative action in the wounded surface but by the lateral
growth of the adjacent bark. Hence, without saying that Waste and Repair do
not go on at all in plants, we may fitly pass them over as of no
importance.
There are but slight indications of waste in those lower orders of animals
which, by their comparative inactivity, show themselves least removed from
vegetal life. Actiniæ kept in an aquarium, do not appreciably diminish in
bulk from prolonged abstinence. Even fish, though much more active than
most other aquatic creatures, appear to undergo but little loss of
substance when kept unfed during considerable periods. Reptiles, too,
maintaining no great temperature, and passing their lives mostly in a state
of torpor, suffer but little diminution of mass by waste. When, however, we
turn to those higher orders of animals which are active and hot-blooded, we
see that waste is rapid: producing, when unchecked, a notable decrease in
bulk and weight, ending very shortly in death. Besides finding that waste
is inconsiderable in creatures which produce but little insensible and
sensible motion, and that it becomes conspicuous in creatures which produce
much insensible and sensible motion; we find that in the same creatures
there is most waste when most motion is generated. This is clearly proved
by hybernating animals. "Valentin found that the waking marmot excreted in
the average 75 times more carbonic acid, and inhaled 41 times more oxygen
than the same animal in the most complete state of hybernation. The stages
between waking and most profound hybernation yielded intermediate figures.
A waking hedgehog yielded about 20.5 times more carbonic acid, and consumed
18.4 times more oxygen than one in the state of hybernation."[22] If we
take these quantities of absorbed oxygen and excreted carbonic acid, as
indicating something like the relative amounts of consumed organic
substance, we see that there is a striking contrast between the waste
accompanying the ordinary state of activity, and the waste accompanying
complete quiescence and reduced temperature. This difference is still more
definitely shown by the fact, that the mean daily loss from starvation in
rabbits and guinea-pigs, bears to that from hybernation, the proportion of
18.3:1. Among men and domestic animals, the relation between degree of
waste and amount of expended energy, though one respecting which there is
little doubt, is less distinctly demonstrable; since waste is not allowed
to go on uninterfered with. We have, however, in the lingering lives of
invalids who are able to take scarcely any nutriment but are kept warm and
still, an illustration of the extent to which waste diminishes as the
expenditure of energy declines.
Besides the connexion between the waste of the organism as a whole and the
production of sensible and insensible motion by the organism as a whole,
there is a traceable connexion between the waste of special parts and the
activities of such special parts. Experiments have shown that "the starving
pigeon daily consumes in the average 40 times more muscular substance that
the marmot in the state of torpor, and only 11 times more fat, 33 times
more of the tissue of the alimentary canal, 18.3 times more liver, 15 times
more lung, 5 times more skin." That is to say, in the hybernating animal
the parts least consumed are the almost totally quiescent motor-organs, and
the part most consumed is the hydro-carbonaceous deposit serving as a store
of energy; whereas in the pigeon, similarly unsupplied with food but awake
and active, the greatest loss takes place in the motor-organs. The
relation between special activity and special waste, is illustrated, too,
in the daily experiences of all: not indeed in the amount of decrease of
the active parts in bulk or weight, for this we have no means of
ascertaining; but in the diminished ability of such parts to perform their
functions. That legs exerted for many hours in walking and arms long
strained in rowing, lose their powers--that eyes become enfeebled by
reading or writing without intermission--that concentrated attention,
unbroken by rest, so prostrates the brain as to incapacitate it for
thinking; are familiar truths. And though we have no direct evidence to
this effect, there is little danger in concluding that muscles exercised
until they ache or become stiff, and nerves of sense rendered weary or
obtuse by work, are organs so much wasted by action as to be partially
incompetent.
Repair is everywhere and always making up for waste. Though the two
processes vary in their relative rates both are constantly going on. Though
during the active, waking state of an animal waste is in excess of repair,
yet repair is in progress; and though during sleep repair is in excess of
waste, yet some waste is necessitated by the carrying on of certain
never-ceasing functions. The organs of these never-ceasing functions
furnish, indeed, the most conclusive proofs of the simultaneity of repair
and waste. Day and night the heart never stops beating, but only varies in
the rapidity and vigour of its beats; and hence the loss of substance which
its contractions from moment to moment entail, must from moment to moment
be made good. Day and night the lungs dilate and collapse; and the muscles
which make them do this must therefore be kept in a state of integrity by a
repair which keeps pace with waste, or which alternately falls behind and
gets in advance of it to a very slight extent.
On a survey of the facts we see, as we might expect to see, that the
progress of repair is most rapid when activity is most reduced. Assuming
that the organs which absorb and circulate nutriment are in proper order,
the restoration of the body to a state of integrity, after the
disintegration consequent on expenditure of energy, is proportionate to the
diminution in expenditure of energy. Thus we all know that those who are in
health, feel the greatest return of vigour after profound sleep--after
complete cessation of motion. We know that a night during which the
quiescence, bodily and mental, has been less decided, is usually not
followed by that spontaneous overflow of energy which indicates a high
state of efficiency throughout the organism. We know, again, that
long-continued recumbency, even with wakefulness (providing the wakefulness
is not the result of disorder), is followed by a certain renewal of
strength; though a renewal less than that which would have followed the
greater inactivity of slumber. We know, too, that when exhausted by labour,
sitting brings a partial return of vigour. And we also know that after the
violent exertion of running, a lapse into the less violent exertion of
walking, results in a gradual disappearance of that prostration which the
running produced. This series of illustrations conclusively proves that the
rebuilding of the organism is ever making up for the pulling down of it
caused by action; and that the effect of this rebuilding becomes more
manifest, in proportion as the pulling down is less rapid. From each
digested meal there is every few hours absorbed into the mass of prepared
nutriment circulating through the body, a fresh supply of the needful
organic compounds; and from the blood, thus occasionally re-enriched, the
organs through which it passes are ever taking up materials to replace the
materials used up in the discharge of functions. During activity the
reintegration falls in arrear of the disintegration; until, as a
consequence, there presently comes a general state of functional languor;
ending, at length, in a quiescence which permits the reintegration to
exceed the disintegration, and restore the parts to their state of
integrity. Here, as wherever there are antagonistic actions, we see
rhythmical divergences on opposite sides of the medium state--changes which
equilibrate each other by their alternate excesses. (_First Principles_,
§§ 85, 173.)
Illustrations are not wanting of special repair that is similarly ever in
progress, and similarly has intervals during which it falls below waste and
rises above it. Every one knows that a muscle, or a set of muscles,
continuously strained, as by holding out a weight at arm's length, soon
loses its power; and that it recovers its power more or less fully after a
short rest. The several organs of the special sensations yield us like
experiences. Strong tastes, powerful odours, loud sounds, temporarily unfit
the nerves impressed by them for appreciating faint tastes, odours, or
sounds; but these incapacities are remedied by brief intervals of repose.
Vision still better illustrates this simultaneity of waste and repair.
Looking at the Sun so affects the eyes that, for a short time, they cannot
perceive the things around with the usual clearness. After gazing at a
bright light of a particular colour, we see, on turning the eyes to
adjacent objects, an image of the complementary colour; showing that the
retina has, for the moment, lost the power to feel small amounts of those
rays which have strongly affected it. Such inabilities disappear in a few
seconds or a few minutes, according to circumstances. And here, indeed, we
are introduced to a conclusive proof that special repair is ever
neutralizing special waste. For the rapidity with which the eyes recover
their sensitiveness, varies with the reparative power of the individual. In
youth the visual apparatus is so quickly restored to its state of
integrity, that many of these _photogenes_, as they are called, cannot be
perceived. When sitting on the far side of a room, and gazing out of the
window against a light sky, a person who is debilitated by disease or
advancing years, perceives, on transferring the gaze to the adjacent wall,
a momentary negative image of the window--the sash-bars appearing light and
the squares dark; but a young and healthy person has no such experience.
With a rich blood and vigorous circulation, the repair of the visual nerves
after impressions of moderate intensity, is nearly instantaneous.
Function carried to excess may produce waste so great that repair cannot
make up for it during the ordinary daily periods of rest; and there may
result incapacities of the over-taxed organs, lasting for considerable
periods. We know that eyes strained by long-continued minute work lose
their power for months or years: perhaps suffering an injury from which
they never wholly recover. Brains, too, are often so unduly worked that
permanent relaxation fails to restore them to vigour. Even of the motor
organs the like holds. The most frequent cause of what is called "wasting
palsy," or atrophy of the muscles, is habitual excess of exertion: the
proof being that the disease occurs most frequently among those engaged in
laborious handicrafts, and usually attacks first the muscles which have
been most worked.
There has yet to be noticed another kind of repair--that, namely, by which
injured or lost parts are restored. Among the _Hydrozoa_ it is common for
any portion of the body to reproduce the rest; even though the rest to be
so reproduced is the greater part of the whole. In the more
highly-organized _Actinozoa_ the half of an individual will grow into a
complete individual. Some of the lower Annelids, as the _Nais_, may be cut
into thirty or forty pieces and each piece will eventually become a perfect
animal. As we ascend to higher forms we find this reparative power much
diminished, though still considerable. The reproduction of a lost claw by a
lobster or crab, is a familiar instance. Some of the inferior _Vertebrata_
also, as lizards, can develop new limbs or new tails, in place of those
which have been cut off; and can even do this several times over, though
with decreasing completeness. The highest animals, however, thus repair
themselves to but a very small extent. Mammals and birds do it only in the
healing of wounds; and very often but imperfectly even in this. For in
muscular and glandular organs the tissues destroyed are not properly
reproduced, but are replaced by tissue of an irregular kind which serves to
hold the parts together. So that the power of reproducing lost parts is
greatest where the organization is lowest; and almost disappears where the
organization is highest. And though we cannot say that in the intermediate
stages there is a constant inverse relation between reparative power and
degree of organization; yet we may say that there is some approach to such
a relation.
§ 63. There is an obvious and complete harmony between the first of the
above inductions and the deduction which follows immediately from first
principles. We have already seen (§ 23) "that whatever amount of power an
organism expends in any shape, is the correlate and equivalent of a power
that was taken into it from without." Motion, sensible or insensible,
generated by an organism, is insensible motion which was absorbed in
producing certain chemical compounds appropriated by the organism under the
form of food. As much energy as was required to raise the elements of these
complex atoms to their state of unstable equilibrium, is given out in their
falls to a state of stable equilibrium; and having fallen to a state of
stable equilibrium they can give out no further energy, but have to be got
rid of as inert and useless. It is an inevitable corollary "from the
persistence of force, that each portion of mechanical or other energy which
an organism exerts, implies the transformation of as much organic matter as
contained this energy in a latent state;" and that this organic matter in
yielding up its latent energy, loses its value for the purposes of life,
and becomes waste matter needing to be excreted. The loss of these complex
unstable substances must hence be proportionate to the quantity of expended
force. Here, then, is the rationale of certain general facts lately
indicated. Plants do not waste to any considerable degree, for the obvious
reason that the sensible and insensible motions they generate are
inconsiderable. Between the small waste, small activity, and low
temperature of the inferior animals, the relation is similarly one
admitting of _a priori_ establishment. Conversely, the rapid waste of
energetic, hot-blooded animals might be foreseen with equal certainty. And
not less manifestly necessary is the variation in waste which, in the same
organism, attends the variation in the heat and mechanical motion produced.
Between the activity of a special part and the waste of that part, a like
relation may be deductively inferred; though it cannot be inferred that
this relation is equally definite. Were the activity of every organ quite
independent of the activities of other organs, we might expect to trace out
this relation distinctly; but since increased activity in any organ or
group of organs, as the muscles, necessarily entails increased activity in
other organs, as in the heart, lungs, and nervous system, it is clear that
special waste and general waste are too much entangled to admit of a
definite relation being established between special waste and special
activity. We may fairly say, however, that this relation is quite as
manifest as we can reasonably anticipate.
§ 64. Deductive interpretation of the phenomena of Repair, is by no means
so easy. The tendency displayed by an animal organism, as well as by each
of its organs, to return to a state of integrity by the assimilation of new
matter, when it has undergone the waste consequent on activity, is a
tendency which is not manifestly deducible from first principles; though it
appears to be in harmony with them. If in the blood there existed
ready-formed units exactly like in kind to those of which each organ
consists, the sorting of these units, ending in the union of each kind with
already existing groups of the same kind, would be merely a good example of
Segregation (_First Principles_, § 163). It would be analogous to the
process by which, from a mixed solution of salts, there are, after an
interval, deposited separate masses of these salts in the shape of
different crystals. But as already said (§ 54), though the selective
assimilation by which the repair of organs is effected, may result in part
from an action of this kind, the facts cannot be thus wholly accounted for;
since organs are in part made up of units which do not exist as such in the
circulating fluids. We must suppose that, as suggested in § 54, groups of
compound units have a certain power of moulding adjacent fit materials into
units of their own form. Let us see whether there is not reason to think
such a power exists.
"The poison of small-pox or of scarlatina," remarks Mr. (now Sir James)
Paget, "being once added to the blood, presently affects the composition of
the whole: the disease pursues its course, and, if recovery ensue, the
blood will seem to have returned to its previous condition: yet it is not
as it was before; for now the same poison may be added to it with
impunity." ... "The change once effected, may be maintained through life.
And herein seems to be a proof of the assimilative force in the blood: for
there seems no other mode of explaining these cases than by admitting that
the altered particles have the power of assimilating to themselves all
those by which they are being replaced: in other words, all the blood that
is formed after such a disease deviates from the natural composition, so
far as to acquire the peculiarity engendered by the disease: it is formed
according to the altered model." Now if the compound molecules of the
blood, or of an organism considered in the aggregate, have the power of
moulding into their own type the matters which they absorb as nutriment;
and if they have the power when their type has been changed by disease, of
moulding materials afterwards received into the modified type; may we not
reasonably suspect that the more or less specialized molecules of each
organ have, in like manner, the power of moulding the materials which the
blood brings to them into similarly specialized molecules? The one
conclusion seems to be a corollary from the other. Such a power cannot be
claimed for the component units of the blood without being conceded to the
component units of every tissue. Indeed the assertion of this power is
little more than an assertion of the fact that organs composed of
specialized units _are_ capable of resuming their structural integrity
after they have been wasted by function. For if they do this, they must do
it by forming from the materials brought to them, certain specialized units
like in kind to those of which they are composed; and to say that they do
this, is to say that their component units have the power of moulding fit
materials into other units of the same order.
§ 65. What must we say of the ability an organism has to re-complete itself
when one of its parts has been cut off? Is it of the same order as the
ability of an injured crystal to re-complete itself. In either case new
matter is so deposited as to restore the original outline. And if in the
case of the crystal we say that the whole aggregate exerts over its parts a
force which constrains the newly-integrated molecules to take a certain
definite form, we seem obliged, in the case of the organism, to assume an
analogous force. If when the leg of a lizard has been amputated there
presently buds out the germ of a new one, which, passing through phases of
development like those of the original leg, eventually assumes a like shape
and structure, we assert only what we see, when we assert that the entire
organism, or the adjacent part of it, exercises such power over the forming
limb as makes it a repetition of its predecessor. If a leg is reproduced,
where there was a leg, and a tail where there was a tail, there seems no
alternative but to conclude that the forces around it control the formative
processes going on in each part. And on contemplating these facts in
connexion with various kindred ones, there is suggested the hypothesis,
that the form of each species of organism is determined by a peculiarity in
the constitution of its units--that these have a special structure in which
they tend to arrange themselves; just as have the simpler units of
inorganic matter. Let us glance at the evidences which more especially
thrust this conclusion upon us.
A fragment of a Begonia-leaf imbedded in fit soil and kept at an
appropriate temperature, will develop a young Begonia; and so small is the
fragment which is thus capable of originating a complete plant, that
something like a hundred plants may be produced from a single leaf. The
friend to whom I owe this observation, tells me that various succulent
plants have like powers of multiplication. Illustrating a similar power
among animals, we have the often-cited experiments of Trembley on the
common polype. Each of the four pieces into which one of these creatures
was cut, grew into a perfect individual. In each of these, again, bisection
and tri-section were followed by like results. And so with their segments,
similarly produced, until as many as fifty polypes had resulted from the
original one. Bodies when cut off regenerated heads; heads regenerated
bodies; and when a polype had been divided into as many pieces as was
practicable, nearly every piece survived and became a complete animal.
What, now, is the implication? We cannot say that in each portion of a
Begonia-leaf, and in every fragment of a Hydra's body, there exists a
ready-formed model of the entire organism. Even were there warrant for the
doctrine that the germ of every organism contains the perfect organism in
miniature, it still could not be contended that each considerable part of
the perfect organism resulting from such a germ, contains another such
miniature. Indeed the one hypothesis negatives the other. The implication
seems, therefore, to be that the living particles composing one of these
fragments, have an innate tendency to arrange themselves into the shape of
the organism to which they belong. We must infer that the active units
composing a plant or animal of any species have an intrinsic aptitude to
aggregate into the form of that species. It seems difficult to conceive
that this can be so; but we see that it _is_ so. Groups of units taken from
an organism (providing they are of a certain bulk and not much
differentiated into special structures) _have_ this power of re-arranging
themselves. Manifestly, too, if we are thus to interpret the reproduction
of an organism from one of its amorphous fragments, we must thus interpret
the reproduction of any minor portion of an organism by the remainder. When
in place of its lost claw a lobster puts forth a cellular mass which, while
increasing in bulk, assumes the form and structure of the original claw, we
cannot avoid ascribing this result to a play of forces like that which
moulds the materials contained in a piece of Begonia-leaf into the shape of
a young Begonia.
§ 66. As we shall have frequent occasion hereafter to refer to these units
which possess the property of arranging themselves into the special
structures of the organisms to which they belong; it will be well here to
ask by what name they may be most fitly called.
On the one hand, it cannot be in those chemical compounds characterizing
organic bodies that this specific property dwells. It cannot be that the
molecules of albumin, or fibrin, or gelatine, or other proteid, possess
this power of aggregating into these specific shapes; for in such case
there would be nothing to account for the unlikenesses of different
organisms. If the proclivities of proteid molecules determined the forms of
the organisms built up of them or by them, the occurrence of such endlessly
varied forms would be inexplicable. Hence what we may call the _chemical
units_ are clearly not the possessors of this property.
On the other hand, this property cannot reside in what may be roughly
distinguished as the _morphological units_. The germ of every organism is a
minute portion of encased protoplasm commonly called a cell. It is by
multiplication of cells that all the early developmental changes are
effected. The various tissues which successively arise in the unfolding
organism, are primarily cellular; and in many of them the formation of
cells continues to be, throughout life, the process by which repair is
carried on. But though cells are so generally the ultimate visible
components of organisms, that they may with some show of reason be called
the morphological units; yet we cannot say that this tendency to aggregate
into special forms dwells in them. In many cases a fibrous tissue arises
out of a nucleated blastema, without cell-formation; and in such cases
cells cannot be regarded as units possessing the structural proclivity. But
the conclusive proof that the morphological units are not the building
factors in an organism composed of them, is yielded by their independent
homologues the so-called unicellular organisms. For each of these displays
the power to assume its specific structure. Clearly, if the ability of a
multicellular organism to assume its specific structure resulted from the
cooperation of its component cells, then a single cell, or the independent
homologue of a single cell, having no other to cooperate with, could
exhibit no structural traits. Not only, however, do single-celled organisms
exhibit structural traits, but these, even among the simplest, are so
distinct as to originate classification into orders, genera, and species;
and they are so constant as to remain the same from generation to
generation.
If, then, this organic polarity (as we might figuratively call this
proclivity towards a specific structural arrangement) can be possessed
neither by the chemical units nor the morphological units, we must conceive
it as possessed by certain intermediate units, which we may term
_physiological_. There seems no alternative but to suppose that the
chemical units combine into units immensely more complex than themselves,
complex as they are; and that in each organism the physiological units
produced by this further compounding of highly compound molecules, have a
more or less distinctive character. We must conclude that in each case some
difference of composition in the units, or of arrangement in their
components, leading to some difference in their mutual play of forces,
produces a difference in the form which the aggregate of them assumes.
The facts contained in this chapter form but a small part of the evidence
which thrusts this assumption upon us. We shall hereafter find various
reasons for inferring that such physiological units exist, and that to
their specific properties, more or less unlike in each plant and animal,
various organic phenomena are due.
CHAPTER V.
ADAPTATION.
§ 67. In plants waste and repair being scarcely appreciable, there are not
likely to arise appreciable changes in the proportions of already-formed
parts. The only divergences from the average structures of a species, which
we may expect particular conditions to produce, are those producible by the
action of these conditions on parts in course of formation; and such
divergences we do find. We know that a tree which, standing alone in an
exposed position, has a short and thick stem, has a tall and slender stem
when it grows in a wood; and that also its branches then take a different
inclination. We know that potato-sprouts which, on reaching the light,
develop into foliage, will, in the absence of light, grow to a length of
several feet without foliage. And every in-door plant furnishes proof that
shoots and leaves, by habitually turning themselves to the light, exhibit a
certain adaptation--an adaptation due, as we must suppose; to the special
effects of the special conditions on the still growing parts. In animals,
however, besides analogous structural changes wrought during the period of
growth, by subjection to circumstances unlike the ordinary circumstances,
there are structural changes similarly wrought after maturity has been
reached. Organs that have arrived at their full sizes possess a certain
modifiability; so that while the organism as a whole retains pretty nearly
the same bulk, the proportions of its parts may be considerably varied.
Their variations, here treated of under the title Adaptation, depend on
specialities of individual action. In the last chapter we saw that the
actions of organisms entail re-actions on them; and that specialities of
action entail specialities of re-action. Here it remains to be pointed out
that these special actions and re-actions do not end with temporary
changes, but work permanent changes.
If, in an adult animal, the waste and repair in all parts were exactly
balanced--if each organ daily gained by nutrition exactly as much as it
lost daily by the discharge of its function--if excess of function were
followed only by such excess of nutrition as balanced the extra waste; it
is clear that there would occur no change in the relative sizes of organs.
But there is no such exact balance. If the excess of function, and
consequent excess of waste, is moderate, it is not simply compensated by
repair but more than compensated--there is a certain increase of bulk. This
is true to some degree of the organism as a whole, when the organism is
framed for activity. A considerable waste giving considerable power of
assimilation, is more favourable to accumulation of tissue than is
quiescence with its comparatively feeble assimilation: whence results a
certain adaptation of the whole organism to its requirements. But it is
more especially true of the parts of an organism in relation to one
another. The illustrations fall into several groups. The growth of muscles
exercised to an unusual degree is a matter of common observation. In the
often-cited blacksmith's arm, the dancer's legs and the jockey's crural
adductors, we have marked examples of a modifiability which almost every
one has to some extent experienced. It is needless to multiply proofs. The
occurrence of changes in the structure of the skin, where the skin is
exposed to unusual stress of function, is also familiar. That thickening of
the epidermis on a labourer's palm results from continual pressure and
friction, is certain. Those who have not before exerted their hands, find
that such an exercise as rowing soon begins to produce a like thickening.
This relation of cause and effect is still better shown by the marked
indurations at the ends of a violinist's fingers. Even in mucous membrane,
which ordinarily is not subject to mechanical forces of any intensity,
similar modifications are possible: witness the callosity of the gums which
arises in those who have lost their teeth, and have to masticate without
teeth. The vascular system furnishes good instances of the increased growth
that follows increased function. When, because of some permanent
obstruction to the circulation, the heart has to exert a greater
contractile force on the mass of blood which it propels at each pulsation,
and when there results the laboured action known as palpitation, there
usually occurs dilatation, or hypertrophy, or a mixture of the two: the
dilatation, which is a yielding of the heart's structure under the
increased strain, implying a failure to meet the emergency; but the
hypertrophy, which consists in a thickening of the heart's muscular walls,
being an adaptation of it to the additional effort required. Again, when an
aneurism in some considerable artery has been obliterated, either
artifically or by a natural inflammatory process; and when this artery has
consequently ceased to be a channel for the blood; some of the adjacent
arteries which anastomose with it become enlarged, so as to carry the
needful quantity of blood to the parts supplied. Though we have no direct
proof of analogous modifications in nervous structures, yet indirect proof
is given by the greater efficiency that follows greater activity. This is
manifested alike in the senses and the intellect. The palate may be
cultivated into extreme sensitiveness, as in professional tea-tasters. An
orchestral conductor gains, by continual practice, an unusually great
ability to discriminate differences of sound. In the finger-reading of the
blind we have evidence that the sense of touch may be brought by exercise
to a far higher capability than is ordinary.[23] The increase of power
which habitual exertion gives to mental faculties needs no illustration:
every person of education has personal experience of it. Even from the
osseous structures evidence may be drawn. The bones of men accustomed to
great muscular action are more massive, and have more strongly marked
processes for the attachment of muscles, than the bones of men who lead
sedentary lives; and a like contrast holds between the bones of wild and
tame animals of the same species. Adaptations of another order, in which
there is a qualitative rather than a quantitative modification, arise after
certain accidents to which the skeleton is liable. When the hip-joint has
been dislocated, and long delay has made it impossible to restore the parts
to their proper places, the head of the thigh-bone, imbedded in the
surrounding muscles, becomes fixed in its new position by attachments of
fibrous tissue, which afford support enough to permit a halting walk. But
the most remarkable modification of this order occurs in united ends of
fractured bones. "False joints" are often formed--joints which rudely
simulate the hinge structure or the ball-and-socket structure, according as
the muscles tend to produce a motion of flexion and extension or a motion
of rotation. In the one case, according to Rokitansky, the two ends of the
broken bone become smooth and covered with periosteum and fibrous tissue,
and are attached by ligaments that allow a certain backward and forward
motion; and in the other case the ends, similarly clothed with the
appropriate membranes, become the one convex and the other concave, are
inclosed in a capsule, and are even occasionally supplied with synovial
fluid!
The general truth that extra function is followed by extra growth, must be
supplemented by the equally general truth, that beyond a limit, usually
soon reached, very little, if any, further modification can be produced.
The experiences which we colligate into the one induction thrust the other
upon us. After a time no training makes the pugilist or the athlete any
stronger. The adult gymnast at last acquires the power to perform certain
difficult feats; but certain more difficult feats no additional practice
enables him to perform. Years of discipline give the singer a particular
loudness and range of voice, beyond which further discipline does not give
greater loudness or wider range: on the contrary, increased vocal exercise,
causing a waste in excess of repair, is often followed by decrease of
power. In the exaltation of the perceptions we see similar limits. The
culture which raises the susceptibility of the ear to the intervals and
harmonies of notes, will not turn a bad ear into a good one. Lifelong
effort fails to make this artist a correct draftsman or that a fine
colourist: each does better than he did at first, but each falls short of
the power attained by some other artists. Nor is this truth less clearly
illustrated among the more complex mental powers. A man may have a
mathematical faculty, a poetical faculty, or an oratorical faculty, which
special education improves to a certain extent. But unless he is unusually
endowed in one of those directions, no amount of education will make him a
first-rate mathematician, a first-rate poet, or a first-rate orator. Thus
the general fact appears to be that while in each individual certain
changes in the proportions of parts may be caused by variations of
functions, the congenital structure of each individual puts a limit to the
modifiability of every part. Nor is this true of individuals only: it
holds, in a sense, of species. Leaving open the question whether, in
indefinite times, indefinite modifications may not be produced by
inheritance of functionally wrought adaptations; experience proves that
within assigned times, the changes wrought in races of organisms by changes
of conditions fall within narrow limits. Though by discipline, aided by
selective breeding, one variety of horse has had its locomotive power
increased considerably beyond the locomotive powers of other varieties; yet
further increase takes place, if at all, at an inappreciable rate. The
different kinds of dogs, too, in which different forms and capacities have
been established, do not now show aptitudes for diverging in the same
directions at considerable rates. In domestic animals generally, certain
accessions of intelligence have been produced by culture; but accessions
beyond these are inconspicuous. It seems that in each species of organism
there is a margin for functional oscillations on all sides of a mean state,
and a consequent margin for structural variations; that it is possible
rapidly to push functional and structural changes towards the extreme of
this margin in any direction, both in an individual and in a race; but that
to push these changes further in any direction, and so to alter the
organism as to bring its mean state up to the extreme of the margin in that
direction, is a comparatively slow process.[24]
We also have to note that the limited increase of size produced in any
organ by a limited increase of its function, is not maintained unless the
increase of function is permanent. A mature man or other animal, led by
circumstances into exerting particular members in unusual degrees, and
acquiring extra sizes in these members, begins to lose such extra sizes on
ceasing to exert the members; and eventually lapses more or less nearly
into the original state. Legs strengthened by a pedestrian tour, become
relatively weak again after a prolonged return to sedentary life. The
acquired ability to perform feats of skill disappears in course of time, if
the performance of them be given up. For comparative failure in executing a
piece of music, in playing a game at chess, or in anything requiring
special culture, the being out of practice is a reason which every one
recognizes as valid. It is observable, too, that the rapidity and
completeness with which an artificial power is lost, is proportionate to
the shortness of the cultivation which evoked it. One who has for many
years persevered in habits which exercise special muscles or special
faculties of mind, retains the extra capacity produced, to a very
considerable degree, even after a long period of desistance; but one who
has persevered in such habits for but a short time has, at the end of a
like period, scarcely any of the facility he had gained. Here too, as
before, successions of organisms present an analogous fact. A species in
which domestication continued through many generations, has organized
certain peculiarities; and which afterwards, escaping domestic discipline,
returns to something like its original habits; soon loses, in great
measure, such peculiarities. Though it is not true, as alleged, that it
resumes completely the structure it had before domestication, yet it
approximates to that structure. The Dingo, or wild dog of Australia, is one
of the instances given of this; and the wild horse of South America is
another. Mankind, too, supplies us with instances. In the Australian bush
and in the backwoods of America, the Anglo-Saxon race, in which
civilization has developed the higher feelings to a considerable degree,
rapidly lapses into comparative barbarism: adopting the moral code, and
sometimes the habits, of savages.
§ 68. It is important to reach, if possible, some rationale of these
general truths--especially of the last two. A right understanding of these
laws of organic modification underlies a right understanding of the great
question of species. While, as before hinted (§ 40), the action of
structure on function is one of the factors in that process of
differentiation by which unlike forms of plants and animals are produced,
the reaction of function on structure is another factor. Hence, it is well
worth while inquiring how far these inductions are deductively
interpretable.
The first of them is the most difficult to deal with. Why an organ exerted
somewhat beyond its wont should presently grow, and thus meet increase of
demand by increase of supply, is not obvious. We know, indeed, (_First
Principles_, §§ 85, 173,) that of necessity, the rhythmical changes
produced by antagonistic organic actions cannot any of them be carried to
an excess in one direction, without there being produced an equivalent
excess in the opposite direction. It is a corollary from the persistence of
force, that any deviation effected by a disturbing cause, acting on some
member of a moving equilibrium, must (unless it altogether destroys the
moving equilibrium) be eventually followed by a compensating deviation.
Hence, that excess of repair should succeed excess of waste, is to be
expected. But how happens the mean state of the organ to be changed? If
daily extra waste naturally brings about daily extra repair only to an
equivalent extent, the mean state of the organ should remain constant. How
then comes the organ to augment in size and power?
Such answer to this question as we may hope to find, must be looked for in
the effects wrought on the organism as a whole by increased function in one
of its parts. For since the discharge of its function by any part is
possible only on condition that those various other functions on which its
own is immediately dependent are also discharged, it follows that excess in
its function presupposes some excess in their functions. Additional work
given to a muscle implies additional work given to the branch arteries
which bring it blood, and additional work, smaller in proportion, to the
arteries from which these branch arteries come. Similarly, the smaller and
larger veins which take away the blood, as well as those structures which
deal with effete products, must have more to do. And yet further, on the
nervous centres which excite the muscle a certain extra duty must fall. But
excess of waste will entail excess of repair, in these parts as well as in
the muscle. The several appliances by which the nutrition and excitation of
an organ are carried on, must also be influenced by this rhythm of action
and reaction; and therefore, after losing more than usual by the
destructive process they must gain more than usual by the constructive
process. But temporarily-increased efficiency in these appliances by which
blood and nervous force are brought to an organ, will cause extra
assimilation in the organ, beyond that required to balance its extra
expenditure. Regarding the functions as constituting a moving equilibrium,
we may say that divergence of any function in the direction of increase,
causes the functions with which it is bound up to diverge in the same
direction; that these, again, cause the functions which they are bound up
with, also to diverge in the same direction; and that these divergences of
the connected functions allow the specially-affected function to be carried
further in this direction than it could otherwise be--further than the
perturbing force could carry it if it had a fixed basis.
It must be admitted that this is but a vague explanation. Among actions so
involved as these, we can scarcely expect to do more than dimly discern a
harmony with first principles. That the facts are to be interpreted in some
such way, may, however, be inferred from the circumstance that an extra
supply of blood continues for some time to be sent to an organ that has
been unusually exercised; and that when unusual exercise is long continued
a permanent increase of vascularity results.
§ 69. Answers to the questions--Why do these adaptive modifications in an
individual animal soon reach a limit? and why, in the descendants of such
animal, similarly conditioned, is this limit very slowly extended?--are to
be found in the same direction as was the answer to the last question. And
here the connexion of cause and consequence is more manifest.
Since the function of any organ is dependent on the functions of the organs
which supply it with materials and stimuli; and since the functions of
these subsidiary organs are dependent on the functions of organs which
supply them with materials and stimuli; it follows that before any great
extra power of discharging its function can be gained by a
specially-exercised organ, a considerable extra power must be gained by a
series of immediately-subservient organs, and some extra power by a
secondary series of remotely-subservient organs. Thus there are required
numerous and wide-spread modifications. Before the artery which feeds a
hard-worked muscle can permanently furnish a large additional quantity of
blood, it must increase in diameter; and that its increase of diameter may
be of use, the main artery from which it diverges must also be so far
modified as to bring this additional quantity of blood to the branch
artery. Similarly with the veins; similarly with the structures which
remove waste-products; similarly with the nerves. And when we ask what
these subsidiary changes imply, we are forced to conclude that there must
be an analogous group of more numerous changes ramifying throughout the
system. The growth of the arteries primarily and secondarily implicated,
cannot go to any extent without growth in the minor blood-vessels on which
their nutrition depends; while their greater contractile power involves
enlargement of the nerves which excite them, and some modification of that
part of the spinal cord whence these nerves proceed. Thus, without tracing
the like remote alterations implied by extra growth of the veins,
lymphatics, glandular organs, and other agencies, it is manifest that a
large amount of rebuilding must be done throughout the organism, before any
organ of importance can be permanently increased in size and power to a
great extent. Hence, though such extra growth in any part as does not
necessitate considerable changes throughout the rest of the organism, may
rapidly take place; a further growth in this part, requiring a re-modelling
of numerous parts remotely and slightly affected, must take place but
slowly.
We have before found our conceptions of vital processes made clearer by
studying analogous social processes. In societies there is a mutual
dependence of functions, essentially like that which exists in organisms;
and there is also an essentially like reaction of functions on structures.
From the laws of adaptive modification in societies, we may therefore hope
to get a clue to the laws of adaptive modification in organisms. Let us
suppose, then, that a society has arrived at a state of equilibrium
analogous to that of a mature animal--a state not like our own, in which
growth and structural development are rapidly going on, but a state of
settled balance among the functional powers of the various classes and
industrial bodies, and a consequent fixity in the relative sizes of such
classes and bodies. Further, let us suppose that in a society thus balanced
there occurs something which throws an unusual demand on one industry--say
an unusual demand for ships (which we will assume to be built of iron) in
consequence of a competing mercantile nation having been prostrated by
famine or pestilence. The immediate result of this additional demand for
iron ships is the employment of more workmen, and the purchase of more
iron, by the ship-builders; and when, presently, the demand continuing, the
ship-builders find their premises and machinery insufficient, they enlarge
them. If the extra requirement persists, the high interest and high wages
bring such extra capital and labour into the business as are needed for new
ship-building establishments. But such extra capital and labour do not come
quickly; since, in a balanced community, not increasing in population and
wealth, labour and capital have to be drawn from other industries, where
they are already yielding the ordinary returns. Let us now go a step
further. Suppose that this iron-ship-building industry, having enlarged as
much as the available capital and labour permit, is still unequal to the
demand; what limits its immediate further growth? The lack of iron. By the
hypothesis, the iron-producing industry, like all the other industries
throughout the community, yields only as much iron as is habitually
required for all the purposes to which iron is applied: ship-building being
only one. If, then, extra iron is required for ship-building, the first
effect is to withdraw part of the iron habitually consumed for other
purposes, and to raise the price of iron. Presently, the iron-makers feel
this change and their stocks dwindle. As, however, the quantity of iron
required for ship-building forms but a small part of the total quantity
required for all purposes, the extra demand on the iron-makers can be
nothing like so great in proportion as is the extra demand on the
ship-builders. Whence it follows that there will be much less tendency to
an immediate enlargement of the iron-producing industry; since the extra
quantity will for some time be obtained by working extra hours.
Nevertheless if, as fast as more iron can be thus supplied, the
ship-building industry goes on growing--if, consequently, the iron-makers
experience a permanently-increased demand, and out of their greater profits
get higher interest on capital, as well as pay higher wages; there will
eventually be an abstraction of capital and labour from other industries to
enlarge the iron-producing industry: new blast-furnaces, new rolling-mills,
new cottages for workmen, will be erected. But obviously, the inertia of
capital and labour to be overcome before the iron-producing industry can
grow by a decrease of certain other industries, will prevent its growth
from taking place until long after the increased ship-building industry has
demanded it; and meanwhile, the growth of the ship-building industry must
be limited by the deficiency of iron. A remoter restraint of the same
nature meets us if we go a step further--a restraint which can be overcome
only in a still longer time. For the manufacture of iron depends on the
supply of coal. The production of coal being previously in equilibrium with
the consumption; and the consumption of coal for the manufacture of iron
being but a small part of the total consumption; it follows that a
considerable extension of the iron manufacture, when it at length takes
place, will cause but a comparatively small additional demand on the
coal-owners and coal-miners--a demand which will not, for a long period,
suffice to cause enlargement of the coal-trade, by drawing capital and
labour from other investments and occupations. And until the permanent
extra demand for coal has become great enough to draw from other
investments and occupations sufficient capital and labour to sink new
mines, the increasing production of iron must be restricted by the scarcity
of coal, and the multiplication of ship-yards and ship-builders must be
checked by the want of iron. Thus, in a community which has reached a state
of moving equilibrium, though any one industry directly affected by an
additional demand may rapidly undergo a small extra growth, yet a growth
beyond this, requiring as it does the building-up of subservient
industries, less directly and strongly affected, as well as the partial
unbuilding of other industries, can take place only with comparative
slowness. And a still further growth, requiring structural modifications of
industries still more distantly affected, must take place still more
slowly.
On returning from this analogy, we see more clearly the truth that any
considerable member of an animal organism, cannot be greatly enlarged
without some general reorganization. Besides a building up of the primary,
secondary, and tertiary groups of the subservient parts, there must be an
unbuilding of sundry non-subservient parts; or, at any rate, there must be
permanently established a lower nutrition of such non-subservient parts.
For it must be remembered that in a mature animal, or one which has reached
a balance between assimilation and expenditure, there cannot (supposing
general conditions to remain constant) be an increase in the nutrition of
some organs without a decrease in the nutrition of others; and an organic
establishment of the increase implies an organic establishment of the
decrease--implies more or less change in the processes and structures
throughout the entire system. And here, indeed, is disclosed one reason why
growing animals undergo adaptations so much more readily than adult ones.
For while there is surplus nutrition, it is possible for
specially-exercised parts to be specially enlarged without any positive
deduction from other parts. There is required only that negative deduction
implied in the diminished growth of other parts.
§ 70. Pursuing the argument further, we reach an explanation of the third
general truth; namely that organisms, and species of organisms, which,
under new conditions, have undergone adaptive modifications, soon return to
something like their original structures when restored to their original
conditions. Seeing, as we have done, how excess of action and excess of
nutrition in any part of an organism, must affect action and nutrition in
subservient parts, and these again in other parts, until the re-action has
divided and subdivided itself throughout the organism, affecting in
decreasing degrees the more and more numerous parts more and more remotely
implicated; we see that the consequent changes in the parts remotely
implicated, constituting the great mass of the organism, must be extremely
slow. Hence, if the need for the adaptive modification ceases before the
great mass of the organism has been much altered in its structure by these
ramified but minute reactions, we shall have a condition in which the
specially-modified part is not in equilibrium with the rest. All the
remotely-affected organs, as yet but little changed, will, in the absence
of the perturbing cause, resume very nearly their previous actions. The
parts that depend on them will consequently by and by do the same. Until at
length, by a reversal of the adaptive process, the organ at first affected
will be brought back almost to its original state. Reconsidering the
above-drawn analogy between an organism and a society, will enable us
better to recognize this necessity. If, in the case supposed, the extra
demand for iron ships, after causing the erection of some additional
ship-yards and the drawing of iron from other manufactures, were to cease;
the old dimensions of the ship-building trade would be quickly returned to:
discharged workmen would seek fresh occupations, and the new yards would be
devoted to other uses. But if the increased need for ships lasted long
enough, and became great enough, to cause a flow of capital and labour from
other industries into the iron-manufacture, a falling off in the demand for
ships, would much less rapidly entail a dwindling of the ship-building
industry. For iron being now produced in greater quantity, a diminished
consumption of it for ships would cause a fall in its price, and a
consequent fall in the cost of ships: thus enabling the ship-builders to
meet the competition which we may suppose led to a decrease in the orders
they received. And since, when new blast-furnaces and rolling-mills, &c.,
had been built with capital drawn from other industries, its transference
back into other industries would involve great loss; the owners, rather
than transfer it, would accept unusually low interest, and an excess of
iron would continue to be produced; resulting in an undue cheapness of
ships, and a maintenance of the ship-building industry at a size beyond the
need. Eventually, however, if the number of ships required still
diminished, the production of iron in excess would become very
unremunerative: some of the blast-furnaces would be blown out; and as much
of the capital and labour as remained available would be re-distributed
among other occupations. Without repeating the steps of the argument, it
will be clear that were the enlargement of the ship-building industry great
enough, and did it last long enough to cause an increase in the number of
coal-mines, the ship-building industry would be still better able to
maintain itself under adverse circumstances; but that it would, though at a
more distant period, end by sinking down to the needful dimensions. Thus
our conclusions are:--First, that if the extra growth caused by extra
activity in a particular industry has lasted long enough only to remodel
the proximately-affected industries; it will dwindle away again after a
moderate period, if the need for it disappears. Second, that a long period
must be required before the re-actions produced by an enlarged industry can
cause a re-construction of the whole society, and before the countless
re-distributions of capital and labour can again reach a state of
equilibrium. And third, that only when such a new state of equilibrium is
eventually reached, can the adaptive modification become a permanent one.
How, in animal organisms the like argument holds, need not be pointed out.
The reader will readily follow the parallel.
That organic types should be comparatively stable, might be anticipated on
the hypothesis of Evolution. The structure of any organism being a product
of the almost infinite series of actions and reactions to which ancestral
organisms have been exposed; any unusual actions and reactions brought to
bear on an individual, can have but an infinitesimal effect in permanently
changing the structure of the organism as a whole. The new set of forces,
compounded with all the antecedent sets of forces, can but inappreciably
modify that moving equilibrium of functions which all these antecedent sets
of forces have established. Though there may result a considerable
perturbation of certain functions--a considerable divergence from their
ordinary rhythms--yet the general centre of equilibrium cannot be sensibly
changed. On the removal of the perturbing cause the previous balance will
be quickly restored: the effect of the new forces being almost obliterated
by the enormous aggregate of forces which the previous balance expresses.
§ 71. As thus understood, the phenomena of adaptation fall into harmony
with first principles. The inference that organic types are fixed, because
the deviations from them which can be produced within assignable periods
are relatively small, and because, when a force producing deviation ceases,
there is a return to something like the original state; proves to be an
invalid inference. Without assuming fixity of species, we find good reasons
for anticipating that kind and degree of stability which is observed. We
find grounds for concluding, _a priori_, that an adaptive change of
structure will soon reach a point beyond which further adaptation will be
slow; for concluding that when the modifying cause has been but a short
time in action, the modification generated will be evanescent; for
concluding that a modifying cause acting even for many generations, will do
but little towards permanently altering the organic equilibrium of a race;
and for concluding that on the cessations of such cause, its effects will
become unapparent in the course of a few generations.
CHAPTER VI.
INDIVIDUALITY.
§ 72. What is an individual? is a question which many readers will think it
easy to answer. Yet it is a question that has led to much controversy among
Zoologists and Botanists, and no quite satisfactory reply to it seems
possible. As applied to a man, or to any one of the higher animals, which
are all sharply-defined and independent, the word individual has a clear
meaning: though even here, when we turn from average cases to exceptional
cases--as a calf with two heads and two pairs of fore-limbs--we find
ourselves in doubt whether to predicate one individuality or two. But when
we extend our range of observation to the organic world at large, we find
that difficulties allied to this exceptional one meets us everywhere under
every variety of form.
Each uniaxial plant may perhaps fairly be regarded as a distinct
individual; though there are botanists who do not make even this admission.
What, however, are we to say of a multiaxial plant? It is, indeed, usual to
speak of a tree with its many branches and shoots as singular; but strong
reasons may be urged for considering it as plural. Every one of its axes
has a more or less independent life, and when cut off and planted may grow
into the likeness of its parent; or, by grafting and budding, parts of this
tree may be developed upon another tree, and there manifest their specific
peculiarities. Shall we regard all the growing axes thus resulting from
slips and grafts and buds, as parts of one individual or as distinct
individuals? If a strawberry-plant sends out runners carrying buds at their
ends, which strike root and grow into independent plants that separate from
the original one by decay of the runners, must we not say that they possess
separate individualities; and yet if we do this, are we not at a loss to
say when their separate individualities were established, unless we admit
that each bud was from the beginning an individual? Commenting on such
perplexities Schleiden says--"Much has been written and disputed concerning
the conception of the individual, without, however, elucidating the
subject, principally owing to the misconception that still exists as to the
origin of the conception. Now the individual is no conception, but the mere
subjective comprehension of an actual object, presented to us under some
given specific conception, and on this latter it alone depends whether the
object is or is not an individual. Under the specific conception of the
solar system, ours is an individual: in relation to the specific conception
of a planetary body, it is an aggregate of many individuals." ... "I think,
however, that looking at the indubitable facts already mentioned, and the
relations treated of in the course of these considerations, it will appear
most advantageous and most useful, in a scientific point of view, to
consider the vegetable cell as the general type of the plant (simple plant
of the first order). Under this conception, _Protococcus_ and other plants
consisting of only one cell, and the spore and pollen-granule, will appear
as individuals. Such individuals may, however, again, with a partial
renunciation of their individual independence, combine under definite laws
into definite forms (somewhat as the individual animals do in the globe of
the _Volvox globator_[25]). These again appear empirically as individual
beings, under a conception of a species (simple plants of the second order)
derived from the form of the normal connexion of the elementary
individuals. But we cannot stop here, since Nature herself combines these
individuals, under a definite form, into larger associations, whence we
draw the third conception of the plant, from a connexion, as it were, of
the second power (compound plants--plants of the third order). The simple
plant proceeding from the combination of the elementary individuals is then
termed a bud (_gemma_), in the composition of plants of the third order."
The animal kingdom presents still greater difficulties. When, from sundry
points on the body of a common polype, there bud out young polypes which,
after acquiring mouths and tentacles and closing up the communications
between their stomachs and the stomach of the parent, finally separate from
the parent; we may with propriety regard them as distinct individuals. But
when in the allied compound _Hydrozoa_, we find that these young polypes
continue permanently connected with the parent; and when by this continuous
budding-out there is presently produced a tree-like aggregation, having a
common alimentary canal into which the digestive cavity of each polype
opens; it is no longer so clear that these little sacs, furnished with
mouths and tentacles, are severally to be regarded as distinct individuals.
We cannot deny a certain individuality to the polypedom. And on discovering
that some of the buds, instead of unfolding in the same manner as the rest,
are transformed into capsules in which eggs are developed--on discovering
that certain of the incipient polypes thus become wholly dependent on the
aggregate for their nutrition, and discharge functions which have nothing
to do with their own maintenance, we have still clearer proof that the
individualities of the members are partially merged in the individuality of
the group. Other organisms belonging to the same order, display still more
decidedly this transition from simple individualities to a complex
individuality. In the _Diphyes_ there is a special modification of one or
more members of the polypedom into a swimming apparatus which, by its
rhythmical contractions, propels itself through the water, drawing the
polypedom after it. And in the more differentiated _Physalia_ various
organs result from the metamorphosis of parts which are the homologues of
individual polypes. In this last instance, the individuality of the
aggregate is so predominant that the individualities of its members are
practically lost. This combination of individualities in such way as to
produce a composite individual, meets us in other forms among the
ascidians. While in some of these, as in the _Clavelina_ and in the
_Botryllidæ_, the animals associated are but little subordinated to the
community they form, in others they are so combined as to form a compound
individual. The pelagic ascidian _Doliolum_ is an example. "Here we find a
large individual which swims by contractions of circular muscular bands,
carries a train of smaller individuals attached to a long dorsal process of
the test. These are arranged in three rows: those constituting the lateral
row have wide mouths and no sexual organs or organs of locomotion--they
subserve the nutrition of the colony, a truth which is illustrated by the
fact that as soon as they are properly developed the large individual (the
mother) loses her alimentary canal;" while from the median row are
eventually derived the sexual zoids.
On the hypothesis of Evolution, perplexities of this nature are just such
as we might anticipate. If Life in general commenced with minute and simple
forms, like those out of which all organisms, however complex, now
originate; and if the transitions from these primordial units to organisms
made up of groups of such units, and to higher organisms made up of groups
of such groups took place by degrees; it is clear that individualities of
the first and simplest order would merge gradually in those of a larger and
more complex order, and these again in others of an order having still
greater bulk and organization. Hence it would be impossible to say where
the lower individualities ceased and the higher individualities commenced.
§ 73. To meet these difficulties, it has been proposed that the whole
product of a single fertilized germ shall be regarded as a single
individual; whether such whole product be organized into one mass, or
whether it be organized into many masses that are partially or completely
separate. It is urged that whether the development of the fertilized germ
be continuous or discontinuous (§ 50) is a matter of secondary importance;
that the totality of living tissue to which the fertilized germ gives rise
in any one case, is the equivalent of the totality to which it gives rise
in any other case; and that we must recognize this equivalence, whether
such totality of living tissue takes a concrete or a discrete arrangement.
In pursuance of this view, a zoological individual is constituted either by
any such single animal as a mammal or bird, which may properly claim the
title of a _zoon_, or by any such group of animals as the numerous _Medusæ_
that have been developed from the same egg, which are to be severally
distinguished as _zooids_.
Admitting it to be very desirable that there should be words for expressing
these relations and this equivalence, it may be objected that to apply the
word individual to a number of separate living bodies, is inconvenient:
conflicting so much, as it does, with the ordinary conception which this
word suggests. It seems a questionable use of language to say that the
countless masses of _Anacharis Alsinastrum_ (now _Eloidea canadensis_)
which, within these few years, have grown up in our rivers, canals, and
ponds, are all parts of one individual: and yet as this plant does not seed
in England, these countless masses, having arisen by discontinuous
development, must be so regarded if we accept the above definition.
It may be contended, too, that while it does violence to our established
way of thinking, this mode of interpreting the facts is not without its
difficulties. Something seems to be gained by restricting the application
of the title individual, to organisms which, being in all respects fully
developed, possess the power of producing their kind after the ordinary
sexual method, and denying this title to those incomplete organisms which
have not this power. But the definition does not really establish this
distinction for us. On the one hand, we have cases in which, as in the
working bee, the whole of the germ-product is aggregated into a single
organism; and yet, though an individual according to the definition, this
organism has no power of reproducing its kind. On the other hand, we have
cases like that of the perfect _Aphis_, where the organism is but an
infinitesimal part of the germ product, and yet has that completeness
required for sexual reproduction. Further, it might be urged with some show
of reason, that if the conception of individuality involves the conception
of completeness, then, an organism which possesses an independent power of
reproducing itself, being more complete than an organism in which this
power is dependent on the aid of another organism, is more individual.
§ 74. There is, indeed, as already implied, no definition of individuality
that is unobjectionable. All we can do is to make the best practicable
compromise.
As applied either to an animate or an inanimate object, the word individual
ordinarily connotes union among the parts of the object and separateness
from other objects. This fundamental element in the conception of
individuality, we cannot with propriety ignore in the biological
application of the word. That which we call an individual plant or animal
must, therefore, be some concrete whole and not a discrete whole. If,
however, we say that each concrete living whole is to be regarded as an
individual, we are still met by the question--What constitutes a concrete
living whole? A young organism arising by internal or external gemmation
from a parent organism, passes gradually from a state in which it is an
indistinguishable part of the parent organism to a state in which it is a
separate organism of like structure with the parent. At what stage does it
become an individual? And if its individuality be conceded only when it
completely separates from the parent, must we deny individuality to all
organisms thus produced which permanently retain their connexions with
their parents? Or again, what must we say of the _Hectocotylus_, which is
an arm of the Cuttle-fish that undergoes a special development and then,
detaching itself, lives independently for a considerable period? And what
must we say of the larval nemertine worm the pilidium of which with its
nervous system is left to move about awhile after the developing worm has
dropped out of it?
To answer such questions we must revert to the definition of life. The
distinction between individual in its biological sense, and individual in
its more general sense, must consist in the manifestation of Life, properly
so called. Life we have seen to be, "the definite combination of
heterogeneous change, both simultaneous and successive, in correspondence
with external co-existences and sequences." Hence, a biological individual
is any concrete whole having a structure which enables it, when placed in
appropriate conditions, to continuously adjust its internal relations to
external relations, so as to maintain the equilibrium of its functions. In
pursuance of this conception, we must consider as individuals all those
wholly or partially independent organized masses which arise by
multicentral and multiaxial development that is either continuous or
discontinuous (§ 50). We must accord the title to each separate aphis, each
polype of a polypedom, each bud or shoot of a lowering plant, whether it
detaches itself as a bulbil or remains attached as a branch.
By thus interpreting the facts we do not, indeed, avoid all anomalies.
While, among flowering plants, the power of independent growth and
development is usually possessed only by shoots or axes; yet, in some
cases, as in that of the Begonia-leaf awhile since mentioned, the appendage
of an axis, or even a small fragment of such appendage, is capable of
initiating and carrying on the functions of life; and in other cases, as
shown by M. Naudin in the _Drosera intermedia_, young plants are
occasionally developed from the surfaces of leaves. Nor among forms like
the compound _Hydrozoa_, does the definition enable us to decide where the
line is to be drawn between the individuality of the group and the
individualities of the members: merging into each other, as these do, in
different degrees. But, as before said, such difficulties must necessarily
present themselves if organic forms have arisen by insensible gradations.
We must be content with a course which commits us to the smallest number of
incongruities; and this course is, to consider as an individual any
organized mass which is capable of independently carrying on that
continuous adjustment of inner to outer relations which constitutes Life.
CHAPTER VI^A.
CELL-LIFE AND CELL-MULTIPLICATION.
§ 74a. The progress of science is simultaneously towards simplification and
towards complication. Analysis simplifies its conceptions by resolving
phenomena into their factors, and by then showing how each simple mode of
action may be traced under multitudinous forms; while, at the same time,
synthesis shows how each factor, by cooperation with various other factors
in countless modes and degrees, produces different results innumerable in
their amounts and varieties. Of course this truth holds alike of processes
and of products. Observation and the grouping into classes make it clear
that through multitudinous things superficially unlike there run the same
cardinal traits of structure; while, along with these major unities,
examination discloses innumerable minor diversities.
A concomitant truth, or the same truth under another aspect, is that Nature
everywhere presents us with complexities within complexities, which go on
revealing themselves as we investigate smaller and smaller objects. In a
preceding chapter (§§ 54a, 54b) it was pointed out that each primitive
organism, in common with each of the units out of which the higher and
larger organisms are built, was found a generation ago to consist of
nucleus, protoplasm, and cell-wall. This general conception of a cell
remained for a time the outcome of inquiry; but with the advance of
microscopy it became manifest that within these minute structures processes
and products of an astonishing nature are to be seen. These we have now to
contemplate.
In the passages just referred to it was said that the external layer or
cell-wall is a non-essential, inanimate part produced by the animate
contents. Itself a product of protoplasmic action, it takes no part in
protoplasmic changes, and may therefore here be ignored.
§ 74b. One of the complexities within complexities was disclosed when it
was found that the protoplasm itself has a complicated structure. Different
observers have described it as constituted by a network or reticulum, a
sponge-work, a foam-work. Of these the first may be rejected; since it
implies a structure lying in one plane. If we accept the second we have to
conceive the threads of protoplasm, corresponding to the fibres of the
sponge, as leaving interstices filled either with liquid or solid. They
cannot be filled with a continuous solid, since all motion of the
protoplasm would be negatived; and that their content is not liquid seems
shown by the fact that its parts move about under the form of granules or
microsomes. But the conception of moving granules implies the conception of
immersion in a liquid or semi-liquid substance in which they move--not a
sponge-work of threads but a foam-work, consisting everywhere of septa
interposed among the granules. This is the hypothesis which sundry
microscopists espouse, and which seems mechanically the most feasible: the
only one which consists with the "streaming" of protoplasm. Ordinarily the
name protoplasm is applied to the aggregate mass--the semi-liquid, hyaline
substance and the granules or microsomes it contains.
What these granules or microsomes are--whether, as some have contended,
they are the essential living elements of the protoplasm, or whether, as is
otherwise held, they are nutritive particles, is at present undecided. But
the fact, alleged by sundry observers, that the microsomes often form rows,
held together by intervening substance, seems to imply that these minute
bodies are not inert. Leaving aside unsettled questions, however, one fact
of significance is manifest--an immense multiplication of surfaces over
which inter-action may take place. Anyone who drops into dilute sulphuric
acid a small nail and then drops a pinch of iron filings, will be shown, by
the rapid disappearance of the last and the long continuance of the first,
how greatly the increasing of surfaces by multiplication of fragments
facilitates change. The effect of subdivision in producing a large area in
a small space, is shown in the lungs, where the air-cells on the sides of
which the blood-vessels ramify, are less than 1/100th of an inch in
diameter, while they number 700,000,000. In the composition of every tissue
we see the same principle. The living part, or protoplasm, is divided into
innumerable protoplasts, among which are distributed the materials and
agencies producing changes. And now we find this principle carried still
deeper in the structure of the protoplasm itself. Each microscopic portion
of it is minutely divided in such ways that its threads or septa have
multitudinous contacts with those included portions of matter which take
part in its activities.
Concerning the protoplasm contained in each cell, named by some cytoplasm,
it remains to say that it always includes a small body called the
centrosome, which appears to have a directive function. Usually the
centrosome lies outside the nucleus, but is alleged to be sometimes within
it. During what is called the "resting stage," or what might more properly
be called the growing stage (for clearly the occasional divisions imply
that in the intervals between them there has been increase) the centrosome
remains quiescent, save in the respect that it exercises some coercive
influence on the protoplasm around. This results in the radially-arranged
lines constituting an "aster." What is the nature of the coercion exercised
by the centrosome--a body hardly distinguishable in size from the
microsomes or granules of protoplasm around--is not known. It can scarcely
be a repelling force; since, in a substance of liquid or semi-liquid kind,
this could not produce approximately straight lines. That it is an
attractive force seems more probable; and the nature of the attraction
would be comprehensible did the centrosome augment in bulk with rapidity.
For if integration were in progress, the drawing in of materials might well
produce converging lines. But this seems scarcely a tenable interpretation;
since, during the so-called "resting stage," this star-like structure
exists--exists, that is, while no active growth of the centrosome is going
on.
Respecting this small body we have further to note that, like the cell as a
whole, it multiplies by fission, and that the bisection of it terminates
the resting or growing stage and initiates those complicated processes by
which two cells are produced out of one: the first step following the
fission being the movement of the halves, with their respective completed
asters, to the opposite sides of the nucleus.
§ 74c. With the hypothesis, now general, that the nucleus or kernel of a
cell is its essential part, there has not unnaturally grown up the dogma
that it is always present; but there is reason to think that the evidence
is somewhat strained to justify this dogma.
In the first place, beyond the cases in which the nucleus, though
ordinarily invisible, is said to have been rendered visible by a re-agent,
there are cases, as in the already-named _Archerina_, where no re-agent
makes one visible. In the second place, there is the admitted fact that
some nuclei are diffused; as in _Trachelocerca_ and some other Infusoria.
In them the numerous scattered granules are supposed to constitute a
nucleus: an interpretation obviously biassed by the desire to save the
generalization. In the third place, the nucleus is frequently multiple in
cells of low types; as in some families of Algæ and predominantly among
Fungi. Once more, the so-called nucleus is occasionally a branching
structure scarcely to be called a "kernel."
The facts as thus grouped suggest that the nucleus has arisen in conformity
with the law of evolution--that the primitive protoplast, though not
homogeneous in the full sense, was homogeneous in the sense of being a
uniformly granular protoplasm; and that the protoplasts with diffused
nuclei, together with those which are multi-nucleate, and those which have
nuclei of a branching form, represent stages in that process by which the
relatively homogeneous protoplast passed into the relatively heterogeneous
one now almost universal.
Concerning the structure and composition of the developed nucleus, the
primary fact to be named is that, like the surrounding granular cytoplasm,
it is formed of two distinct elements. It has a groundwork or matrix not
differing much from that of the cytoplasm, and at some periods continuous
with it; and immersed in this it has a special matter named chromatin,
distinguished from its matrix by becoming dyed more or less deeply when
exposed to fit re-agents. During the "resting stage," or period of growth
and activity which comes between periods of division, the chromatin is
dispersed throughout the ground-substance, either in discrete portions or
in such way as to form an irregular network or sponge-work, various in
appearance. When the time for fission is approaching this dispersed
chromatin begins to gather itself together: reaching its eventual
concentration through several stages. By its concentration are produced the
chromosomes, constant in number in each species of plant or animal. It is
alleged that the substance of the chromosomes is not continuous, but
consists of separate elements or granules, which have been named
chromomeres; and it is also alleged that, whether in the dispersed or
integrated form, each chromosome retains its individuality--that the
chromomeres composing it, now spreading out into a network and now uniting
into a worm-like body, form a group which never loses its identity. Be this
as it may, however, the essential fact is that during the growth-period the
chromatin substance is widely distributed, and concentration of it is one
of the chief steps towards a division of the nucleus and presently of the
cell.
During this process of mitosis or karyokinesis, the dispersed chromatin
having passed through the coil-stage, reaches presently the star-stage, in
which the chromosomes are arranged symmetrically about the equatorial plane
of the nucleus. Meanwhile in each of them there has been a preparation for
splitting longitudinally in such way that the halves when separated contain
(or are assumed to contain) equal numbers of the granules or chromomeres,
which some think are the ultimate morphological units of the chromosomes. A
simultaneous change has occurred: there has been in course of formation a
structure known as the _amphiaster_. The two centrosomes which, as before
said, place themselves on opposite sides of the nucleus, become the
terminal poles of a spindle-shaped arrangement of fibres, arising mainly
from the groundwork of the nucleus, now continuous with the groundwork of
the cytoplasm. A conception of this structure may be formed by supposing
that the radiating fibres of the respective asters, meeting one another and
uniting in the intermediate space, thereafter exercise a tractive force;
since it is clear that, while the central fibres of the bundle will form
straight lines, the outer ones, pulling against one another not in straight
lines, will form curved lines, becoming more pronounced in their curvatures
as the distance from the axis increases. That a tractive force is at work
seems inferable from the results. For the separated halves of the split
chromosomes, which now form clusters on the two sides of the equatorial
plane, gradually part company, and are apparently drawn as clusters towards
the opposing centrosomes. As this change progresses the original nucleus
loses its individuality. The new chromosomes, halves of the previous
chromosomes, concentrate to found two new nuclei; and, by something like a
reversal of the stages above described, the chromatin becomes dispersed
throughout the substance of each new nucleus. While this is going on the
cell itself, undergoing constriction round its equator, divides into two.
Many parts of this complex process are still imperfectly understood, and
various opinions concerning them are current. But the essential facts are
that this peculiar substance, the chromatin, at other times existing
dispersed, is, when division is approaching, gathered together and dealt
with in such manner as apparently to insure equal quantities being
bequeathed by the mother-cell to the two daughter-cells.
§ 74d. What is the physiological interpretation of these structures and
changes? What function does the nucleus discharge; and, more especially,
what is the function discharged by the chromatin? There have been to these
questions sundry speculative answers.
The theory espoused by some, that the nucleus is the regulative organ of
the cell, is met by difficulties. One of them is that, as pointed out in
the chapter on "Structure," the nucleus, though morphologically central, is
not central geometrically considered; and that its position, often near to
some parts of the periphery and remote from others, almost of itself
negatives the conclusion that its function is directive in the ordinary
sense of the word. It could not well control the cytoplasm in the same ways
in all directions and at different distances. A further difficulty is that
the cytoplasm when deprived of its nucleus can perform for some time
various of its actions, though it eventually dies without reproducing
itself.
For the hypothesis that the nucleus is a vehicle for transmitting
hereditary characters, the evidence seems strong. When it was shown that
the head of a spermatozoon is simply a detached nucleus, and that its
fusion with the nucleus of an ovum is the essential process initiating the
development of a new organism, the legitimate inference appeared to be that
these two nuclei convey respectively the paternal and maternal traits which
are mingled in the offspring. And when there came to be discerned the
karyokinesis by which the chromatin is, during cell-fission, exactly halved
between the nuclei of the daughter-cells, the conclusion was drawn that the
chromatin is more especially the agent of inheritance. But though, taken by
themselves, the phenomena of fertilization seem to warrant this inference,
the inference does not seem congruous with the phenomena of ordinary
cell-multiplication--phenomena which have nothing to do with fertilization
and the transmission of hereditary characters. No explanation is yielded of
the fact that ordinary cell-multiplication exhibits an elaborate process
for exact halving of the chromatin. Why should this substance be so
carefully portioned out among the cells of tissues which are not even
remotely concerned with propagation of the species? If it be said that the
end achieved is the conveyance of paternal and maternal qualities in equal
degrees to every tissue; then the reply is that they do not seem to be
conveyed in equal degrees. In the offspring there is not a uniform
diffusion of the two sets of traits throughout all parts, but an irregular
mixture of traits of the one with traits of the other.
In presence of these two suggested hypotheses and these respective
difficulties, may we not suspect that the action of the chromatin is one
which in a way fulfils both functions? Let us consider what action may do
this.
§ 74e. The chemical composition of chromatin is highly complex, and its
complexity, apart from other traits, implies relative instability. This is
further implied by the special natures of its components. Various analyses
have shown that it consists of an organic acid (which has been called
nucleic acid) rich in phosphorus, combined with an albuminous substance:
probably a combination of various proteids. And the evidence, as summarised
by Wilson, seems to show that where the proportion of phosphorized acid is
high the activity of the substance is great, as in the heads of
spermatozoa; while, conversely, where the quantity of phosphorus is
relatively small, the substance approximates in character to the cytoplasm.
Now (like sulphur, present in the albuminoid base), phosphorus is an
element which, besides having several allotropic forms, has a great
affinity for oxygen; and an organic compound into which it enters, beyond
the instability otherwise caused, has a special instability caused by its
presence. The tendency to undergo change will therefore be great when the
proportion of the phosphorized component is great. Hence the statement that
"the chemical differences between chromatin and cytoplasm, striking and
constant as they are, are differences of degree only;" and the conclusion
that the activity of the chromatin is specially associated with the
phosphorus.[26]
What, now, are the implications? Molecular agitation results from
decomposition of each phosphorized molecule: shocks are continually
propagated around. From the chromatin, units of which are thus ever falling
into stabler states, there are ever being diffused waves of molecular
motion, setting up molecular changes in the cytoplasm. The chromatin stands
towards the other contents of the cell in the same relation that a
nerve-element stands to any element of an organism which it excites: an
interpretation congruous with the fact that the chromatin is as near to as,
and indeed nearer than, a nerve-ending to any minute structure stimulated
by it.
Several confirmatory facts may be named. During the intervals between
cell-fissions, when growth and the usual cell-activities are being carried
on, the chromatin is dispersed throughout the nucleus into an irregular
network: thus greatly increasing the surface of contact between its
substance and the substances in which it is imbedded. As has been remarked,
this wide distribution furthers metabolism--a metabolism which in this case
has, as we infer, the function of generating, not special matters but
special motions. Moreover, just as the wave of disturbance a nerve carries
produces an effect which is determined, not by anything which is peculiar
in itself, but by the peculiar nature of the organ to which it is
carried--muscular, glandular or other; so here, the waves diffused from the
chromatin do not determine the kinds of changes in the cytoplasm, but
simply excite it: its particular activities, whether of movement,
absorption, or structural excretion, being determined by its constitution.
And then, further, we observe a parallelism between the metabolic changes
in the two cases; for, on the one hand, "diminished staining capacity of
the chromatin [implying a decreased amount of phosphorus, which gives the
staining capacity] occurs during a period of intense constructive activity
in the cytoplasm;" and, on the other hand, in high organisms having nervous
systems, the intensity of nervous action is measured by the excretion of
phosphates--by the using up of the phosphorus contained in nerve-cells.
For thus interpreting the respective functions of chromatin and cytoplasm,
yet a further reason may be given. One of the earliest general steps in the
evolution of the _Metazoa_, is the differentiation of parts which act from
parts which make them act. The _Hydrozoa_ show us this. In the hydroid
stage there are no specialized contractile organs: these are but incipient:
individual ectoderm cells have muscular processes. Nor is there any
"special aggregation of nerve-cells." If any stimulating units exist they
are scattered. But in the _Medusa_-stage nerve-matter is collected into a
ring round the edge of the umbrella. That is to say, in the undeveloped
form such motor action as occurs is not effected by a specialized part
which excites another part; but in the developed form a differentiation of
the two has taken place. All higher types exhibit this differentiation. Be
it muscle or gland or other operating organ, the cause of its activity lies
not in itself but in a nervous agent, local or central, with which it is
connected. Hence, then, there is congruity between the above interpretation
and certain general truths displayed by animal organization at large. We
may infer that in a way parallel to that just indicated, cell-evolution
was, under one of its aspects, a change from a stage in which the exciting
substance and the substance excited were mingled with approximate
uniformity, to a stage in which the exciting substance was gathered
together into the nucleus and finally into the chromosomes: leaving behind
the substance excited, now distinguished as cytoplasm.
§ 74f. Some further general aspects of the phenomena appear to be in
harmony with this interpretation. Let us glance at them.
In Chapters III and IIIA of the First Part, reasons were given for
concluding that in the animal organism nitrogenous substances play the part
of decomposing agents to the carbo-hydrates--that the molecular disturbance
set up by the collapse of a proteid molecule destroys the equilibrium of
sundry adjacent carbo-hydrate molecules, and causes that evolution of
energy which accompanies their fall into molecules of simpler compounds.
Here, if the foregoing argument is valid, we may conclude that this highly
complex phosphorized compound which chromatin contains, plays the same part
to the adjacent nitrogenous compounds as these play to the carbo-hydrates.
If so, we see arising a stage earlier that "general physiological method"
illustrated in § 23f. It was there pointed out that in animal organisms the
various structures are so arranged that evolution of a small amount of
energy in one, sets up evolution of a larger amount of energy in another;
and often this multiplied energy undergoes a second multiplication of like
kind. If this view is tenable, we may now suspect that this method
displayed in the structures of the _Metazoa_ was initiated in the
structures of the _Protozoa_, and consequently characterizes those
homologues of them which compose the _Metazoa_.
When contemplated from the suggested point of view, karyokinesis appears to
be not wholly incomprehensible. For if the chromatin yields the energy
which initiates changes throughout the rest of the cell, we may see why
there eventually arises a process for exact halving of the chromatin in a
mother-cell between two daughter-cells. To make clear the reason, let us
suppose the portioning out of the chromatin leaves one of the two with a
sensibly smaller amount than the other. What must result? Its source of
activity being relatively less, its rate of growth and its energy of action
will be less. If a protozoon, the weaker progeny arising by division of it
will originate an inferior stirp, unable to compete successfully with that
arising from the sister-cell endowed with a larger portion of chromatin. By
continual elimination of the varieties which produce unequal halving,
necessarily at a disadvantage if a moiety of their members tend continually
to disappear, there will be established a variety in which the halving is
exact: the character of this variety being such that all its members aid
the permanent multiplication of the species. If, again, the case is that of
a metazoon, there will be the same eventual result. An animal or plant in
which the chromatin is unequally divided among the cells, must have tissues
of uncertain formation. Assume that an organ has, by survival of the
fittest, been adjusted in the proportions and qualities of its parts to a
given function. If the multiplying protoplasts, instead of taking equal
portions of chromatin, have some of them smaller portions, the parts of the
organ formed of these, developing less rapidly and having inferior
energies, will throw the organ out of adjustment, and the individual will
suffer in the struggle for life. That is to say, irregular division of the
chromatin will introduce a deranging factor and natural selection will weed
out individuals in which it occurs. Of course no interpretation is thus
yielded of the special process known as karyokinesis. Probably other modes
of equal division might have arisen. Here the argument implies merely that
the tendency of evolution is to establish _some_ mode. In verification of
the view that equal division arises from the cause named, it is pointed out
to me that amitosis, which is a negation of mitosis or karyokinesis, occurs
in transitory tissues or diseased tissues or where degeneracy is going on.
But how does all this consist with the conclusion that the chromatin
conveys hereditary traits--that it is the vehicle in which the
constitutional structure, primarily of the species and secondarily of
recent ancestors and parents, is represented? To this question there seems
to be no definite answer. We may say only that this second function is not
necessarily in conflict with the first. While the unstable units of
chromatin, ever undergoing changes, diffuse energy around, they may also be
units which, under the conditions furnished by fertilization, gravitate
towards the organization of the species. Possibly it may be that the
complex combination of proteids, common to chromatin and cytoplasm, is that
part in which the constitutional characters inhere; while the phosphorized
component, falling from its unstable union and decomposing, evolves the
energy which, ordinarily the cause of changes, now excites the more active
changes following fertilization. This suggestion harmonizes with the fact
that the fertilizing substance which in animals constitutes the head of the
spermatozoon, and in plants that of the spermatozoid or antherozoid, is
distinguished from the other agents concerned by having the highest
proportion of the phosphorized element; and it also harmonizes with the
fact that the extremely active changes set up by fertilization are
accompanied by decrease of this phosphorized element. Speculation aside,
however, we may say that the two functions of the chromatin do not exclude
one another, but that the general activity which originates from it may be
but a lower phase of that special activity caused by fertilization.[27]
§ 74g. Here we come unawares to the remaining topic embraced under the
title Cell-Life and Cell-Multiplication. We pass naturally from asexual
multiplication of cells to sexual multiplication--from cell-reproduction to
cell-generation. The phenomena are so numerous and so varied that a large
part of them must be passed over. Conjugation among the _Protophyta_ and
_Protozoa_, beginning with cases in which there is a mingling of the
contents of two cells in no visible respect different from one another, and
developing into a great variety of processes in which they differ, must be
left aside, and attention limited to the terminal process of fertilization
as displayed in higher types of organisms.
Before fertilization there occurs in the ovum an incidental process of a
strange kind--"strange" because it is a collateral change taking no part in
subsequent changes. I refer to the production and extrusion of the "polar
bodies." It is recognized that the formation of each is analogous to
cell-formation in general; though process and product are both dwarfed.
Apart from any ascribed meaning, the fact itself is clear. There is an
abortive cell-formation. Abortiveness is seen firstly in the diminutive
size of the separated body or cell, and secondly in the deficient number of
its chromosomes: a corresponding deficiency being displayed in the group of
chromosomes remaining in the egg--remaining, that is (on the hypothesis
here to be suggested), in the sister-cell, supposing the polar body to be
an aborted cell. It is currently assumed that the end to be achieved by
thus extruding part of the chromosomes, is to reduce the remainder to half
the number characterizing the species; so that when, to this group in the
germ-cell, the sperm-cell brings a similarly-reduced group, union of the
two shall bring the chromosomes to the normal number. I venture to suggest
another interpretation. In doing this, however, I must forestall a
conclusion contained in the next chapter; namely, the conclusion that
gamogenesis begins when agamogenesis is being arrested by unfavourable
conditions, and that the failing agamogenesis initiates the gamogenesis. Of
numerous illustrations to be presently given I will, to make clear the
conception, name only one--the formation of fructifying organs in plants at
times when, and in places where, shoots are falling off in vigour and
leaves in size. Here the successive foliar organs, decreasingly fitted
alike in quality and dimensions for carrying on their normal lives, show us
an approaching cessation of asexual multiplication, ending in the aborted
individuals we call stamens; and the fact that sudden increase of nutrition
while gamogenesis is being thus initiated, causes resumption of
agamogenesis, shows that the gamogenesis is consequent upon the failing
agamogenesis. See then the parallel. On going back from multicellular
organisms to unicellular organisms (or those homologues of them which form
the reproductive agents in multicellular organisms), we find the same law
hold. The polar bodies are aborted cells, indicating that asexual
multiplication can no longer go on, and that the conditions leading to
sexual multiplication have arisen. If this be so, decrease in the chromatin
becomes an initial cause of the change instead of an accompanying incident;
and we need no longer assume that a quantity of precious matter is lost,
not by passive incapacity, but by active expulsion. Another anomaly
disappears. If from the germ-cell there takes place this extrusion of
superfluous chromatin, the implication would seem to be that a parallel
extrusion takes place from the sperm-cell. But this is not true. In the
sperm-cell there occurs just that failure in the production of chromatin
which, according to the hypothesis above sketched out, is to be expected;
for, in the process of cell-multiplication, the cells which become
spermatozoa are _left_ with half the number of chromosomes possessed by
preceding cells: there is actually that impoverishment and declining vigour
here suggested as the antecedent of fertilization. It needs only to imagine
the ovum and the polar body to be alike in size, to see the parallelism;
and to see that obscuration of it arises from the accumulation of cytoplasm
in the ovum.
A test fact remains. Sometimes the first polar body extruded undergoes
fission while the second is being formed. This can have nothing to do with
reducing the number of chromosomes in the ovum. Unquestionably, however,
this change is included with the preceding changes in one transaction,
effected by one influence. If, then, it is irrelevant to the decrease of
chromosomes, so must the preceding changes be irrelevant: the hypothesis
lapses. Contrariwise this fact supports the view suggested above. That
extrusion of a polar body is a process of cell-fission is congruous with
the fact that another fission occurs after extrusion. And that this occurs
irregularly shows that the vital activities, seen in cell-growth and
cell-multiplication, now succeed in producing further fission of the
dwarfed cell and now fail: the energies causing asexual multiplication are
exhausted and there arises the state which initiates sexual multiplication.
Maturation of the ovum having been completed, entrance of the spermatozoon,
sometimes through the limiting membrane and sometimes through a micropyle
or opening in it, takes place. This instantly initiates a series of
complicated changes: not many seconds passing before there begins the
formation of an aster around one end of the spermatozoon-head. The growth
of this aster, apparently by linear rangings of the granules composing the
reticulum of the germ-cell, progresses rapidly; while the whole structure
hence arising moves inward. Soon there takes place the fusion of this
sperm-nucleus with the germ-nucleus to form the cleavage-nucleus, which,
after a pause, begins to divide and subdivide in the same manner as cells
at large: so presently forming a cluster of cells out of which arise the
layers originating the embyro. The details of this process do not concern
us. It suffices to indicate thus briefly its general nature.
And now ending thus the account of genesis under its histological aspect,
we pass to the account of genesis under its wider and more significant
aspects.
CHAPTER VII.
GENESIS.
§ 75. Having, in the last chapter but one, concluded what constitutes an
individual, and having, in the last chapter, contemplated the histological
process which initiates a new individual, we are in a position to deal with
the multiplication of individuals. For this, the title Genesis is here
chosen as being the most comprehensive title--the least specialized in its
meaning. By some biologists Generation has been used to signify one method
of multiplication, and Reproduction to signify another method; and each of
these words has been thus rendered in some degree unfit to signify
multiplication in general.
Here the reader is indirectly introduced to the fact that the production of
new organisms is carried on in fundamentally unlike ways. Up to quite
recent times it was believed, even by naturalists, that all the various
processes of multiplication observable in different kinds of organisms,
have one essential character in common: it was supposed that in every
species the successive generations are alike. It has now been proved,
however, that in many plants and in numerous animals, the successive
generations are not alike; that from one generation there proceeds another
whose members differ more or less in structure from their parents; that
these produce others like themselves, or like their parents, or like
neither; but that eventually, the original form re-appears. Instead of
there being, as in the cases most familiar to us, a constant recurrence of
the same form, there is a cyclical recurrence of the same form. These two
distinct processes of multiplication, may be aptly termed _homogenesis_ and
_heterogenesis_.[28] Under these heads let us consider them.
There are two kinds of homogenesis, the simplest of them, probably once
universal but now exceptional, being that in which there is no other form
of multiplication than one resulting from perpetual spontaneous fission.
The rise of distinct sexes was doubtless a step in evolution, and before it
took place the formation of new individuals could have arisen only by
division of the old, either into two or into many. At present this process
survives, so far as appears, among _Bacteria_, certain _Algæ_, and sundry
_Protozoa_; though it is possible that a rarely-occurring conjugation has
in these cases not yet been observed. It is a probable conclusion, however,
that in the _Bacteria_ at any rate, the once universal mode of
multiplication still survives as an exceptional mode. But now passing over
these cases, we have to note that the kind of genesis (once supposed to be
the sole kind), in which the successive generations are alike, is sexual
genesis, or, as it has been otherwise called--_gamogenesis_. In every
species which multiplies by this kind of homogenesis, each generation
consists of males and females; and from the fertilized germs they produce
the next generation of similar males and females arises: the only needful
qualification of this statement being that in many _Protophyta_ and
_Protozoa_ the conjugating cells or protoplasts are not distinguishable in
character. This mode of propagation has the further trait, that each
fertilized germ usually gives rise to but one individual--the product of
development is organized round one axis and not round several axes,
Homogenesis in contrast with heterogenesis as exhibited in species which
display distinct sexuality, has also the characteristic that each new
individual begins as an egg detached from the maternal tissues, instead of
being a portion of protoplasm continuous with them, and that its
development proceeds independently. This development may be carried on
either internally or externally; whence results the division into the
oviparous and the viviparous. The oviparous kind is that in which the
fertilized germ is extruded from the parent before it has undergone any
considerable development. The viviparous kind is that in which development
is considerably advanced, or almost completed, before extrusion takes
place. This distinction is, however, not a sharply-defined one: there are
transitions between the oviparous and the viviparous processes. In
ovo-viviparous genesis there is an internal incubation; and though the
young are in this case finally extruded from the parent in the shape of
eggs, they do not leave the parent's body until after they have assumed
something like the parental form. Looking around, we find that homogenesis
is universal among the _Vertebrata_. Every vertebrate animal arises from a
fertilized germ, and unites into its single individuality the whole product
of this fertilized germ. In the mammals or highest _Vertebrata_, this
homogenesis is in every case viviparous; in birds it is uniformly
oviparous; and in reptiles and fishes it is always essentially oviparous,
though there are cases of the kind above referred to, in which viviparity
is simulated. Passing to the _Invertebrata_, we find oviparous homogenesis
universal among the _Arachnida_ (except the Scorpions, which are
ovo-viviparous); universal among the higher _Crustacea_, but not among the
lower; extremely general, though not universal, among Insects; and
universal among the higher _Mollusca_ though not among the lower. Along
with extreme inferiority among animals, we find homogenesis to be the
exception rather than the rule; and in the vegetal kingdom there appear to
be no cases, except among the _Algæ_ and a few aberrant parasites like the
_Rafflesiaceæ_, in which the centre or axis which arises from a fertilized
germ becomes the immediate producer of fertilized germs.
In propagation characterized by unlikeness of the successive generations,
there is asexual genesis with occasionally-recurring sexual genesis; in
other words--_agamogenesis_ interrupted more or less frequently by
_gamogenesis_. If we set out with a generation of perfect males and
females, then, from their ova arise individuals which are neither males nor
females, but which produce the next generation from buds. By this method of
multiplication many individuals originate from a single fertilized germ.
The product of development is organized round more than one centre or axis.
The simplest form of heterogenesis is that seen in most uniaxial plants.
If, as we find ourselves obliged to do, we regard each separate shoot or
axis of growth as a distinct individual, homogenesis is seen in those which
have absolutely terminal flowers; but in all other uniaxial plants, the
successive individuals are not represented by the series A, A, A, A, &c.,
but they are represented by the series A, B, A, B, A, B, &c. For in the
majority of plants which were classed as uniaxial (§ 50), and which may be
conveniently so distinguished from other plants, the axis which shoots up
from the seed, and substantially constitutes the plant, does not itself
flower but gives lateral origin to flowering axes. Though in ordinary
uniaxial plants the fructifying apparatus _appears_ to be at the end of the
primary, vertical axis; yet dissection shows that, morphologically
considered, each fructifying axis is an offspring from the primary axis.
There arises from the seed a sexless individual, from which spring by
gemmation individuals having reproductive organs; and from these there
result fertilized germs or seeds that give rise to sexless individuals.
That is to say, gamogenesis and agamogenesis alternate: the peculiarity
being that the sexual individuals arise from the sexless ones by continuous
development. The _Salpæ_ show us an allied form of heterogenesis in the
animal kingdom. Individuals developed from fertilized ova, instead of
themselves producing fertilized ova, produce, by gemmation, strings of
individuals from which fertilized ova again originate. In multiaxial
plants, we have a succession of generations represented by the series A, B,
B, B, &c., A, B, B, B, &c. Supposing A to be a flowering axis or sexual
individual, then, from any fertilized germ it casts off, there grows up a
sexless individual, B; from this there bud-out other sexless individuals,
B, and so on for generations more or less numerous, until at length, from
some of these sexless individuals, there bud-out seed-bearing individuals
of the original form A. Branched herbs, shrubs, and trees, exhibit this
form of heterogenesis: the successive generations of sexless individuals
thus produced being, in most cases, continuously developed, or aggregated
into a compound individual, but being in some cases discontinuously
developed. Among animals a kind of heterogenesis represented by the same
succession of letters, occurs in such compound polypes as the _Sertularia_,
and in those of the _Hydrozoa_ which assume alternately the polypoid form
and the form of the _Medusa_. The chief differences presented by these
groups arise from the fact that the successive generations of sexless
individuals produced by budding, are in some cases continuously developed,
and in others discontinuously developed; and from the fact that, in some
cases, the sexual individuals give off their fertilized germs while still
growing on the parent-polypedom, but in other cases not until after leaving
the parent-polypedom and undergoing further development. Where, as in all
the foregoing kinds of agamogenesis, the new individuals bud out, not from
any specialized reproductive organs but from unspecialized parts of the
parent, the process has been named, by Prof. Owen, _metagenesis_. In most
instances the individuals thus produced grow from the outsides of the
parents--the metagenesis is external. But there is also a kind of
metagenesis which we may distinguish as internal. Certain _entozoa_ of the
genus _Distoma_ exhibit it. From the egg of a _Distoma_ there results a
rudely-formed creature known as a sporocyst and from this a redia.
Gradually, as this divides and buds, the greater part of the inner
substance is transformed into young animals called _Cercariæ_ (which are
the larvæ of _Distomata_); until at length it becomes little more than a
living sac full of living offspring. In the _Distoma pacifica_, the brood
of young animals thus arising by internal gemmation are not _Cercariæ_, but
are like their parent: themselves becoming the producers of _Cercariæ_,
after the same manner, at a subsequent period. So that now the succession
of forms is represented by the series A, B, A, B, &c., now by the series A,
B, B, A, B, B, &c., and now by A, B, B, C, A. Both cases, however,
exemplify internal metagenesis in contrast with the several kinds of
external metagenesis described above. That agamogenesis which is carried on
in a reproductive organ--either an ovarium or the homologue of one--has
been called, by Prof. Owen, _parthenogenesis_. It is the process familiarly
exemplified in the _Aphides_. Here, from the fertilized eggs laid by
perfect females there grow up imperfect females, in the ovaria of which are
developed ova that though unfertilized, rapidly assume the organization of
other imperfect females, and are born viviparously. From this second
generation of imperfect females, there by-and-by arises, in the same
manner, a third generation of the same kind; and so on for many
generations: the series being thus symbolized by the letters A, B, B, B, B,
B, &c., A. Respecting this kind of heterogenesis it should be added that,
in animals as in plants, the number of generations of sexless individuals
produced before the re-appearance of sexual ones, is indefinite; both in
the sense that in the same species it may go on to a greater or less extent
according to circumstances, and in the sense that among the generations of
individuals proceeding from the same fertilized germ, a recurrence of
sexual individuals takes place earlier in some of the diverging lines of
multiplication than in others. In trees we see that on some branches
flower-bearing axes arise while other branches are still producing only
leaf-bearing axes; and in the successive generations of _Aphides_ a
parallel fact has been observed. Lastly has to be set down that kind of
heterogenesis in which, along with gamogenesis, there occurs a form of
agamogenesis exactly like it, save in the absence of fecundation. This is
called true parthenogenesis--reproduction carried on by virgin mothers
which are in all respects like other mothers. Among silk-worm-moths this
parthenogenesis is exceptional rather than ordinary. Usually the eggs of
these insects are fertilized; but if they are not they are still laid, and
some of them produce larvæ. In certain _Lepidoptera_, however, of the
groups _Psychidæ_ and _Tineidæ_, parthenogenesis appears to be a normal
process--indeed, so far as is known, the only process; for of some species
the males have never been found.
A general conception of the relations among the different modes of Genesis,
thus briefly described, will be best given by the following tabular
statement.
GENESIS is
{ Oviparous
{ or
Homogenesis, which is usually Gamogenesis { Ovo-viviparous
{ or
{ Viviparous
or
{ Gamogenesis
{ alternating
Heterogenesis, which is { with { Parthenogenesis
{ Agamogenesis { or { Internal
{ Metagenesis { or
{ External
This, like all other classifications of such phenomena, presents anomalies.
It may be justly objected that the processes here grouped under the head
agamogenesis, are the same as those before grouped under the head of
discontinuous development (§ 50): thus making development and genesis
partially coincident. Doubtless it seems awkward that what are from one
point of view considered as structural changes are from another point of
view considered as modes of multiplication.[29] There is, however, nothing
for us but a choice of imperfections. We cannot by any logical dichotomies
accurately express relations which, in Nature, graduate into one another
insensibly. Neither the above, nor any other scheme, can do more than give
an approximate idea of the truth.
§ 76. Genesis under every form is a process of negative or positive
disintegration; and is thus essentially opposed to that process of
integration which is the primary process in individual evolution. Negative
disintegration occurs in those cases where, as among the compound
_Hydrozoa_, there is a continuous development of new individuals by budding
from the bodies of older individuals; and where the older individuals are
thus prevented from growing to a greater size, or reaching a higher degree
of integration. Positive disintegration occurs in those forms of
agamogenesis where the production of new individuals is discontinuous, as
well as in all cases of gamogenesis. The degrees of disintegration are
various. At the one extreme the parent organism is completely broken up, or
dissolved into new individuals; and at the other extreme each new
individual forms but a small deduction from the parent organism. _Protozoa_
and _Protophyta_ show us that form of disintegration called spontaneous
fission: two or more individuals being produced by the splitting-up of the
original one. The _Volvox_ and the _Hydrodictyon_ are plants which, having
developed broods within themselves, give them exit by bursting; and among
animals the one lately referred to which arises from the _Distoma_ egg,
entirely loses its individuality in the individualities of the numerous
_Distoma_-larvæ with which it becomes filled. Speaking generally, the
degree of disintegration becomes less marked as we approach the higher
organic forms. Plants of superior types throw off from themselves, whether
by gamogenesis or agamogenesis, parts that are relatively small; and among
superior animals there is no case in which the parent individuality is
habitually lost in the production of new individuals. To the last, however,
there is of necessity a greater or less disintegration. The seeds and
pollen-grains of a flowering plant are disintegrated portions of tissue; as
are also the ova and spermatozoa of animals. And whether the fertilized
germs carry away from their parents small or large quantities of nutriment,
these quantities in all cases involve further negative or positive
disintegrations of the parents.
Except in spore-producing plants, new individuals which result from
agamogenesis usually do not separate from the parent-individuals until they
have undergone considerable development, if not complete development. The
agamogenetic offspring of those lowest organisms which develop centrally,
do not, of course, pass beyond central structure; but the agamogenetic
offspring of organisms which develop axially, commonly assume an axial
structure before they become independent. The vegetal kingdom shows us this
in the advanced organization of detached bulbils, and of buds that root
themselves before separating. Of animals, the _Hydrozoa_, the _Trematoda_,
and the _Salpæ_, present us with different kinds of agamogenesis, in all of
which the new individuals are organized to a considerable extent before
being cast off. This rule is not without exceptions, however. The
statoblasts of the _Plumatella_ (which play the part of winter eggs),
developed in an unspecialized part of the body, furnish a case of
metagenesis in which centres of development, instead of axes, are detached;
and in the above-described parthenogenesis of moths and bees, such centres
are detached from an ovarium.
When produced by gamogenesis, the new individuals become (in a
morphological sense) independent of the parents while still in the shape of
centres of development, rather than axes of development; and this even
where the reverse is apparently the case. The fertilized germs of those
inferior plants which are central, or multicentral, in their development,
are of course thrown off as centres; and the same is usually the case even
in those which are uniaxial or multiaxial. In the higher plants, of the two
elements that go to the formation of the fertilized germ, the pollen-cell
is absolutely separated from the parent-plant under the shape of a centre,
and the egg-cell, though not absolutely separated from the parent, is still
no longer subordinate to the organizing forces of the parent. So that when,
after the egg-cell has been fertilized by matter from the pollen-tube, the
development commences, it proceeds without parental control: the new
individual, though remaining physically united with the old individual,
becomes structurally and functionally separate: the old individual doing no
more than supply materials. Throughout the animal kingdom, the new
individuals produced by gamogenesis are obviously separated in the shape of
centres of development wherever the reproduction is oviparous: the only
conspicuous variation being in the quantity of nutritive matter bequeathed
by the parent at the time of separation. And though, where the reproduction
is viviparous, the process appears to be different, and in one sense is so,
yet, intrinsically, it is the same. For in these cases the new individual
really detaches itself from the parent while still only a centre of
development; but instead of being finally cast off in this state it is
re-attached, and supplied with nutriment until it assumes a more or less
complete axial structure.
§ 77. As we have lately seen, the essential act in gamogenesis is the union
of two cell-nuclei, produced in the great majority of cases by different
parent organisms. Nearly always the containing cells, often called
_gametes_, are unlike: the sperm-cell being the male product, and the
germ-cell the female. But among some _Protozoa_ and many of the lower
_Algæ_ and _Fungi_, the uniting cells show no differentiation. Sexuality is
only nascent.
There are very many modes and modifications of modes in which these cells
are produced; very many modes and modifications of modes by which they are
brought into contact; and very many modes and modifications of modes by
which the resulting fertilized germs have secured to them the fit
conditions for their development. But passing over these divergent and
re-divergent kinds of sexual multiplication, which it would take too much
space here to specify, the one universal trait is this coalescence of a
detached portion of one organism with a more or less detached portion of
another.
Such simple _Algæ_ as the _Desmidieæ_, which are sometimes called
unicellular plants, show us a coalescence, not of detached portions of two
organisms, but of two entire organisms: the entire contents of the
individuals uniting to form the germ-mass. Where, as among the
_Confervoideæ_, we have aggregated cells whose individualities are scarcely
at all subordinate to that of the aggregate, the gamogenetic act is often
effected by the union "of separate motile protoplasmic masses produced by
the division of the contents of any cell of the aggregate. These
free-swimming masses of protoplasm, which are quite similar to (but
generally smaller than) the agamogenetic 'zoospores' of the same plants,
and to the free-swimming individuals of many _Protophyta_, are apparently
the primitive type of gametes (conjugating cells); but it is noteworthy
that such a gamete nearly always unites with one derived from another cell
or from another individual. The same fact holds with regard to the gametes
of the Protophytes themselves, which are formed in the same way from the
single cell of the mother individual. In the higher types of
_Confervoideæ_, and in _Vaucheria_, we find these equivalent,
free-swimming, gametes replaced by sexually differentiated sperm- and
germ-cells, in some cases arising in different organs set apart for their
production, and essentially representing those found in the higher plants.
Transitional forms, intermediate between these and the cases where
equivalent gametes are formed from any cell of the plant are also known."
Recent investigations concerning the conjugation of _Protozoa_ have shown
that there is not, as was at one time thought, a fusion of two
individualities, but a fusion of parts of their nuclei. The macro-nucleus
having disappeared, and the micro-nucleus having broken up into portions,
each individual receives from the other one of these portions, which
becomes fused with its own nuclear matter. So that even in these humble
forms, where there is no differentiation of sexes, the union is not between
elements that have arisen in the same individual but between those which
have arisen in different individuals: the parts being in this case alike.
The marvellous phenomena initiated by the meeting of sperm-cell and
germ-cell, or rather of their nuclei, naturally suggest the conception of
some quite special and peculiar properties possessed by these cells. It
seems obvious that this mysterious power which they display of originating
a new and complex organism, distinguishes them in the broadest way from
portions of organic substance in general. Nevertheless, the more we study
the evidence the more are we led towards the conclusion that these cells
are not fundamentally different from other cells. The first fact which
points to this conclusion is the fact recently dwelt upon (§ 63), that in
many plants and inferior animals, a small fragment of tissue which is but
little differentiated, is capable of developing into an organism like that
from which it was taken. This implies that the component units of tissues
have inherent powers of arranging themselves into the forms of the
organisms which originated them. And if in these component units, which we
distinguished as physiological, such powers exist,--if, under fit
conditions, and when not much specialized, they manifest such powers in a
way as marked as that in which the contents of sperm-cells and germ-cells
manifest them; then, it becomes clear that the properties of sperm-cells
and germ-cells are not so peculiar as we are apt to assume. Again, the
organs emitting sperm-cells and germ-cells have none of the specialities of
structure which might be looked for, did sperm-cells and germ-cells need
endowing with properties unlike those of all other organic agents. On the
contrary, these reproductive centres proceed from tissues characterized by
their low organization. In plants, for example, it is not appendages that
have acquired considerable structure which produce the fructifying
particles: these arise at the extremities of the axes where the degree of
structure is the least. The cells out of which come the egg and the
pollen-grains, are formed from undifferentiated tissue in the interior of
the ovule and of the stamen. Among many inferior animals devoid of special
reproductive organs, such as the _Hydra_, the ova and spermatozoa originate
from the interstitial cells of the ectoderm, which lie among the bases of
the functional cells--have not been differentiated for function; and in the
_Medusæ_, according to Weismann, they arise in the homologous layer, save
where the medusoid form remains attached, and then they arise in the
endoderm and migrate to the ectoderm: lack of specialization being in all
cases implied. Then in the higher animals these same generative agents
appear to be merely modified epithelium-cells--cells not remarkable for
their complexity of structure but rather for their simplicity. If, by way
of demurrer to this view, it be asked why other epithelium-cells do not
exhibit like properties; there are two replies. The first is that other
epithelium-cells are usually so far changed to fit them to their special
functions that they are unfitted for assuming the reproductive function.
The second is that in some cases, where they are but little specialized,
they _do_ exhibit the like properties: not, indeed, by uniting with other
cells to produce new germs but by producing new germs without such union. I
learn from Dr. Hooker that the _Begonia phyllomaniaca_ habitually develops
young plants from the scales of its stem and leaves--nay, that many young
plants are developed by a single scale. The epidermal cells composing one
of these scales swell, here and there, into large globular cells; form
chlorophyll in their interiors; shoot out rudimentary axes; and then, by
spontaneous constrictions, cut themselves off; drop to the ground; and grow
into Begonias. Moreover, in a succulent English plant, the _Malaxis
paludosa_, a like process occurs: the self-detached cells being, in this
case, produced by the surfaces of the leaves.[30] Thus, there is no
warrant for the assumption that sperm-cells and germ-cells possess powers
fundamentally unlike those of other cells. The inference to which the facts
point, is, that they differ from the rest mainly in not having undergone
functional adaptations. They are cells which have departed but little from
the original and most general type: such specializations as some of them
exhibit in the shape of locomotive appliances, being interpretable as
extrinsic modifications which have reference to nothing beyond certain
mechanical requirements. Sundry facts tend likewise to show that there does
not exist the profound distinction we are apt to assume between the male
and female reproductive elements. In the common polype sperm-cells and
germ-cells are developed in the same layer of indifferent tissue; and in
_Tethya_, one of the sponges, Prof. Huxley has observed that they occur
mingled together in the general parenchyma. The pollen-grains and
embryo-cells of plants arise in adjacent parts of the meristematic tissue
of the flower-bud; and from the description of a monstrosity in the
Passion-flower, recently given by Mr. Salter to the Linnæan Society, it
appears both that ovules may, in their general structure, graduate into
anthers, and that they may produce pollen in their interiors. Moreover,
among the lower _Algæ_, which show the beginning of sexual differentiation,
the smaller gametes, which we must regard as incipient sperm-cells, are
sometimes able to fuse _inter se_, and give rise to a zygote which will
produce a new plant. All which evidence is in perfect harmony with the
foregoing conclusion; since, if sperm-cells and germ-cells have natures not
essentially unlike those of unspecialized cells in general, their natures
cannot be essentially unlike each other.
The next general fact to be noted is that these cells whose union
constitutes the essential act of gamogenesis, are cells in which the
developmental changes have come to a close--cells which are incapable of
further evolution. Though they are not, as many cells are, unfitted for
growth and metamorphosis by being highly specialized, yet they have lost
the power of growth and metamorphosis. They have severally reached a state
of equilibrium. And while the internal balance of forces prevents a
continuance of constructive changes, it is readily overthrown by external
destructive forces. For it almost uniformly happens that sperm-cells and
germ-cells which are not brought in contact disappear. In a plant, the
egg-cell, if not fertilized, is absorbed or dissipated, while the ovule
aborts; and the unimpregnated ovum eventually decomposes: save, indeed, in
those types in which parthenogenesis is a part of the normal cycle.
Such being the characters of these cells, and such being their fates if
kept apart, we have now to observe what happens when they are united. In
plants the extremity of the elongated pollen-cell applies itself to the
surface of the embryo-sac, and one of its nuclei having, with some
protoplasm, passed into the egg-cell, there becomes fused with the nucleus
of the egg-cell. Similarly in animals, the spermatozoon passes through the
limiting membrane of the ovum, and a mixture takes place between the
substance of its nucleus and the substance of the nucleus of the ovum. But
the important fact which it chiefly concerns us to notice, is that on the
union of these reproductive elements there begins, either at once or on the
return of favourable conditions, a new series of developmental changes. The
state of equilibrium at which each had arrived is destroyed by their mutual
influence, and the constructive changes, which had come to a close,
recommence. A process of cell-multiplication is set up; and the resulting
cells presently begin to aggregate into the rudiment of a new organism.
Thus, passing over the variable concomitants of gamogenesis, and confining
our attention to what is constant in it, we see:--that there is habitually,
if not universally, a fusion of two portions of organic substance which are
either themselves distinct individuals, or are thrown off by distinct
individuals; that these portions of organic substance, which are severally
distinguished by their low degree of specialization, have arrived at states
of structural quiescence or equilibrium; that if they are not united this
equilibrium ends in dissolution; but that by the mixture of them this
equilibrium is destroyed and a new evolution initiated.
§ 78. What are the conditions under which Genesis takes place? How does it
happen that some organisms multiply by homogenesis and others by
heterogenesis? Why is it that where agamogenesis prevails it is usually
from time to time interrupted by gamogenesis? A survey of the facts
discloses certain correlations which, if not universal, are too general to
be without significance.
Where multiplication is carried on by heterogenesis we find, in numerous
cases, that agamogenesis continues as long as the forces which result in
growth are greatly in excess of the antagonist forces. Conversely, we find
that the recurrence of gamogenesis takes place when the conditions are no
longer so favourable to growth. In like manner where there is homogenetic
multiplication, new individuals are usually not formed while the preceding
individuals are still rapidly growing--that is, while the forces producing
growth exceed the opposing forces to a great extent; but the formation of
new individuals begins when nutrition is nearly equalled by expenditure. A
few out of the many facts which seem to warrant these inductions must
suffice.
The relation in plants between fructification and innutrition (or rather,
between fructification and such diminished nutrition as makes growth
relatively slow) was long ago asserted by a German biologist--Wolff, I am
told. Since meeting with this assertion I have examined into the facts for
myself. The result has been a conviction, strengthened by every inquiry,
that some such relation exists. Uniaxial plants begin to produce their
lateral, flowering axes, only after the main axis has developed the great
mass of its leaves, and is showing its diminished nutrition by smaller
leaves, or shorter internodes, or both. In multiaxial plants two, three, or
more generations of leaf-bearing axes, or sexless individuals, are produced
before any seed-bearing individuals show themselves. When, after this first
stage of rapid growth and agamogenetic multiplication, some gamogenetic
individuals arise, they do so where the nutrition is least;--not on the
main axis, or on secondary axes, or even on tertiary axes, but on axes that
are the most removed from the channels which supply nutriment. Again, a
flowering axis is commonly less bulky than the others: either much shorter
or, if long, much thinner. And further, it is an axis of which the terminal
internodes are undeveloped: the foliar organs, which instead of becoming
leaves become sepals, and petals, and stamens, follow each other in close
succession, instead of being separated by portions of the still-growing
axis. Another group of evidences meets us when we observe the variations of
fruit-bearing which accompany variations of nutrition in the plant regarded
as a whole. Besides finding, as above, that gamogenesis commences only when
growth has been checked by extension of the remoter parts to some distance
from the roots, we find that gamogenesis is induced at an earlier stage
than usual by checking the nutrition. Trees are made to fruit while still
quite small by cutting their roots or putting them into pots; and luxuriant
branches which have had the flow of sap into them diminished, by what
gardeners call "ringing," begin to produce flower-shoots instead of
leaf-shoots. Moreover, it is to be remarked that trees which, by flowering
early in the year, seem to show a direct relation between gamogenesis and
increasing nutrition, really do the reverse; for in such trees the
flower-buds are formed in the autumn. That structure which determines these
buds into sexual individuals is given when the nutrition is declining.
Conversely, very high nutrition in plants prevents, or arrests,
gamogenesis. It is notorious that unusual richness of soil, or too large a
quantity of manure, results in a continuous production of leaf-bearing or
sexless shoots; and a like result happens when the cutting down of a tree,
or of a large part of it, is followed by the sending out of new shoots:
these, supplied with excess of sap, are luxuriant and sexless. Besides
being prevented from producing sexual individuals by excessive nutrition,
plants are, by excessive nutrition, made to change the sexual individuals
they were about to produce, into sexless ones. This arrest of gamogenesis
may be seen in various stages. The familiar instance of flowers made barren
by the transformation of their stamens into petals, shows us the lowest
degree of this reversed metamorphosis. Where the petals and stamens are
partially changed into green leaves, the return towards the agamogenetic
structure is more marked; and it is still more marked when, as occasionally
happens in luxuriantly-growing plants, new flowering axes, and even
leaf-bearing axes, grow out of the centres of flowers.[31] The anatomical
structure of the sexual axis affords corroborative evidence: giving the
impression, as it does, of an aborted sexless axis. Besides lacking those
internodes which the leaf-bearing axis commonly possesses, the flowering
axis differs by the absence of rudimentary lateral axes. In a leaf-bearing
shoot the axil of every leaf usually contains a small bud, which may or may
not develop into a lateral shoot; but though the petals of a flower are
homologous with leaves, they do not bear homologous buds at their bases.
Ordinarily, too, the foliar appendages of sexual axes are much smaller than
those of sexless ones--the stamens and pistils especially, which are the
last formed, being extremely dwarfed; and it may be that the absence of
chlorophyll from the parts of fructification is a fact of like meaning.
Moreover, the formation of the seed-vessel appears to be a direct
consequence of arrested nutrition. If a gloved-finger be taken to represent
a growing shoot, (the finger standing for the pith of the shoot and the
glove for the peripheral layers of meristem and young tissue, in which the
process of growth takes place); and if it be supposed that there is a
diminished supply of material for growth; then, it seems a fair inference
that growth will first cease at the apex of the axis, represented by the
end of the glove-finger; and supposing growth to continue in those parts of
the peripheral layers of young tissue that are nearer to the supply of
nutriment, their further longitudinal extension will lead to the formation
of a cavity at the extremity of the shoot, like that which results in a
glove-finger when the finger is partially withdrawn and the glove sticks to
its end. Whence it seems, both that this introversion of the apical
meristem may be considered as due to failing nutrition, and that the ovules
growing from its introverted surface (which would have been its outer
surface but for the defective nutrition) are extremely aborted homologues
of external appendages: both they and the pollen-grains being either
morphologically or literally quite terminal, and the last showing by their
dehiscence the exhaustion of the organizing power.[32]
Those kinds of animals which multiply by heterogenesis, present us with a
parallel relation between the recurrence of gamogenesis and the recurrence
of conditions checking rapid growth: at least, this is shown where
experiments have thrown light on the connexion of cause and effect; namely,
among the _Aphides_. These creatures, hatched from eggs in the spring,
multiply by agamogenesis, which in this case is parthenogenesis, throughout
the summer. When the weather becomes cold and plants no longer afford
abundant sap, perfect males and females are produced; and from gamogenesis
result fertilized ova. But beyond this evidence we have much more
conclusive evidence. For it has been shown, both that the rapidity of the
agamogenesis is proportionate to the warmth and nutrition, and that if the
temperature and supply of food be artificially maintained, the agamogenesis
continues through the winter. Nay more--it not only, under these
conditions, continues through one winter, but it has been known to continue
for four successive years: some forty or fifty sexless generations being
thus produced. And those who have investigated the matter see no reason to
doubt the indefinite continuance of this agamogenetic multiplication, so
long as the external requirements are duly met. Evidence of another kind,
complicated by special influences, is furnished by the heterogenesis of the
_Daphnia_--a small crustacean commonly known as the Water-flea, which
inhabits ponds and ditches. From the nature of its habitat this little
creature is exposed to very variable conditions. Besides being frozen in
winter, the small bodies of water in which it lives are often unduly heated
by the summer Sun, or dried up by continued drought. The circumstances
favourable to the _Daphnia's_ life and growth, being thus liable to
interruptions which, in our climate, have a regular irregularity of
recurrence; we may, in conformity with the hypothesis, expect to find both
that the gamogenesis recurs along with declining physical prosperity and
that its recurrence is very variable. I use the expression "declining
physical prosperity" advisedly; since "declining nutrition," as measured by
supply of food, does not cover all the conditions. This is shown by the
experiments of Weismann (abstracted for me by Mr. Cunningham) who found
that in various _Daphnideæ_ which bring forth resting eggs, sexual and
asexual reproduction go on simultaneously, as well as separately, in the
spring and summer: these variable results being adapted to variable
conditions. For not only are these creatures liable to die from lack of
food, from the winter's cold, and from the drying up of their ditches, &c.,
as well as from the over-heating of them, but during this period of
over-heating they are liable to die from that deoxygenation of the water
which heat causes. Manifestly the favourable and unfavourable conditions
recurring in combinations that are rarely twice alike, cannot be met by any
regularly recurring form of heterogenesis; and it is interesting to see how
survival of the fittest has established a mixed form. In the spring, as
well as in the autumn, there is in some cases a formation of resting or
winter eggs; and evidently these provide against the killing off of the
whole population by summer drought. Meanwhile, by ordinary males and
females there is a production of summer eggs adapted to meet the incident
of drying up by drought and subsequent re-supply of water. And all along
successive generations of parthenogenetic females effect a rapid
multiplication as long as conditions permit. Since life and growth are
impeded or arrested not by lack of food only, but by other unfavourable
conditions, we may understand how change in one or more of these may set up
one or other form of genesis, and how the mixture of them may cause a mixed
mode of multiplication which, originally initiated by external causes,
becomes by inheritance and selection a trait of the species.[33] And then
in proof that external causes initiate these peculiarities, we have the
fact that in certain _Daphnideæ_ "which live in places where existence and
parthenogenesis are possible throughout the year, the sexual period has
disappeared:" there are no males.
Passing now to animals which multiply by homogenesis--animals in which the
whole product of a fertilized germ aggregates round a single centre or axis
instead of round many centres or axes--we see, as before, that so long as
the conditions allow rapid increase in the mass of this germ-product, the
formation of new individuals by gamogenesis does not take place. Only when
growth is declining in relative rate, do perfect sperm-cells and germ-cells
begin to appear; and the fullest activity of the reproductive function
arises as growth ceases: speaking generally, at least; for though this
relation is tolerably definite in the highest orders of animals which
multiply by gamogenesis, it is less definite in the lower orders. This
admission does not militate against the hypothesis, as it seems to do; for
the indefiniteness of the relation occurs where the limit of growth is
comparatively indefinite. We saw (§ 46) that among active, hot-blooded
creatures, such as mammals and birds, the inevitable balancing of
assimilation by expenditure establishes, for each species, an almost
uniform adult size; and among creatures of these kinds (birds especially,
in which this restrictive effect of expenditure is most conspicuous), the
connexion between cessation of growth and commencement of reproduction is
distinct. But we also saw (§ 46) that where, as in the Crocodile and the
Pike, the conditions and habits of life are such that expenditure does not
overtake assimilation as size increases, there is no precise limit of
growth; and in creatures thus circumstanced we may naturally look for a
comparatively indeterminate relation between declining growth and
commencing reproduction.[34] There is, indeed, among fishes, at least one
case which appears very anomalous. The male parr, or young of the male
salmon, a fish of four or five inches in length, is said to produce milt.
Having, at this early stage of its growth, not one-hundredth of the weight
of a full-grown salmon, how does its production of milt consist with the
alleged general law? The answer must be in great measure hypothetical. If
the salmon is (as it appears to be in its young state) a species of
fresh-water trout that has contracted the habit of annually migrating to
the sea, where it finds a food on which it thrives--if the original size of
this species was not much greater than that of the parr (which is nearly as
large as some varieties of trout)--and if the limit of growth in the trout
tribe is very indefinite, as we know it to be; then we may reasonably infer
that the parr has nearly the adult form and size which this species of
trout had before it acquired its migratory habit; and that this production
of milt is, in such case, a concomitant of the incipient decline of growth
naturally arising in the species when living under the conditions of the
ancestral species. Should this be so, the immense subsequent growth of the
parr into the salmon, consequent on a suddenly-increased facility in
obtaining food, removes to a great distance the limit at which assimilation
is balanced by expenditure; and has the effect, analogous to that produced
in plants, of arresting the incipient reproductive process. A confirmation
of this view may be drawn from the fact that when the parr, after its first
migration to the sea, returns to fresh water, having increased in a few
months from a couple of ounces to five or six pounds, it no longer shows
any fitness for propagation: the grilse, or immature salmon, does not
produce milt or spawn.
We conclude, then, that the products of a fertilized germ go on
accumulating by simple growth, so long as the forces whence growth results
are greatly in excess of the antagonist forces; but that when diminution of
the one set of forces or increase of the other, causes a considerable
decline in this excess and an approach towards equilibrium, fertilized
germs are again produced. Whether the germ-product be organized round one
axis or round the many axes that arise by agamogenesis, matters not.
Whether, as in the higher animals, this approach to equilibrium results
from that disproportionate increase of expenditure entailed by increase of
size; or whether, as in most plants and many inferior animals, it results
from absolute or relative decline of nutrition; matters not. In any case
the recurrence of gamogenesis is associated with a decrease in the excess
of tissue-producing power. We cannot say, indeed, that this decrease always
results in gamogenesis: some organisms multiply for an indefinite period by
agamogenesis only. The Weeping Willow, which has been propagated throughout
Europe, does not seed in Europe; and yet, as the Weeping Willow, by its
large size and the multiplication of generation upon generation of lateral
axes, presents the same causes of local innutrition as other trees, we
cannot ascribe the absence of sexual axes to the continued predominance of
nutrition. Among animals, too, the anomalous case of the _Tineidæ_, a group
of moths in which parthenogenetic multiplication goes on for generation
after generation, seems to imply that gamogenesis does not necessarily
result from an approximate balance of assimilation by expenditure. What we
must say is that an approach towards equilibrium between the forces which
cause growth and the forces which oppose growth, is the chief condition to
the recurrence of gamogenesis; but that there appear to be other
conditions, in the absence of which approach to equilibrium is not followed
by gamogenesis.
§ 79. The above induction is an approximate answer to the question--_When_
does gamogenesis recur? but not to the question which was propounded--_Why_
does gamogenesis recur?--_Why_ cannot multiplication be carried on in all
cases, as it is in many cases, by agamogenesis? As already said, biologic
science is not yet advanced enough to reply. Meanwhile, the evidence above
brought together suggests a certain hypothetical answer.
Seeing, on the one hand, that gamogenesis recurs only in individuals which
are approaching a state of organic equilibrium; and seeing, on the other
hand, that the sperm-cells and germ-cells thrown off by such individuals
are cells in which developmental changes have ended in quiescence, but in
which, after their union, there arises a process of active cell-formation;
we may suspect that the approach towards a state of general equilibrium in
such gamogenetic individuals, is accompanied by an approach towards
molecular equilibrium in them; and that the need for this union of
sperm-cell and germ-cell is the need for overthrowing this equilibrium, and
re-establishing active molecular change in the detached germ--a result
probably effected by mixing the slightly different physiological units of
slightly different individuals. The several arguments which support this
view, cannot be satisfactorily set forth until after the topics of Heredity
and Variation have been dealt with. Leaving it for the present, I propose
hereafter to re-consider it in connexion with sundry others raised by the
phenomena of Genesis.
But before ending the chapter, it may be well to note the relations between
these different modes of multiplication, and the conditions of existence
under which they are respectively habitual. While the explanation of the
teleologist is untrue, it is often an obverse to the truth; for though, on
the hypothesis of Evolution, it is clear that things are not arranged thus
or thus for the securing of special ends, it is also clear that
arrangements which _do_ secure these special ends tend to establish
themselves--are established by their fulfilment of these ends. Besides
insuring a structural fitness between each kind of organism and its
circumstances, the working of "natural selection" also insures a fitness
between the mode and rate of multiplication of each kind of organism and
its circumstances. We may, therefore, without any teleological implication,
consider the fitness of homogenesis and heterogenesis to the needs of the
different classes of organisms which exhibit them.
Heterogenesis prevails among organisms of which the food, though abundant
compared with their expenditure, is dispersed in such a way that it cannot
be appropriated in a wholesale manner. _Protophyta_, subsisting on diffused
gases and decaying organic matter in a state of minute subdivision, and
_Protozoa_, to which food comes in the shape of extremely small floating
particles, are enabled, by their rapid agamogenetic multiplication, to
obtain materials for growth better than they would do did they not thus
continually divide and disperse in pursuit of it. The higher plants, having
for nutriment the carbonic acid of the air and certain mineral components
of the soil, show us modes of multiplication adapted to the fullest
utilization of these substances. A herb with but little power of forming
the woody fibre requisite to make a stem that can support wide-spreading
branches, after producing a few sexless axes produces sexual ones; and
maintains its race better, by the consequent early dispersion of seeds,
than by a further production of sexless axes. But a tree, able to lift its
successive generations of sexless axes high into the air, where each gets
carbonic acid and light almost as freely as if it grew by itself, may with
advantage go on budding-out sexless axes year after year; since it thereby
increases its subsequent power of budding-out sexual axes. Meanwhile it may
advantageously transform into seed-bearers those axes which, in consequence
of their less direct access to materials absorbed by the roots, are failing
in their nutrition; for it thus throws off from a point at which sustenance
is deficient, a migrating group of germs that may find sustenance
elsewhere. The heterogenesis displayed by animals of the Coelenterate type
has evidently a like utility. A polype, feeding on minute annelids and
crustaceans which, flitting through the water, come in contact with its
tentacles, and limited to that quantity of prey which chance brings within
its grasp, buds out young polypes which, either as a colony or as dispersed
individuals, spread their tentacles through a larger space of water than
the parent alone can; and by producing them, the parent better insures the
continuance of its species than it would do if it went on slowly growing
until its nutrition was nearly balanced by its waste, and then multiplied
by gamogenesis. Similarly with the _Aphis_. Living on sap sucked from
tender shoots and leaves, and able thus to take in but a very small
quantity in a given time, this creature's race is more likely to be
preserved by a rapid asexual propagation of small individuals, which
disperse themselves over a wide area of nutrition, than it would be did the
individual growth continue so as to produce large individuals multiplying
sexually. And then when autumnal cold and diminishing supply of sap put a
check to growth, the recurrence of gamogenesis, or production of fertilized
ova which remain dormant through the winter, is more favourable to the
preservation of the race than would be a further continuance of
agamogenesis. On the other hand, among the higher animals living on food
which, though dispersed, is more or less aggregated into large masses, this
alternation of gamic and agamic reproduction ceases to be useful. The
development of the germ-product into a single organism of considerable
bulk, is in many cases a condition without which these large masses of
nutriment could not be appropriated; and here the formation of many
individuals instead of one would be fatal. But we still see the beneficial
results of the general law--the postponement of gamogenesis until the rate
of growth begins to decline. For so long as the rate of growth continues
rapid, there is proof that the organism gets food with facility--that
expenditure does not seriously check accumulation; and that the size
reached is as yet not disadvantageous: or rather, indeed, that it is
advantageous. But when the rate of growth is much decreased by the increase
of expenditure--when the excess of assimilative power is diminishing so
fast as to indicate its approaching disappearance--it becomes needful, for
the maintenance of the species, that this excess shall be turned to the
production of new individuals; since, did growth continue until there was a
complete balancing of assimilation and expenditure, the production of new
individuals would be either impossible or fatal to the parent. And it is
clear that "natural selection" will continually tend to determine the
period at which gamogenesis commences, in such a way as most favours the
maintenance of the race.
Here, too, may fitly be pointed out the fact that, by "natural selection,"
there will in every case be produced the most advantageous proportion of
males and females. If the conditions of life render numerical inequality of
the sexes beneficial to the species, in respect either of the number of the
offspring or the character of the offspring; then, those varieties of the
species which approach more than other varieties towards this beneficial
degree of inequality, will be apt to supplant other varieties. And
conversely, where equality in the number of males and females is
beneficial, the equilibrium will be maintained by the dying out of such
varieties as produce offspring among which the sexes are not balanced.
NOTE.--Such alterations of statement in this chapter as have been made
necessary by the advance of biological knowledge since 1864 have not, I
think, tended to invalidate its main theses, but have tended to verify
them. Some explanations to be here added may remove remaining difficulties.
Certain types, which are transitional between _Protozoa_ and _Metazoa_,
exhibit under its simplest form the relation between self-maintenance and
race-maintenance--the integration primarily effecting the one and the
disintegration primarily effecting the other. Among the _Mycetozoa_ a
number of amoeba-like individuals aggregate into what is called a
plasmodium; and while, in some orders, they become fused into a mass of
protoplasm through which their nuclei are dispersed, in other orders
(_Sorophora_) they retain their individualities and simply form a coherent
aggregate. These last, presumably the earliest in order of evolution,
remain united so long as the plasmodium, having a small power of
locomotion, furthers the general nutrition; but when this is impeded by
drought or cold, there arise spores. Each spore contains an amoeboid
individual; and this, escaping when favourable conditions return,
establishes by fission and by union with others like itself a new colony or
plasmodium. Reduced to its lowest terms, we here see the antagonism between
that growth of the coherent mass of units which accompanies its physical
prosperity, and that incoherence and dispersion of the units which follows
unfavourable conditions and arrest of growth, and which presently initiates
new plasmodia.
This antagonism, seen in these incipient _Metazoa_ which show us none of
that organization characterizing the _Metazoa_ in general, is everywhere in
more or less disguised forms exhibited by them--must necessarily be so if
growth of the individual is a process of integration while formation of new
individuals is a process of disintegration. And, primarily, it is an
implication that whatever furthers the one impedes the other.
But now while recognizing the truth that nutrition and innutrition (using
these words to cover not supply of nutriment only but the presence of other
influences favourable or unfavourable to the vital processes) primarily
determine the alternations of these; we have also to recognize the truth
that from the beginning survival of the fittest has been shaping the forms
and effects of their antagonism. By inheritance a physiological habit which
modifies the form of the antagonism in a way favourable to the species,
will become established. Especially will this be the case where the lives
of the individuals have become relatively definite and where special organs
have been evolved for casting off reproductive centres. The resulting
physiological rhythm may in such cases become so pronounced as greatly to
obscure the primitive relation. Among plants we see this in the fact that
those which have been transferred from one habitat to another having widely
different seasons, long continue their original time of flowering, though
it is inappropriate to the new circumstances--the reproductive periodicity
has become organic. Similarly in each species of higher animal, development
of the reproductive organs and maturation of reproductive cells take place
at a settled age, whether the conditions have been favourable or
unfavourable to physical prosperity. The established constitutional
tendency, adapted to the needs of the species, over-rides the
constitutional needs of the individual.
Even here, however, the primitive antagonism, though greatly obscured,
occasionally shows itself. Instance the fact that in plants where
gamogenesis is commencing a sudden access of nutrition will cause
resumption of agamogenesis; and I suspect that an illustration may be found
among human beings in the earlier establishment of the reproductive
function among the ill-fed poor than among the well-fed rich.
One other qualification has to be added. In plants and animals which have
become so definitely constituted that at an approximately fixed stage, the
proclivity towards the production of new individuals becomes pronounced, it
naturally happens that good nutrition aids it. Surplus nutriment being
turned into the reproductive channel, the reproduction is efficient in
proportion as the surplus is great. Hence the fact that in fruit trees
which have reached the flowering stage, manuring has the effect that though
it does not increase the quantity of blossoms it increases the quantity of
fruit; and hence the fact that well-fed and easy-living races of men are
prolific.
CHAPTER VIII.
HEREDITY.
§ 80. Already, in the last two chapters, the law of hereditary transmission
has been tacitly assumed; as, indeed, it unavoidably is in all such
discussions. Understood in its entirety, the law is that each plant or
animal, if it reproduces, gives origin to others like itself: the likeness
consisting, not so much in the repetition of individual traits as in the
assumption of the same general structure. This truth has been rendered so
familiar by daily illustration as almost to have lost its significance.
That wheat produces wheat--that existing oxen have descended from ancestral
oxen--that every unfolding organism eventually takes the form of the class,
order, genus, and species from which it sprang; is a fact which, by force
of repetition, has acquired in our minds almost the aspect of a necessity.
It is in this, however, that Heredity is principally displayed: the
manifestations of it commonly referred to being quite subordinate. And, as
thus understood, Heredity is universal. The various instances of
heterogenesis lately contemplated seem, indeed, to be at variance with this
assertion. But they are not really so. Though the recurrence of like forms
is, in these instances, not direct but cyclical, still, the like forms do
recur; and, when taken together, the group of forms produced during one of
the cycles is as much like the groups produced in preceding cycles, as the
single individual arising by homogenesis is like ancestral individuals.
While, however, the general truth that organisms of a given type uniformly
descend from organisms of the same type, is so well established by infinite
illustrations as to have assumed the character of an axiom; it is not
universally admitted that non-typical peculiarities are inherited. Many
entertain a vague belief that the law of Heredity applies only to main
characters of structure and not to details; or, at any rate, that though it
applies to such details as constitute differences of species, it does not
apply to smaller details. The circumstance that the tendency to repetition
is in a slight degree qualified by the tendency to variation (which, as we
shall hereafter see, is but an indirect result of the tendency to
repetition), leads some to doubt whether Heredity is unlimited. A careful
weighing of the evidence, however, and a due allowance for the influences
by which the minuter manifestations of Heredity are obscured, may remove
this scepticism.
First in order of importance comes the fact that not only are there
uniformly transmitted from an organism to its offspring, those traits of
structure which distinguish the class, order, genus, and species; but also
those which distinguish the variety. We have numerous cases, among both
plants and animals, where, by natural or artificial conditions, there have
been produced divergent modifications of the same species; and abundant
proof exists that the members of any one sub-species habitually transmit
their distinctive peculiarities to their descendants. Agriculturists and
gardeners can furnish unquestionable illustrations. Several varieties of
wheat are known, of which each reproduces itself. Since the potato was
introduced into England there have been formed from it a number of
sub-species; some of them differing greatly in their forms, sizes,
qualities, and periods of ripening. Of peas, also, the like may be said.
And the case of the cabbage-tribe is often cited as showing the permanent
establishment of races which have diverged widely from a common stock.
Among fruits and flowers the multiplication of kinds, and the continuance
of each kind with certainty by agamogenesis, and to some extent by
gamogenesis, might be exemplified without end. From all sides evidence may
be gathered showing a like persistence of varieties among animals. We have
our distinct breeds of sheep, our distinct breeds of cattle, our distinct
breeds of horses: each breed maintaining its characteristics. The many
sorts of dogs which, if we accept the physiological test, we must consider
as all of one species, show us in a marked manner the hereditary
transmission of small differences--each sort, when kept pure, reproducing
itself not only in size, form, colour, and quality of hair, but also in
disposition and speciality of intelligence. Poultry, too, have their
permanently-established races. And the Isle of Man sends us a tail-less
kind of cat. Even in the absence of other evidence, that which ethnology
furnishes would suffice. Grant them to be derived from one stock, and the
varieties of man yield proof upon proof that non-specific traits of
structure are bequeathed from generation to generation. Or grant only their
derivation from several stocks, and we still have, between races descended
from a common stock, distinctions which prove the inheritance of minor
peculiarities. Besides seeing the Negroes continue to produce Negroes,
copper-coloured men to produce men of a copper colour, and the fair-skinned
races to perpetuate their fair skins--besides seeing that the broad-faced
and flat-nosed Calmuck begets children with broad faces and flat noses,
while the Jew bequeaths to his offspring the features which have so long
characterized Jews; we see that those small unlikenesses which distinguish
more nearly-allied varieties of men, are maintained from generation to
generation. In Germany, the ordinary shape of skull is appreciably
different from that common in Britain: near akin though the Germans are to
the British. The average Italian face continues to be unlike the faces of
northern nations. The French character is now, as it was centuries ago,
contrasted in sundry respects with the characters of neighbouring peoples.
Nay, even between races so closely allied as the Scotch Celts, the Welsh
Celts, and the Irish Celts, appreciable differences of form and nature have
become established.
The fact that sub-species and sub-sub-species thus exemplify the general
law of inheritance which shows itself in the perpetuation of ordinal,
generic, and species peculiarities, is strong reason for the belief that
this general lay is unlimited in its application. This has the support of
still more special evidences. They are divisible into two classes. In the
one come cases where congenital peculiarities, not traceable to any obvious
causes, are bequeathed to descendants. In the other come cases where the
peculiarities thus bequeathed are not congenital, but have resulted from
changes of functions during the lives of the individuals bequeathing them.
We will consider first the cases that come in the first class.
§ 81. Note at the outset the character of the chief testimony. Excluding
those inductions that have been so fully verified as to rank with exact
science, there are no inductions so trustworthy as those which have
undergone the mercantile test. When we have thousands of men whose profit
or loss depends on the truth of their inferences from perpetually-repeated
observations; and when we find that their inferences, handed down from
generation to generation, have generated an unshakable conviction; we may
accept it without hesitation. In breeders of animals we have such a class,
led by such experiences, and entertaining such a conviction--the conviction
that minor peculiarities of organization are inherited as well as major
peculiarities. Hence the immense prices given for successful racers, bulls
of superior forms, sheep that have certain desired peculiarities. Hence the
careful record of pedigrees of high-bred horses and sporting dogs. Hence
the care taken to avoid intermixture with inferior stocks. As quoted by Mr.
Darwin, Youatt says the principle of selection "enables the agriculturist
not only to modify the character of his flock but to change it altogether."
Lord Somerville, speaking of what breeders have done for sheep, says:--"It
would seem that they have chalked upon a wall a form perfect in itself and
then given it existence." That most skilful breeder, Sir John Sebright,
used to say, with respect to pigeons, that "he would produce any given
feather in three years, but it would take him six years to obtain head and
beak." In all which statements the tacit assertion is, that individual
traits are bequeathed from generation to generation, and may be so
perpetuated and increased as to become permanent distinctions.
Of special instances there are many besides that of the often-cited
Otto-breed of sheep, descended from a single short-legged lamb, and that of
the six-fingered Gratio Kelleia, who transmitted his peculiarity, in
different degrees, to several of his children and to some of his
grandchildren. In a paper contributed to the _Edinburgh New Philosophical
Journal_ for July, 1863, Dr. (now Sir John) Struthers gives cases of
hereditary digital variations. Esther P----, who had six fingers on one
hand, bequeathed this malformation along some lines of her descendants for
two, three, and four generations. A---- S---- inherited an extra digit on
each hand and each foot from his father; and C---- G----, who also had six
fingers and six toes, had an aunt and a grandmother similarly formed. A
collection of evidence published by Mr. Sedgwick in the _Medico-Chirurgical
Review_ for April and for July, 1863, in two articles on "The Influence of
Sex in limiting Hereditary Transmission," includes the following
cases:--Augustin Duforet, a pastry-cook of Douai, who had but two instead
of three phalanges to all his fingers and toes, inherited this malformation
from his grandfather and father, and had it in common with an uncle and
numerous cousins. An account has been given by Dr. Lepine, of a man with
only three fingers on each hand and four toes on each foot, and whose
grandfather and son exhibited the like anomaly. Béchet describes Victoire
Barré as a woman who, like her father and sister, had but one developed
finger on each hand and but two toes on each foot, and whose monstrosity
re-appeared in two daughters. And there is a case where the absence of two
distal phalanges on the hands was traced for two generations. The various
recorded instances in which there has been transmission from one generation
to another, of webbed-fingers, of webbed-toes, of hare-lip, of congenital
luxation of the thigh, of absent patellæ, of club-foot, &c., would occupy
more space than can here be spared. Defects in the organs of sense are also
not unfrequently inherited. Four sisters, their mother, and grandmother,
are described by Duval as similarly affected by cataract. Prosper Lucas
details an example of amaurosis affecting the females of a family for three
generations. Duval, Graffe, Dufon, and others testify to like cases coming
under their observation.[35] Deafness, too, is occasionally transmitted
from parent to child. There are deaf-mutes whose imperfections have been
derived from ancestors; and malformations of the external ears have also
been perpetuated in offspring. Of transmitted peculiarities of the skin and
its appendages, many cases have been noted. One is that of a family
remarkable for enormous black eyebrows; another that of a family in which
every member had a lock of hair of a lighter colour than the rest on the
top of the head; and there are also instances of congenital baldness being
hereditary. From one of our leading sculptors I learn that his wife has a
flat mole under the foot near the little toe, and one of her sons has the
same. Entire absence of teeth, absence of particular teeth, and anomalous
arrangements of teeth, are recorded as traits that have descended to
children. And we have evidence that soundness and unsoundness of teeth are
transmissible.
The inheritance of tendencies to such diseases as gout, consumption, and
insanity is universally admitted. Among the less-common diseases of which
the descent has been observed, are ichthyosis, leprosy, pityriasis,
sebaceous tumours, plica polonica, dipsomania, somnambulism, catalepsy,
epilepsy, asthma, apoplexy, elephantiasis. General nervousness displayed by
parents almost always re-appears in their children. Even a bias towards
suicide appears to be sometimes hereditary.
§ 82. To prove the transmission of those structural peculiarities which
have resulted from functional peculiarities, is, for several reasons,
comparatively difficult. Changes produced in the sizes of parts by changes
in their amounts of action, are mostly unobtrusive. A muscle which has
increased in bulk is usually so obscured by natural or artificial clothing,
that unless the alteration is extreme it passes without remark. Such
nervous developments as are possible in the course of a single life, cannot
be seen externally. Visceral modifications of a normal kind are observable
but obscurely, or not at all. And if the changes of structure worked in
individuals by changes in their habits are thus difficult to trace, still
more difficult to trace must be the transmission of them: further hidden,
as this is, by the influences of other individuals who are often otherwise
modified by other habits. Moreover, such specialities of structure as are
due to specialities of function, are usually entangled with specialities of
structure which are, or may be, due to selection, natural or artificial. In
most cases it is impossible to say that a structural peculiarity which
seems to have arisen in offspring from a functional peculiarity in a
parent, is wholly independent of some congenital peculiarity of structure
in the parent, whence this functional peculiarity arose. We are restricted
to cases with which natural or artificial selection can have had nothing to
do, and such cases are difficult to find. Some, however, may be noted.
A species of plant that has been transferred from one soil or climate to
another, frequently undergoes what botanists call "change of habit"--a
change which, without affecting its specific characters, is yet
conspicuous. In its new locality the species is distinguished by leaves
that are much larger or much smaller, or differently shaped, or more
fleshy; or instead of being as before comparatively smooth, it becomes
hairy; or its stem becomes woody instead of being herbaceous; or its
branches, no longer growing upwards, assume a drooping character. Now these
"changes of habit" are clearly determined by functional changes. Occurring,
as they do, in many individuals which have undergone the same
transportation, they cannot be classed as "spontaneous variations." They
are modifications of structure consequent on modifications of function that
have been produced by modifications in the actions of external forces. And
as these modifications re-appear in succeeding generations, we have, in
them, examples of functionally-established variations that are hereditarily
transmitted.
Evidence of analogous changes in animals is difficult to disentangle. Only
among domesticated kinds have we any opportunity of tracing the results of
altered habits; and here, in nearly all cases, artificial selection has
obscured them. Still, there are some facts which seem to the point. Mr.
Darwin, while ascribing almost wholly to "natural selection" the production
of those modifications which eventuate in differences of species,
nevertheless admits the effects of use and disuse. He says--"I find in the
domestic duck that the bones of the wing weigh less and the bones of the
leg more, in proportion to the whole skeleton, than do the same bones in
the wild duck; and I presume that this change may be safely attributed to
the domestic duck flying much less, and walking more, than its wild parent.
The great and inherited development of the udders in cows and goats in
countries where they are habitually milked, in comparison with the state of
these organs in other countries, is another instance of the effect of use.
Not a single domestic animal can be named which has not in some country
drooping ears; and the view suggested by some authors, that the drooping is
due to the disuse of the muscles of the ear, from the animals not being
much alarmed by danger, seems probable." Again--"The eyes of moles and of
some burrowing rodents are rudimentary in size, and in some cases are quite
covered up by skin and fur. This state of the eyes is probably due to
gradual reduction from disuse, but aided perhaps by natural selection." ...
"It is well known that several animals belonging to the most different
classes, which inhabit the caves of Styria and of Kentucky, are blind. In
some of the crabs the footstalk of the eye remains, though the eye is gone;
the stand for the telescope is there, though the telescope with its glasses
has been lost. As it is difficult to imagine that eyes, though useless,
could be in any way injurious to animals living in darkness, I attribute
their loss wholly to disuse."[36] The direct inheritance of an acquired
peculiarity is sometimes observable. Mr. Lewes gives a case. He "had a
puppy taken from its mother at six weeks old, who, although never taught
'to beg' (an accomplishment his mother had been taught), spontaneously took
to begging for everything he wanted when about seven or eight months old:
he would beg for food, beg to be let out of the room, and one day was found
opposite a rabbit hutch begging for rabbits." Instances are on record, too,
of sporting dogs which spontaneously adopted in the field, certain modes of
behaviour which their parents had learnt.
But the best examples of inherited modifications produced by modifications
of function, occur in mankind. To no other cause can be ascribed the rapid
metamorphoses undergone by the British races when placed in new conditions.
In the United States the descendants of the immigrant Irish lose their
Celtic aspect, and become Americanized. This cannot be ascribed to mixture,
since the feeling with which Irish are regarded by Americans prevents any
considerable amount of intermarriage. Equally marked is the case of the
immigrant Germans who, though they keep very much apart, rapidly assume the
prevailing type. To say that "spontaneous variation" increased by natural
selection, can have produced this effect, is going too far. Peoples so
numerous cannot have been supplanted in the course of two or three
generations by varieties springing from them. Hence the implication is that
physical and social conditions have wrought modifications of function and
structure, which offspring have inherited and increased. Similarly with
special cases. In the _Cyclopædia of Practical Medicine_, Vol. II., p. 419,
Dr. Brown states that he "has in many instances observed in the case of
individuals whose complexion and general appearance has been modified by
residence in hot climates, that children born to them subsequently to such
residence, have resembled them rather in their acquired than primary mien."
Some visible modifications of organs caused by changes in their functions,
may be noted. That large hands are inherited by those whose ancestors led
laborious lives, and that those descended from ancestors unused to manual
labour commonly have small hands, are established opinions. It seems very
unlikely that in the absence of any such connexion, the size of the hand
should have come to be generally regarded as some index of extraction. That
there exists a like relation between habitual use of the feet and largeness
of the feet, we have strong evidence in the customs of the Chinese. The
torturing practice of artificially arresting the growth of the feet, could
never have become established among the ladies of China, had they not seen
that a small foot was significant of superior rank--that is of a luxurious
life--that is of a life without bodily labour. There is evidence, too, that
modifications of the eyes, caused by particular uses of the eyes, are
inherited. Short sight appears to be uncommon among peasants; but it is
frequent among classes who use their eyes much for reading and writing, and
is often congenital. Still more marked is this relation in Germany. There,
the educated are notoriously studious, and judging from the numbers of
young Germans who wear spectacles, there is reason to think that congenital
myopia is very frequent among them.
Some of the best illustrations of functional heredity, are furnished by
mental characteristics. Certain powers which mankind have gained in the
course of civilization cannot, I think, be accounted for without admitting
the inheritance of acquired modifications. The musical faculty is one of
these. To say that "natural selection" has developed it by preserving the
most musically endowed, seems an inadequate explanation. Even now that the
development and prevalence of the faculty have made music an occupation by
which the most musical can get sustenance and bring up families; it is very
questionable whether, taking the musical career as a whole, it has any
advantage over other careers in the struggle for existence and
multiplication. Still more if we look back to those early stages through
which the faculty must have passed before definite perception of melody was
arrived at, we fail to see how those possessing the rudimentary faculty in
a somewhat greater degree than the rest, would thereby be enabled the
better to maintain themselves and their children. There is no explanation
but that the habitual association of certain cadences of speech with
certain emotions, has slowly established in the race an organized and
inherited connection between such cadences and such emotions; that the
combination of such cadences, more or less idealized, which constitutes
melody, has all along had a meaning in the average mind, only because of
the meaning which cadences had acquired in the average mind; and that by
the continual hearing and practice of melody there has been gained and
transmitted an increasing musical sensibility. Confirmation of this view
may be drawn from individual cases. Grant that among a people endowed with
musical faculty to a certain degree, spontaneous variation will
occasionally produce men possessing it in a higher degree; it cannot be
granted that spontaneous variation accounts for the frequent production, by
such highly-endowed men, of men still more highly endowed. On the average,
the children of marriages with others not similarly endowed, will be less
distinguished rather than more distinguished. The most that can be expected
is that this unusual amount of faculty shall re-appear in the next
generation undiminished. How then shall we explain cases like those of
Bach, Mozart, and Beethoven, all of them sons of men having unusual musical
powers who were constantly exercising those powers, and who greatly
excelled their fathers in their musical powers? What shall we say to the
facts that Haydn was the son of an organist, that Hummel was born to a
music master, and that Weber's father was a distinguished violinist? The
occurrence of so many cases in one nation within a short period of time,
cannot rationally be ascribed to the coincidence of "spontaneous
variations." It can be ascribed to nothing but inherited developments of
structure caused by augmentations of function.
But the clearest proof that structural alterations caused by alterations of
function are inherited, occurs when the alterations are morbid. I had
originally named in this place the results of M. Brown-Sequard's
experiments on guinea-pigs, showing that those which had been artificially
made epileptic had offspring which were epileptic; and I name them again
though his inference is by many rejected. For, as exemplified a few pages
back, strong evidence is often disregarded for trivial reasons by those who
dislike the conclusion drawn. Just naming this evidence and its possible
invalidity, let me pass to some results of experiences recently set forth
by Dr. Savage, President of the Neurological Society. In an essay on
"Heredity and Neurosis" published in _Brain_, Parts LXXVII, LXXVIII, 1897,
he says:--"We recognise the transmission of a tendency to develop gout, and
we recognise that the disease produced by the individual himself differs
little from that which may have been inherited." [That is, acquired gout
may be transmitted as constitutional gout.] "I have seen several patients
whose history I have been able to examine carefully, in whom mental tricks
have been transmitted from one generation to another." In the "musical
prodigies" descending from musical parents, "there seemed to be a
transmission of a greatly increased aptitude or tendency which is all one
is contending for." "Though there is, in my opinion, power to transmit
acquired peculiarities, yet the tendency is to transmit a predisposition."
(pp. 19-21.) And an authority on nervous diseases who is second to
none--Dr. Hughlings Jackson--takes the same view. The liability to
consumption shown by children of consumptive parents, which no one doubts,
shows us the same thing. It is admitted that consumption may be produced by
conditions very unfavourable to life; and unless it is held that the
disease so produced differs from the disease when inherited, the conclusion
must be that here, too, there is a transmission of functionally-produced
organic changes. This holds true whether the production of tubercle is due
to innate defect or whether it is due to the invasion of a bacillus. For in
this last case the consumptive diathesis must be regarded as a state of
body more than usually liable to invasion by the bacillus, and this is the
same when acquired as when transmitted.
§ 83. Two modified manifestations of Heredity remain to be noticed. The one
is the re-appearance in offspring of traits not borne by the parents, but
borne by the grandparents or by remoter ancestors. The other is the
limitation of Heredity by sex--the restriction of transmitted peculiarities
to offspring of the same sex as the parent possessing them.
Atavism, which is the name given to the recurrence of ancestral traits, is
proved by many and varied facts. In the picture-galleries of old families,
and on the monumental brasses in the adjacent churches, are often seen
types of feature which are still, from time to time, repeated in members of
these families. It is a matter of common remark that some constitutional
diseases, such as gout and insanity, after missing a generation, will show
themselves in the next. Dr. Struthers, in his above-quoted paper "On
Variation in the Number of Fingers and Toes, and in the Phalanges in Man,"
gives cases of malformations common to grandparent and grandchild, but of
which the parent had no trace. M. Girou (as quoted by Mr. Sedgwick)
says--"One is often surprised to see lambs black, or spotted with black,
born of ewes and rams with white wool, but if one takes the trouble to go
back to the origin of this phenomena, it is found in the ancestors."
Instances still more remarkable, in which the remoteness of the ancestors
copied is very great, are given by Mr. Darwin. He points out that in
crosses between varieties of the pigeon, there will sometimes re-appear the
plumage of the original rock-pigeon, from which these varieties descended;
and he thinks the faint zebra-like markings occasionally traceable in
horses have probably a like meaning.
The other modified manifestation of heredity above referred to is the
limitation of heredity by sex. In Mr. Sedgwick's essays, already named,
will be found evidence implying that there exists some such tendency to
limitation, which does or does not show itself distinctly according to the
nature of the organic modification to be conveyed. On joining to the
evidence he gives certain bodies of allied evidence we shall, I think, find
the inconsistences comprehensible.
Beyond the familiar facts that in ourselves, along with the essential
organs of sex there go minor structures and traits distinctive of sex, such
as the beard and the voice in man, we have numerous cases in which, along
with different sex-organs there go general differences, sometimes immense
and often conspicuous. We have those in which (as in sundry parasites) the
male is extremely small compared with the female; we have those in which
the male is winged and the female wingless; we have those, as among birds,
in which the plumage of males contrasts strongly with that of females; and
among butterflies we have kindred instances in which the wings of the two
sexes are wholly unlike--some, indeed, in which there is not simply
dimorphism but polymorphism: two kinds of females both differing from the
male. How shall we range these facts with the ordinary facts of
inheritance? Without difficulty if heredity results from the proclivity
which the component units contained in a germ-cell or a sperm-cell have to
arrange themselves into a structure like that of the structure from which
they were derived. For the obvious corollary is that where there is
gamogenesis there will result partly concurring and partly conflicting
proclivities. In the fertilized germ we have two groups of physiological
units, slightly different in their structures. These slightly-different
units severally multiply at the expense of the nutriment supplied to the
unfolding germ--each kind moulding this nutriment into units of its own
type. Throughout the process of development the two kinds of units, mainly
agreeing in their proclivities and in the form which they tend to build
themselves into, but having minor differences, work in unison to produce an
organism of the species from which they were derived, but work in
antagonism to produce copies of their respective parent-organisms. And
hence ultimately results an organism in which traits of the one are mixed
with traits of the other; and in which, according to the predominance of
one or other group of units, one or other sex with all its concomitants is
produced.
If so, it becomes comprehensible that with the predominance of either
group, and the production of the same sex as that of the parent whence it
was derived, there will go the repetition not only of the minor sex-traits
of that parent but also of any peculiarities he or she possessed, such as
monstrosities. Since the two groups are nearly balanced, and since
inheritance is never an average of the two parents but a mixture of traits
of the one with traits of the other, it is not difficult to see why there
should be some irregularity in the transmission of these monstrosities and
constitutional tendencies, though they are most frequently transmitted only
to those of the same sex.[37]
§ 84. Unawares in the last paragraph there has been taken for granted the
truth of that suggestion concerning Heredity ventured in § 66. Anything
like a positive explanation is not to be expected in the present stage of
Biology, if at all. We can look for nothing beyond a simplification of the
problem; and a reduction of it to the same category with certain other
problems which also admit of hypothetical solutions only. If an hypothesis
which sundry widespread phenomena have already thrust upon us, can be shown
to render the phenomena of Heredity more intelligible than they at present
seem, we shall have reason to entertain it. The applicability of any method
of interpretation to two different but allied classes of facts, is evidence
of its truth.
The power which many animals display of reproducing lost parts, we saw to
be inexplicable except on the assumption that the units of which any
organism is built have a tendency to arrange themselves into the shape of
that organism (§ 65). This power is sufficiently remarkable in cases where
a lost limb or tail is replaced, but it is still more remarkable in cases
where, as among some annelids, the pieces into which an individual is cut
severally complete themselves by developing heads and tails, or in cases
like that of the _Holothuria_, which having, when alarmed, ejected its
viscera, reproduces them. Such facts compel us to admit that the components
of an organism have a proclivity towards a special structure--that the
adult organism when mutilated exhibits that same proclivity which is
exhibited by the young organism in the course of its normal development. As
before said, we may, for want of a better name, figuratively call this
power organic polarity: meaning by this phrase nothing more than the
observed tendency towards a special arrangement. And such facts as those
presented by the fragments of a _Hydra_, and by fragments of leaves from
which complete plants are produced, oblige us to recognize this proclivity
as existing throughout the tissues in general--nay, in the case of the
_Begonia phyllomaniaca_, obliges us to recognize this proclivity as
existing in the physiological units contained in each undifferentiated
cell. Quite in harmony with this conclusion, are certain implications since
noticed, respecting the characters of sperm-cells and germ-cells. We saw
sundry reasons for rejecting the supposition that these are
highly-specialized cells and for accepting the opposite supposition, that
they are cells differing from others rather in being unspecialized. And
here the assumption to which we seem driven by the _ensemble_ of the
evidence, is, that sperm-cells and germ-cells are essentially nothing more
than vehicles in which are contained small groups of the physiological
units in a fit state for obeying their proclivity towards the structural
arrangement of the species they belong to.
If the likeness of offspring to parents is thus determined, it becomes
manifest, _à priori_, that besides the transmission of generic and specific
peculiarities, there will be a transmission of those individual
peculiarities which, arising without assignable causes, are classed as
"spontaneous." For if the assumption of a special arrangement of parts by
an organism, is due to the proclivity of its physiological units towards
that arrangement; then the assumption of an arrangement of parts slightly
different from that of the species, implies physiological units slightly
unlike those of the species; and these slightly-unlike physiological units,
communicated through the medium of sperm-cell or germ-cell, will tend, in
the offspring, to build themselves into a structure similarly diverging
from the average of the species.
But it is not equally manifest that, on this hypothesis, alterations of
structure caused by alterations of function must be transmitted to
offspring. It is not obvious that change in the form of a part, caused by
changed action, involves such change in the physiological units throughout
the organism that these, when groups of them are thrown off in the shape of
reproductive centres, will unfold into organisms that have this part
similarly changed in form. Indeed, when treating of Adaptation (§ 69), we
saw that an organ modified by increase or decrease of function, can but
slowly re-act on the system at large, so as to bring about those
correlative changes required to produce a new equilibrium; and yet only
when such new equilibrium has been established, can we expect it to be
_fully_ expressed in the modified physiological units of which the organism
is built--only then can we count on a complete transfer of the modification
to descendants. Nevertheless, that changes of structure caused by changes
of action must also be transmitted, however obscurely, appears to be a
deduction from first principles--or if not a specific deduction, still, a
general implication. For if an organism A, has, by any peculiar habit or
condition of life, been modified into the form A', it follows that all the
functions of A', reproductive function included, must be in some degree
different from the functions of A. An organism being a combination of
rhythmically-acting parts in moving equilibrium, the action and structure
of any one part cannot be altered without causing alterations of action and
structure in all the rest; just as no member of the Solar System could be
modified in motion or mass, without producing rearrangements throughout the
whole Solar System. And if the organism A, when changed to A', must be
changed in all its functions; then the offspring of A' cannot be the same
as they would have been had it retained the form A. That the change in the
offspring must, other things equal, be in the same direction as the change
in the parent, appears implied by the fact that the change propagated
throughout the parental system is a change towards a new state of
equilibrium--a change tending to bring the actions of all organs,
reproductive included, into harmony with these new actions. Or, bringing
the question to its ultimate and simplest form, we may say that as, on the
one hand, physiological units will, because of their special polarities,
build themselves into an organism of a special structure; so, on the other
hand, if the structure of this organism is modified by modified function,
it will impress some corresponding modification on the structures and
polarities of its units. The units and the aggregate must act and re-act on
each other. If nothing prevents, the units will mould the aggregate into a
form in equilibrium with their pre-existing polarities. If, contrariwise,
the aggregate is made by incident actions to take a new form, its forces
must tend to re-mould the units into harmony with this new form. And to say
that the physiological units are in any degree so re-moulded as to bring
their polar forces towards equilibrium with the forces of the modified
aggregate, is to say that when separated in the shape of reproductive
centres, these units will tend to build themselves up into an aggregate
modified in the same direction.
NOTE.--A large amount of additional evidence supporting the belief that
functionally produced modifications are inherited, will be found in
Appendix B.
CHAPTER IX.
VARIATION.
§ 85. Equally conspicuous with the truth that every organism bears a
general likeness to its parents, is the truth that no organism is exactly
like either parent. Though similar to both in generic and specific traits,
and usually, too, in those traits which distinguish the variety, it
diverges in numerous traits of minor importance. No two plants are
indistinguishable; and no two animals are without differences. Variation is
co-extensive with Heredity.
The degrees of variation have a wide range. There are deviations so small
as to be not easily detected; and there are deviations great enough to be
called monstrosities. In plants we may pass from cases of slight alteration
in the shape of a leaf, to cases where, instead of a flower with its calyx
above the seed-vessel, there is produced a flower with its calyx below the
seed-vessel; and while in one animal there arises a scarcely noticeable
unlikeness in the length or colour of the hair, in another an organ is
absent or a supernumerary organ appears. Though small variations are by far
the most general, yet variations of considerable magnitude are not
uncommon; and even those variations constituted by additions or
suppressions of parts, are not so rare as to be excluded from the list of
causes by which organic forms are changed. Cattle without horns are
frequent. Of sheep there are horned breeds and breeds that have lost their
horns. At one time there existed in Scotland a race of pigs with solid feet
instead of cleft feet. In pigeons, according to Mr. Darwin, "the number of
the caudal and sacral vertebræ vary; as does the number of the ribs,
together with their relative breadth and the presence of processes."
That variations, both small and large, which arise without any specific
assignable cause, tend to become hereditary, was shown in the last chapter.
Indeed the evidence which proves Heredity in its smaller manifestations is
the same evidence which proves Variation; since it is only when there occur
variations that the inheritance of anything beyond the structural
peculiarities of the species can be proved. It remains here, however, to be
observed that the transmission of variations is itself variable; and that
it varies both in the direction of decrease and in the direction of
increase. An individual trait of one parent may be so counteracted by the
influence of the other parent, that it may not appear in the offspring; or,
not being so counteracted, the offspring may possess it, perhaps in an
equal degree or perhaps in a less degree; or the offspring may exhibit the
trait in even a still higher degree. Among illustrations of this, one must
suffice. I quote it from the essay by Sir J. Struthers referred to in the
last chapter.
"The great-great-grandmother, Esther P---- (who married A---- L----), had a
sixth little finger on one hand. Of their eighteen children (twelve
daughters and six sons), only one (Charles) is known to have had digital
variety. We have the history of the descendants of three of the sons,
Andrew, Charles, and James.
"(1.) Andrew L---- had two sons, Thomas and Andrew; and Thomas had two sons
all without digital variety. Here we have three successive generations
without the variety possessed by the great-grandmother showing itself.
"(2.) James L----, who was normal, had two sons and seven daughters, also
normal. One of the daughters became Mrs. J---- (one of the informants), and
had three daughters and five sons, all normal except one of the sons, James
J----, now æt. 17, who had six fingers on each hand....
"In this branch of the descendants of Esther, we see it passing over two
generations and reappearing in one member of the third generation, and now
on both hands.
"(3.) Charles L----, the only child of Esther who had digital variety, had
six fingers on each hand. He had three sons, James, Thomas, and John, all
of whom were born with six fingers on each hand, while John has also a
sixth toe on one foot. He had also five other sons and four daughters, all
of whom were normal.
"(a.) Of the normal children of this, the third generation, the five sons
had twelve sons and twelve daughters, and the four daughters have had four
sons and four daughters, being the fourth generation, all of whom were
normal. A fifth generation in this sub-group consists as yet of only two
boys and two girls who are also normal.
"In this sub-branch, we see the variety of the first generation present in
the second, passing over the third and fourth, and also the fifth as far as
it has yet gone.
"(b.) James had three sons and two daughters, who are normal.
"(c.) Thomas had four sons and five daughters, who are normal; and has two
grandsons, also normal.
"In this sub-branch of the descent, we see the variety of the first
generation, showing itself in the second and third, and passing over the
fourth, and (as far as it yet exists) the fifth generation.
"(d.) John L---- (one of the informants) had six fingers, the additional
finger being attached on the outer side, as in the case of his brothers
James and Thomas. All of them had the additional digits removed. John has
also a sixth toe on one foot, situated on the outer side. The fifth and
sixth toes have a common proximal phalange, and a common integument invests
the middle and distal phalanges, each having a separate nail.
"John L---- has a son who is normal, and a daughter, Jane, who was born
with six fingers on each hand and six toes on each foot. The sixth fingers
were removed. The sixth toes are not wrapped with the fifth as in her
father's case, but are distinct from them. The son has a son and daughter,
who, like himself, are normal.
"In this, the most interesting sub-branch of the descent, we see digital
increase, which appeared in the first generation on one limb, appearing in
the second on two limbs, the hands; in the third on three limbs, the hands
and one foot; in the fourth on all the four limbs. There is as yet no fifth
generation in uninterrupted transmission of the variety. The variety does
not yet occur in any member of the fifth generation of Esther's
descendants, which consists, as yet, only of three boys and one girl, whose
parents were normal, and of two boys and two girls, whose grandparents were
normal. It is not known whether in the case of the great-great-grandmother,
Esther P----, the variety was original or inherited."[38]
§ 86. Where there is great uniformity among the members of a species, the
divergences of offspring from the average type are usually small; but
where, among the members of a species, considerable unlikenesses have once
been established, unlikenesses among the offspring are frequent and great.
Wild plants growing in their natural habitats are uniform over large areas,
and maintain from generation to generation like structures; but when
cultivation has caused appreciable differences among the members of any
species of plant, extensive and numerous deviations are apt to arise.
Similarly, between wild and domesticated animals of the same species, we
see the contrast that though the homogeneous wild race maintains its type
with great persistence, the comparatively heterogeneous domestic race
frequently produces individuals more unlike the average type than the
parents are.
Though unlikeness among progenitors is one antecedent of variation, it is
by no means the sole antecedent. Were it so, the young ones successively
born to the same parents would be alike. If any peculiarity in a new
organism were a direct resultant of the structural differences between the
two organisms which produced it; then all subsequent new organisms produced
by these two would show the same peculiarity. But we know that the
successive offspring have different peculiarities: no two of them are ever
exactly alike.
One cause of such structural variation in progeny, is functional variation
in parents. Proof of this is given by the fact that, among progeny of the
same parents, there is more difference between those begotten under
different constitutional states than between those begotten under the same
constitutional state. It is notorious that twins are more nearly alike than
children borne in succession. The functional conditions of the parents
being the same for twins, but not the same for their brothers and sisters
(all other antecedents being constant), we have no choice but to admit that
variations in the functional conditions of the parents, are the antecedents
of those greater unlikenesses which their brothers and sisters exhibit.
Some other antecedent remains, however. The parents being the same, and
their constitutional states the same, variation, more or less marked, still
manifests itself. Plants grown from seeds out of one pod, or animals
produced at one birth, are not alike. Sometimes they differ considerably.
In a litter of pigs or of kittens, we rarely see uniformity of markings;
and occasionally there are important structural contrasts. I have myself
recently been shown a litter of Newfoundland puppies, some of which had
four digits to their feet, while in others there was present, on each
hind-foot, what is called the "dew-claw"--a rudimentary fifth digit.
Thus, induction points to three causes of variation, all in action
together. We have heterogeneity among progenitors, which, did it act
uniformly and alone in generating, by composition of forces, new
deviations, would impress such new deviations to the same extent on all
offspring of the same parents; which it does not. We have functional
variation in the parents, which, acting either alone or in combination with
the preceding cause, would entail the same structural variations on all
young ones simultaneously produced; which it does not. Consequently there
is some third cause of variation, yet to be found, which acts along with
the structural and functional variations of ancestors and parents.
§ 87. Already, in the last section, there has been implied some relation
between variation and the action of external conditions. The above-cited
contrast between the uniformity of a wild species and the multiformity of
the same species when cultivated or domesticated, thrusts this truth upon
us. Respecting the variations of plants, Mr. Darwin remarks that "'sports'
are extremely rare under nature, but far from rare under cultivation."
Others who have studied the matter assert that if a species of plant which,
up to a certain time, has maintained great uniformity, once has its
constitution thoroughly disturbed, it will go on varying indefinitely.
Though, in consequence of the remoteness of the periods at which they were
domesticated, there is a lack of positive proof that our extremely variable
domestic animals have become variable under the changed conditions implied
by domestication, having been previously constant; yet competent judges do
not doubt that this has been the case.
Now the constitutional disturbance which precedes variation, can be nothing
else than an overthrowing of the pre-established equilibrium of functions.
Transferring a plant from forest lands to a ploughed field or a manured
garden, is altering the balance of forces to which it has been hitherto
subject, by supplying it with different proportions of the assimilable
matters it requires, and taking away some of the positive impediments to
its growth which competing wild plants before offered. An animal taken from
woods or plains, where it lived on wild food of its own procuring, and
placed under restraint while artificially supplied with food not quite like
what it had before, is an animal subject to new outer actions to which its
inner actions must be adjusted. From the general law of equilibration we
found it to follow that "the maintenance of such a moving equilibrium" as
an organism displays, "requires the habitual genesis of internal forces
corresponding in number, directions, and amounts, to the external incident
forces--as many inner functions, single or combined, as there are single or
combined outer actions to be met" (_First Principles_, § 173); and more
recently (§ 27), we have seen that Life itself is "the definite combination
of heterogeneous changes, both simultaneous and successive, in
correspondence with external co-existences and sequences." Necessarily,
therefore, an organism exposed to a permanent change in the arrangement of
outer forces must undergo a permanent change in the arrangement of inner
forces. The old equilibrium has been destroyed; and a new equilibrium must
be established. There must be functional perturbations, ending in a
re-adjusted balance of functions.
If, then, change of conditions is the only known cause by which the
original homogeneity of a species is destroyed; and if change of conditions
can affect an organism only by altering its functions; it follows that
alteration of functions is the only known internal cause to which the
commencement of variation can be ascribed. That such minor functional
changes as parents undergo from year to year are influential on the
offspring, we have seen is proved by the greater unlikeness that exists
between children born to the same parents at different times, than exists
between twins. And here we seem forced to conclude that the larger
functional variations produced by greater external changes, are the
initiators of those structural variations which, when once commenced in a
species, lead by their combinations and antagonisms to multiform results.
Whether they are or are not the direct initiators, they must still be the
indirect initiators.
§ 87a. In the foregoing sentence those pronounced structural variations
from which may presently arise new varieties and eventually species, are
ascribed to "the larger functional variations produced by greater external
changes"; and this limitation is a needful one, since there is a constant
cause of minor variations of a wholly different kind.
There are the variations arising from differences in the conditions to
which the germ is subject, both before detachment from the parent and
after. At first sight it seems that plants grown from seeds out of the same
seed-vessel and animals belonging to the same litter, ought, in the absence
of any differences of ancestral antecedents, to be entirely alike. But this
is not so. Inevitably they are subject from the very outset to slightly
different sets of agencies. The seeds in a seed-vessel do not stand in
exactly the same relations to the sources of nutriment: some are nearer
than others. They are somewhat differently exposed to the heat and light
penetrating their envelope; and some are more impeded in their growth by
neighbours than others are. Similarly with young animals belonging to the
same litter. Their uterine lives are made to some extent unlike by unlike
connexions with the blood-supply, by mutual interferences not all the same,
and even by different relations to the disturbances caused by the mother's
movements. So, too, is it after separation from the parent plant or animal.
Even the biblical parable reminds us that seeds fall into places here
favourable and there unfavourable in various degrees. In respect of soil,
in respect of space for growth, in respect of shares of light, none of them
are circumstanced in quite the same ways. With animals the like holds. In a
litter of pigs some, weaker than others, do not succeed as often in getting
possession of teats. And then in both cases the differences thus initiated
become increasingly pronounced. Among young plants the smaller, outgrown by
their better-placed neighbours, are continually more shaded and more left
behind; and among the litter the weakly ones, continually thrust aside by
the stronger, become relatively more weakly from deficient nutrition.
Differentiations thus arising, both before and after separation from
parents, though primarily differences of growth, entail structural
differences; for it is a general law of nutrition that when there is
deficiency of food the non-essential organs suffer more than the essential
ones, and the unlikenesses of proportion hence arising constitute
unlikenesses of structure. It may be concluded, however, that variations
generated in this manner usually have no permanent results. In the first
place, the individuals which, primarily in growth and secondarily in
smaller developments of less-important organs, are by implication inferior,
are likely to be eliminated from the species. In the second place,
differences of structure produced in the way shown do not express
differences of constitution--are not the effects of somewhat divergent
physiological units; and consequently are not likely to be repeated in
posterity.
§ 88. We have still, therefore, to explain those variations which have no
manifest causes of the kinds thus far considered. These are the variations
termed "spontaneous." Not that those who apply to them this word, or some
equivalent, mean to imply that they are uncaused. Mr. Darwin expressly
guards himself against such an interpretation. He says:--"I have hitherto
sometimes spoken as if the variations--so common and multiform in organic
beings under domestication, and in a lesser degree in those in a state of
nature--had been due to chance. This, of course, is a wholly incorrect
expression, but it serves to acknowledge plainly our ignorance of the cause
of each particular variation." Not only, however, do I hold, in common with
Mr. Darwin, that there must be some cause for these apparently-spontaneous
variations, but it seems to me that a definite cause is assignable. I think
it may be shown that unlikenesses must necessarily arise even between the
new individuals simultaneously produced by the same parents. Instead of the
occurrence of such variations being inexplicable, the absence of them would
be inexplicable.
In any series of dependent changes a small initial difference often works a
marked difference in the results. The mode in which a particular breaker
bursts on the beach, may determine whether the seed of some foreign plant
which it bears is or is not stranded--may cause the presence or absence of
this plant from the Flora of the land; and may so affect, for millions of
years, in countless ways, the living creatures throughout the land. A
single touch, by introducing into the body some morbid matter, may set up
an immensely involved set of functional disturbances and structural
alterations. The whole tenor of a life may be changed by a word of advice;
or a glance may determine an action which alters thoughts, feelings, and
deeds throughout a long series of years. In those still more involved
combinations of changes which societies exhibit, this truth is still more
conspicuous. A hair's-breadth difference in the direction of some soldier's
musket at the battle of Arcola, by killing Napoleon, might have changed
events throughout Europe; and though the type of social organization in
each European country would have been now very much what it is, yet in
countless details it would have been different.
Illustrations like these, with which pages might be filled, prepare us for
the conclusion that organisms produced by the same parents at the same
time, must be more or less differentiated, both by insensible initial
differences and by slight differences in the conditions to which they are
subject during their evolution. We need not, however, rest with assuming
such initial differences: the necessity of them is demonstrable. The
individual germ-cells which, in succession or simultaneously, are separated
from the same parent, can never be exactly alike; nor can the sperm-cells
which fertilize them. When treating of the instability of the homogeneous
(_First Principles_, § 149), we saw that no two parts of any aggregate can
be similarly conditioned with respect to incident forces; and that being
subject to forces that are more or less unlike, they must become more or
less unlike. Hence, no two ova in an ovarium or ovules in a seed-vessel--no
two spermatozoa or pollen-cells, can be identical. Whether or not there
arise other contrasts, there are certain to arise quantitative contrasts;
since the process of nutrition cannot be absolutely alike for all. The
reproductive centres must begin to differentiate from the very outset. Such
being the necessities of the case, what will happen on any successive or
simultaneous fertilizations? Inevitably unlikenesses between the respective
parental influences must result. Quantitative differences among the
sperm-cells and among the germ-cells, will insure this. Grant that the
number of physiological units contained in any one reproductive cell, can
rarely if ever be exactly equal to the number contained in any other,
ripened at the same time or at a different time; and it follows that among
the fertilized germs produced by the same parents, the physiological units
derived from them respectively will bear a different numerical ratio to
each other in every case. If the parents are constitutionally quite alike,
the variation in the ratio between the units they severally bequeath,
cannot cause unlikenesses among the offspring. But if otherwise, no two of
the offspring can be alike. In every case the small initial difference in
the proportions of the slightly-unlike units, will lead, during evolution,
to a continual multiplication of differences. The insensible divergence at
the outset will generate sensible divergences at the conclusion. Possibly
some may hence infer that though, in such case, the offspring must differ
somewhat from each other and from both parents, yet that in every one of
them there must result a homogeneous mixture of the traits of the two
parents. A little consideration shows that the reverse is inferable. If,
throughout the process of development, the physiological units derived from
each parent preserved the same ratio in all parts of the growing organism,
each organ would show as much as every other, the influence of either
parent. But no such uniform distribution is possible. It has been shown
(_First Principles_, § 163), that in any aggregate of mixed units
segregation must inevitably go on. Incident forces will tend ever to cause
separation of the two orders of units from each other--will tend to
integrate groups of the one order in one place and groups of the other
order in another place. Hence there must arise not a homogeneous mean
between the two parents, but a mixture of organs, some of which mainly
follow the one and some the other. And this is the kind of mixture which
observation shows us.
Still it may be fairly objected that however the attributes of the two
parents are variously mingled in their offspring, they must in all of them
fall between the extremes displayed in the parents. In no characteristic
could one of the young exceed both parents, were there no cause of
"spontaneous variation" but the one alleged. Evidently, then, there is a
cause yet unfound.
§ 89. Thus far we have contemplated the process under its simplest aspect.
While we have assumed the two parents to be somewhat unlike, we have
assumed that each parent has a homogeneous constitution--is built up of
physiological units which are exactly alike. But in no case can such a
homogeneity exist. Each parent had parents who were more or less
contrasted--each parent inherited at least two orders of physiological
units not quite identical. Here then we have a further cause of variation.
The sperm-cells or germ-cells which any organism produces, will differ from
each other not quantitatively only but qualitatively. Of the
slightly-unlike physiological units bequeathed to it, the reproductive
cells it casts off cannot habitually contain the same proportions; and we
may expect the proportions to vary not slightly but greatly. Just as,
during the evolution of an organism, the physiological units derived from
the two parents tend to segregate, and produce likeness to the male parent
in this part and to the female parent in that; so, during the formation of
reproductive cells, there will arise in one a predominance of the
physiological units derived from the father, and in another a predominance
of the physiological units derived from the mother. Thus, then, every
fertilized germ, besides containing different _amounts_ of the two parental
influences, will contain different _kinds_ of influences--this having
received a marked impress from one grandparent, and that from another.
Without further exposition the reader will see how this cause of
complication, running back through each line of ancestry, must produce in
every germ numerous minute differences among the units.
Here, then, we have a clue to the multiplied variations, and sometimes
extreme variations, that arise in races which have once begun to vary. Amid
countless different combinations of units derived from parents, and through
them from ancestors, immediate and remote--amid the various conflicts in
their slightly-different organic polarities, opposing and conspiring with
one another in all ways and degrees; there will from time to time arise
special proportions causing special deviations. From the general law of
probabilities it may be concluded that while these involved influences,
derived from many progenitors, must, on the average of cases, obscure and
partially neutralize one another; there must occasionally result such
combinations of them as will produce considerable divergences from average
structures; and, at rare intervals, such combinations as will produce very
marked divergences. There is thus a correspondence between the inferable
results and the results as habitually witnessed.
§ 90. Still there remains a difficulty. It may be said that admitting
functional change to be the initiator of variation--granting that the
physiological units of an organism long subject to new conditions, will
tend to become modified in such way as to cause change of structure in
offspring; yet there will still be no cause of the supposed heterogeneity
among the physiological units of different individuals. There seems
validity in the objection, that as all the members of a species whose
circumstances have been altered will be affected in the same manner, the
results, when they begin to show themselves in descendants, will show
themselves in the same manner: not multiform variations will arise, but
deviations all in one direction.
The reply is simple. The members of a species thus circumstanced will _not_
be similarly affected. In the absence of absolute uniformity among them,
the functional changes caused in them will be more or less dissimilar. Just
as men of slightly-unlike dispositions behave in quite opposite ways under
the same circumstances; or just as men of slightly-unlike constitutions get
diverse disorders from the same cause, and are diversely acted on by the
same medicine; so, the insensibly-differentiated members of a species whose
conditions have been changed, may at once begin to undergo various kinds of
functional changes. As we have already seen, small initial contrasts may
lead to large terminal contrasts. The intenser cold of the climate into
which a species has migrated, may cause in one individual increased
consumption of food to balance the greater loss of heat; while in another
individual the requirement may be met by a thicker growth of fur. Or, when
meeting with the new foods which a new region furnishes, accident may
determine one member of the species to begin with one kind and another
member with another kind; and hence may arise established habits in these
respective members and their descendants. Now when the functional
divergences thus set up in sundry families of a species have lasted long
enough to affect their constitutions, and to modify somewhat the
physiological units thrown off in their reproductive cells, the divergences
produced by these in offspring will be of divers kinds. And the original
homogeneity of constitution having been thus destroyed, variation may go on
with increasing facility. There will result a heterogeneous mixture of
modifications of structure caused by modifications of function; and of
still more numerous correlated modifications, indirectly so caused. By
natural selection of the most divergent forms, the unlikenesses of parents
will be rendered more marked, and the limits of variation wider. Until at
length the divergences of constitutions and modes of life, become great
enough to lead to segregation of the varieties.
§ 91. That variations must occur, and that they must ever tend, both
directly and indirectly, towards adaptive modifications, are conclusions
deducible from first principles; apart from any detailed interpretations
like the above. That the state of homogeneity is an unstable state we have
found to be a universal truth. Each species must pass from the uniform into
the more or less multiform, unless the incidence of external forces is
exactly the same for all its members, which it never can be. Through the
process of differentiation and integration, which of necessity brings
together, or keeps together, like individuals, and separates unlike ones
from them, there must nevertheless be maintained a tolerably uniform
species, so long as there continues a tolerably uniform set of conditions
in which it may exist. But if the conditions change, either absolutely by
some disturbance of the habitat or relatively by spread of the species into
other habitats, then the divergent individuals that result must be
segregated by the divergent sets of conditions into distinct varieties
(_First Principles_, § 166). When, instead of contemplating a species in
the aggregate, we confine our attention to a single member and its
descendants, we see it to be a corollary from the general law of
equilibration that the moving equilibrium constituted by the vital actions
in each member of this family, must remain constant so long as the external
actions to which they correspond remain constant; and that if the external
actions are changed, the disturbed balance of internal changes, if not
overthrown, cannot cease undergoing modification until the internal changes
are again in equilibrium with the external actions: corresponding
structural alterations having arisen.
On passing from these derivative laws to the ultimate law, we see that
Variation is necessitated by the persistence of force. The members of a
species inhabiting any area cannot be subject to like sets of forces over
the whole of that area. And if, in different parts of the area, different
kinds or amounts or combinations of forces act on them, they cannot but
become different in themselves and in their progeny. To say otherwise, is
to say that differences in the forces will not produce differences in the
effects; which is to deny the persistence of force.
CHAPTER X.
GENESIS, HEREDITY, AND VARIATION.
§ 92. A question raised, and hypothetically answered, in §§ 78 and 79, was
there postponed until we had dealt with the topics of Heredity and
Variation. Let us now resume the consideration of this question, in
connexion with sundry others which the facts suggest.
After contemplating the several methods by which the multiplication of
organisms is carried on--after ranging them under the two heads of
Homogenesis, in which the successive generations are similarly produced,
and Heterogenesis, in which they are dissimilarly produced--after observing
that Homogenesis is nearly always sexual genesis, while Heterogenesis is
asexual genesis with occasionally-recurring sexual genesis; we came to the
questions--why is it that some organisms multiply in the one way and some
in the other? and why is it that where agamogenesis prevails it is usually,
from time to time, interrupted by gamogenesis? In seeking answers to these
questions, we inquired whether there are common to both Homogenesis and
Heterogenesis, any conditions under which alone sperm-cells and germ-cells
arise and are united for the production of new organisms; and we reached
the conclusion that, in all cases, they arise only when there is an
approach to equilibrium between the forces which produce growth and the
forces which oppose growth. This answer to the question--_when_ does
gamogenesis recur? still left unanswered the question--_why_ does
gamogenesis recur? And to this the reply suggested was, that the approach
towards general equilibrium in organisms, "is accompanied by an approach
towards molecular equilibrium in them; and that the need for this union of
sperm-cell with germ-cell is the need for overthrowing this equilibrium,
and re-establishing active molecular change in the detached germ--a result
probably effected by mixing the slightly-different physiological units of
slightly-different individuals." This is the hypothesis which we have now
to consider. Let us first look at the evidences which certain inorganic
phenomena furnish.
The molecules of any aggregate which have not a balanced arrangement,
inevitably tend towards a balanced arrangement. As before mentioned (_First
Principles_, § 100), amorphous wrought iron, when subject to continuous
jar, begins to arrange itself into crystals--its atoms assume a condition
of polar equilibrium. The particles of unannealed glass, which are so
unstably arranged that slight disturbing forces make them separate into
small groups, take advantage of that greater freedom of movement given by a
raised temperature, to adjust themselves into a state of relative rest.
During any such re-arrangement the aggregate exercises a coercive force
over its units. Just as in a growing crystal the atoms successively
assimilated from the solution, are made by the already crystallized atoms
to take a certain form, and even to re-complete that form when it is
broken; so in any mass of unstably-arranged atoms which passes into a
stable arrangement, each atom conforms to the forces exercised on it by all
the other atoms. This is a corollary from the general law of equilibration.
We saw (_First Principles_, § 170) that every change is towards
equilibrium; and that change can never cease until equilibrium is reached.
Organisms, above all other aggregates, conspicuously display this
progressive equilibration; because their units are of such kinds, and so
conditioned, as to admit of easy re-arrangement. Those extremely active
changes which go on during the early stages of evolution, imply an immense
excess of the molecular forces over those antagonist forces which the
aggregate exercises on the molecules. While this excess continues, it is
expended in growth, development, and function: expenditure for any of these
purposes being proof that part of the force constituting molecular tensions
remains unbalanced. Eventually, however, this excess diminishes. Either, as
in organisms which do not expend much energy, decrease of assimilation
leads to its decline; or, as in organisms which expend much energy, it is
counterbalanced by the rapidly-increasing reactions of the aggregate
(§ 46). The cessation of growth when followed, as in some organisms, by
death, implies the arrival at an equilibrium between the molecular forces
and those forces which the aggregate opposes to them. When, as in other
organisms, growth ends in the establishment of a moving equilibrium, there
is implied such a decreased preponderance of the molecular forces, as
leaves no surplus beyond that which is used up in functions. The declining
functional activity characteristic of advancing life, expresses a further
decline in this surplus. And when all vital movements come to an end, the
implication is that the actions of the units on the aggregate and the
reactions of the aggregate on the units are completely balanced. Hence,
while a state of rapid growth indicates such a play of forces among the
units of an aggregate as will produce active re-distribution, the
diminution and arrest of growth shows that the units have fallen into such
relative positions that re-distribution is no longer so facile. When,
therefore, we see that gamogenesis recurs only when growth is decreasing,
or has come to an end, we must say that it recurs only when the organic
units are approximating to equilibrium--only when their mutual restraints
prevent them from readily changing their arrangements in obedience to
incident forces.
That units of like forms can be built up into a more stable aggregate than
units of slightly unlike forms, is tolerably manifest _à priori_. And we
have facts which prove that mixing allied but somewhat different units,
_does_ lead to comparative instability. Most metallic alloys exemplify this
truth. Common solder, which is a mixture of lead and tin, melts at a much
lower temperature than either lead or tin. The compound of lead, tin, and
bismuth, called "fusible metal," becomes fluid at the temperature of
boiling water; while the temperatures at which lead, tin, and bismuth
become fluid are, respectively, 612°, 442°, and 497° F. Still more
remarkable is the illustration furnished by potassium and sodium. These
metals are very near akin in all respects--in their specific gravities,
their atomic weights, their chemical affinities, and the properties of
their compounds. That is to say, all the evidences unite to show that their
units, though not identical, have a close resemblance. What now happens
when they are mixed? Potassium alone melts at 136°, sodium alone melts at
190°, but the alloy of potassium and sodium is liquid at the ordinary
temperature of the air. Observe the meaning of these facts, expressed in
general terms. The maintenance of a solid form by any group of units
implies among them an arrangement so stable that it is not overthrown by
the incident forces. Whereas the assumption of a liquid form implies that
the incident forces suffice to destroy the arrangement of the units. In the
one case the thermal undulations fail to dislocate the parts; while in the
other case the parts are so dislocated by the thermal undulations that they
fall into total disorder--a disorder admitting of easy re-arrangement into
any other order. For the liquid state is a state in which the units become
so far free from mutual restraints, that incident forces can change their
relative positions very readily. Thus we have reason to conclude that an
aggregate of units which, though in the main similar to one another, have
minor differences, must be more unstable than an aggregate of homogeneous
units. The one will yield to disturbing forces which the other successfully
resists.
Now though the colloidal molecules of which organisms are mainly built, are
themselves highly composite; and though the physiological units compounded
out of these colloidal molecules must have structures far more involved;
yet it must happen with such units, as with simple units, that those which
have exactly like forms will admit of arrangement into a more stable
aggregate than those which have slightly-unlike forms. Among units of this
order, as among units of a simpler order, imperfect similarity must entail
imperfect balance in anything formed of them, and consequent diminished
ability to withstand disturbing forces. Hence, given two organisms which,
by diminished nutrition or increased expenditure, are being arrested in
their growths--given in each an approaching equilibrium between the forces
of the units and the forces of the aggregate--given, that is, such a
comparatively balanced state among the units that re-arrangement of them by
incident forces is no longer so easy; and it will follow that by uniting a
group of units from the one organism with a group of slightly-different
units from the other, the tendency towards equilibrium will be diminished,
and the mixed units will be rendered more modifiable in their arrangements
by the forces acting on them: they will be so far freed as to become again
capable of that re-distribution which constitutes evolution.
And now let us test this hypothesis by seeing what power it gives us of
interpreting established inductions.
§ 93. The majority of plants being hermaphrodites, it has, until quite
recently, been supposed that the ovules of each flower are fertilized by
pollen from the anthers of the same flower. Mr. Darwin, however, has shown
that the arrangements are generally such as to prevent this. Either the
ovules and the pollen are not ripe simultaneously, or obstacles prevent
access of the one to the other. At the same time he has shown that there
exist arrangements, often of a remarkable kind, which facilitate the
transfer of pollen by insects from the stamens of one flower to the pistil
of another. Similarly, it has been found that among the lower animals,
hermaphrodism does not usually involve the production of fertile ova by the
union of sperm-cells and germ-cells developed in the same individual; but
that the reproductive centres of one individual are united with those of
another to produce fertile ova. Either, as in _Pyrosoma_, _Perophora_, and
in many higher molluscs, the ova and spermatozoa are matured at different
times; or, as in annelids, they are prevented by their relative positions
from coming in contact.
Remembering the fact that among the higher classes of organisms,
fertilization is always effected by combining the sperm-cell of one
individual with the germ-cell of another; and joining with it the above
fact that among hermaphrodite organisms, the germ-cells developed in any
individual are usually not fertilized by sperm-cells developed in the same
individual; we see reason for thinking that the essential thing in
fertilization, is the union of specially-fitted portions of _different_
organisms. If fertilization depended on the peculiar properties of
sperm-cell and germ-cell, as such; then, in hermaphrodite organisms, it
would be a matter of indifference whether the united sperm-cells and
germ-cells were those of the same individual or those of different
individuals. But the circumstance that there exist in such organisms
elaborate appliances for mutual fertilization, shows that unlikeness of
derivation in the united reproductive centres, is the desideratum. Now this
is just what the foregoing hypothesis implies. If, as was concluded,
fertilization has for its object the disturbance of that approaching
equilibrium existing among the physiological units separated from an adult
organism; and if, as we saw reason to think, this object is effected by
mixture with the slightly-different physiological units of another
organism; then, we at the same time see that this object will not be
effected by mixture with physiological units belonging to the same
organism. Thus, the hypothesis leads us to expect such provisions as we
find.
§ 94. But here a difficulty presents itself. These propositions seem to
involve the conclusion that self-fertilization is impossible. It apparently
follows from them, that a group of physiological units from one part of an
organism ought to have no power of altering the state of approaching
balance in a group from another part of it. Yet self-fertilization does
occur. Though the ovules of one plant are generally fertilized by pollen
from another plant of the same kind, yet they may be, some of them,
fertilized by pollen of the same plant; and, indeed, there are plants in
which self-fertilization is the rule: even provision being in some cases
made to prevent fertilization by pollen from other individuals. And though,
among hermaphrodite animals, self-fertilization is usually negatived by
structural or functional arrangements, yet in certain _Entozoa_ there
appear to be special provisions by which the sperm-cells and the germ-cells
of the same individual may be united, when not previously united with those
of another individual. Nay, it has even been shown that in certain
Ascidians the contents of oviduct and spermiduct of the same individual
produce, when united, fertile ova whence evolve perfect individuals.
Certainly, at first sight, these facts do not consist with the above
supposition. Nevertheless there is something like a solution.
In the last chapter, when considering the variations caused in offspring
from uniting elements representing unlike parental constitutions, it was
pointed out that in an unfolding organism, composed of slightly-different
physiological units derived from slightly-different parents, there cannot
be maintained an even distribution of the two orders of units. We saw that
the instability of the homogeneous negatives the uniform blending of them;
and that, by the process of differentiation and integration, they must be
more or less separated; so that in one part of the body the influence of
one parent will predominate, and in another part of the body the influence
of the other parent: an inference which harmonizes with daily observation.
We also saw that the sperm-cells or germ-cells produced by such an organism
must, in virtue of these same laws, be more or less unlike one another. It
was shown that through segregation, some of the sperm-cells or germ-cells
will get an excess of the physiological units derived from one side, and
some of them an excess of those derived from the other side: a cause which
accounts for the unlikenesses among offspring simultaneously produced. Now
from this segregation of the different orders of physiological units,
inherited from different parents and lines of ancestry, there arises the
possibility of self-fertilization in hermaphrodite organisms. If the
physiological units contained in the sperm-cells and germ-cells of the same
flower, are not quite homogeneous--if in some of the ovules the
physiological units derived from the one parent greatly predominate, and in
some of the ovules those derived from the other parent; and if the like is
true of the pollen-cells; then, some of the ovules may be nearly as much
contrasted with some of the pollen-cells in the characters of their
contained units, as were the ovules and pollen-cells of the parents from
which the plant proceeded. Between part of the sperm-cells and part of the
germ-cells, the community of nature will be such that fertilization will
not result from their union; but between some of them, the differences of
constitution will be such that their union will produce the requisite
molecular instability. The facts, so far as they are known, seem in harmony
with this deduction. Self-fertilization in flowers, when it takes place, is
not so efficient as mutual fertilization. Though some of the ovules produce
seeds, yet more of them than usual are abortive. From which, indeed,
results the establishment of varieties that have structures favourable to
mutual fertilization; since, being more prolific, these have, other things
equal, greater chances in the "struggle for existence."
Further evidence is at hand supporting this interpretation. There is reason
to believe that self-fertilization, which at the best is comparatively
inefficient, loses all efficiency in course of time. After giving an
account of the provisions for an occasional, or a frequent, or a constant
crossing between flowers; and after quoting Prof. Huxley to the effect that
among hermaphrodite animals, there is no case in which "the occasional
influence of a distinct individual can be shown to be physically
impossible;" Mr. Darwin writes--"from these several considerations and from
the many special facts which I have collected, but which I am not here able
to give, I am strongly inclined to suspect that, both in the vegetable and
animal kingdoms, an occasional intercross with a distinct individual is a
law of nature ... in none, as I suspect, can self-fertilization go on for
perpetuity." This conclusion, based wholly on observed facts, is just the
conclusion to which the foregoing argument points. That necessary action
and the re-action between the parts of an organism and the organism as a
whole--that power of an aggregate to re-mould the units, which is the
correlative of the power of the units to build up into such an aggregate;
implies that any differences existing among the units inherited by an
organism, must gradually diminish. Being subject in common to the total
forces of the organism, they will in common be modified towards congruity
with these forces, and therefore towards likeness with one another. If,
then, in a self-fertilizing organism and its self-fertilizing descendants,
such contrasts as originally existed among the physiological units are
progressively obliterated--if, consequently, there can no longer be a
segregation of different physiological units in different sperm-cells and
germ-cells; self-fertilization will become impossible. Step by step the
fertility will diminish, and the series will finally die out.
And now observe, in confirmation of this view, that self-fertilization is
limited to organisms in which an approximate equilibrium among the organic
forces is not long maintained. While growth is actively going on, and the
physiological units are subject to a continually-changing distribution of
forces, no decided assimilation of the units can be expected: like forces
acting on the unlike units will tend to segregate them, so long as
continuance of evolution permits further segregation; and only when further
segregation cannot go on, will the like forces tend to assimilate the
units. Hence, where there is no prolonged maintenance of an approximate
organic balance, self-fertilization may be possible for some generations;
but it will be impossible in organisms distinguished by a sustained moving
equilibrium.
§ 95. The interpretation which it affords of sundry phenomena familiar to
breeders of animals, adds probability to the hypothesis. Mr. Darwin has
collected a large "body of facts, showing, in accordance with the almost
universal belief of breeders, that with animals and plants a cross between
different varieties, or between individuals of the same variety but of
another strain, gives vigour and fertility to the offspring; and on the
other hand, that _close_ interbreeding diminishes vigour and fertility,"--a
conclusion harmonizing with the current belief respecting
family-intermarriages in the human race. Have we not here a solution of
these facts? Relations must, on the average of cases, be individuals whose
physiological units are more nearly alike than usual. Animals of different
varieties must be those whose physiological units are more unlike than
usual. In the one case, the unlikeness of the units may frequently be
insufficient to produce fertilization; or, if sufficient to produce
fertilization, not sufficient to produce that active molecular change
required for vigorous development. In the other case, both fertilization
and vigorous development will be made probable.
Nor are we without a cause for the irregular manifestations of these
general tendencies. The mixed physiological units composing any organism
being, as we have seen, more or less segregated in the reproductive centres
it throws off; there may arise various results according to the degrees of
difference among the units, and the degrees in which the units are
segregated. Of two cousins who have married, the common grandparents may
have had either similar or dissimilar constitutions; and if their
constitutions were dissimilar, the probability that their married
grandchildren will have offspring will be greater than if their
constitutions were similar. Or the brothers and sisters from whom these
cousins descended, instead of severally inheriting the constitutions of
their parents in tolerably equal degrees, may have severally inherited them
in very different degrees: in which last case, intermarriages among the
cousins will be less likely to prove infertile. Or the brothers and sisters
from whom these cousins descended, may severally have married persons very
like, or very unlike, themselves; and from this cause there may have
resulted, either an undue likeness, or a due unlikeness, between the
married cousins.[39] These several causes, conspiring and conflicting in
endless ways and degrees, will work multiform effects. Moreover,
differences of segregation will make the reproductive centres produced by
the same nearly-related organisms, vary considerably in their amounts of
unlikeness; and therefore, supposing their amounts of unlikeness great
enough to cause fertilization, this fertilization will be effective in
various degrees. Hence it may happen that among offspring of nearly-related
parents, there may be some in which the want of vigour is not marked, and
others in which there is decided want of vigour. So that we are alike shown
why in-and-in breeding tends to diminish both fertility and vigour: and why
the effect cannot be a uniform effect, but only an average effect.
§ 96. While, if the foregoing arguments are valid, gamogenesis has for its
main result the initiation of a new development by the overthrow of that
approximate equilibrium arrived at among the molecules of the
parent-organisms, a further result appears to be subserved by it. Those
inferior organisms which habitually multiply by agamogenesis, have
conditions of life that are simple and uniform; while those organisms which
have highly-complex and variable conditions of life, habitually multiply by
gamogenesis. Now if a species has complex and variable conditions of life,
its members must be severally exposed to sets of conditions that are
slightly different: the aggregates of incident forces cannot be alike for
all the scattered individuals. Hence, as functional deviation must ever be
inducing structural deviation, each individual throughout the area occupied
tends to become fitted for the particular habits which its particular
conditions necessitate; and in so far, _un_fitted for the average habits
proper to the species. But these undue specializations are continually
checked by gamogenesis. As Mr. Darwin remarks, "intercrossing plays a very
important part in nature in keeping the individuals of the same species, or
of the variety, true and uniform in character:" the idiosyncratic
divergences obliterate one another. Gamogenesis, then, is a means of
turning to positive advantage the individual differentiations which, in its
absence, would result in positive disadvantage. Were it not that
individuals are ever being made unlike one another by their unlike
conditions, there would not arise in them those contrasts of molecular
constitution, which we have seen to be needful for producing the fertilized
germs of new individuals. And were not these individual differentiations
ever being mutually cancelled, they would end in a fatal narrowness of
adaptation.
This truth will be most clearly seen if we reduce it to its purely abstract
form, thus:--Suppose a quite homogeneous species, placed in quite
homogeneous conditions; and suppose the constitutions of all its members in
complete concord with their absolutely-uniform and constant conditions;
what must happen? The species, individually and collectively, is in a state
of perfect moving equilibrium. All disturbing forces have been eliminated.
There remains no force which can, in any way, change the state of this
moving equilibrium; either in the species as a whole or in its members. But
we have seen (_First Principles_, § 173) that a moving equilibrium is but a
transition towards complete equilibration, or death. The absence of
differential or un-equilibrated forces among the members of a species, is
the absence of all forces which can cause changes in the conditions of its
members--is the absence of all forces which can initiate new organisms. To
say, as above, that complete molecular homogeneity existing among the
members of a species, must render impossible that mutual molecular
disturbance which constitutes fertilization, is but another way of saying
that the actions and re-actions of each organism, being in perfect balance
with the actions and re-actions of the environment upon it, there remains
in each organism no force by which it differs from any other--no force
which any other does not meet with an equal force--no force which can set
up a new evolution among the units of any other.
And so we reach the remarkable conclusion that the life of a species, like
the life of an individual, is maintained by the unequal and ever-varying
actions of incident forces on its different parts.[40] An individual
homogeneous throughout, and having its substance everywhere continuously
subject to like actions, could undergo none of those changes which life
consists of; and similarly, an absolutely-uniform species, having all its
members exposed to identical influences, would be deprived of that
initiator of change which maintains its existence as a species. Just as, in
each organism, incident forces constantly produce divergences from the mean
state in various directions, which are constantly balanced by opposite
divergences indirectly produced by other incident forces; and just as the
combination of rhythmical functions thus maintained, constitutes the life
of the organism; so, in a species, there is, through gamogenesis, a
perpetual neutralization of those contrary deviations from the mean state
which are caused in its different parts by different sets of incident
forces; and it is similarly by the rhythmical production and compensation
of these contrary deviations, that the species continues to live. The
moving equilibrium in a species, like the moving equilibrium in an
individual, would rapidly end in complete equilibration, or death, were not
its continually-dissipated forces continually re-supplied from without.
Besides owing to the external world those energies which, from moment to
moment, keep up the lives of its individual members, every species owes to
certain more indirect actions of the external world, those energies which
enable it to perpetuate itself in successive generations.
§ 97. What evidence still remains may be conveniently woven up along with a
recapitulation of the argument pursued through the last three chapters. Let
us contemplate the facts in their synthetic order.
That compounding and re-compounding through which we pass from the simplest
inorganic substances to the most complex organic substances, has several
concomitants. Each successive stage of composition presents us with
molecules that are severally larger or more integrated, that are severally
more heterogeneous, that are severally more unstable, and that are more
numerous in their kinds (_First Principles_, § 151). And when we come to
the substances of which living bodies are formed, we find ourselves among
innumerable divergent groups and sub-groups of compounds, the units of
which are large, heterogeneous, and unstable, in high degrees. There is no
reason to assume that this process ends with the formation of those complex
colloids which constitute organic matter. A more probable assumption is
that out of the complex colloidal molecules there are evolved, by a still
further integration, molecules which are still more heterogeneous, and of
kinds which are still more multitudinous. What must be their properties?
Already the colloidal molecules are extremely unstable--capable of being
variously modified in their characters by very slight incident forces; and
already the complexity of their polarities prevents them from readily
falling into such positions of equilibrium as results in crystallization.
Now the organic molecules composed of these colloidal molecules, must be
similarly characterized in far higher degrees. Far more numerous must be
the minute changes that can be wrought in them by minute external forces;
far more free must they remain for a long time to obey forces tending to
re-distribute them; and far greater must be the number of their kinds.
Setting out with these physiological units, the existence of which various
organic phenomena compel us to recognize, and the production of which the
general law of Evolution thus leads us to anticipate; we get an insight
into the phenomena of Genesis, Heredity, and Variation. If each organism is
built of certain of these highly-plastic units peculiar to its
species--units which slowly work towards an equilibrium of their complex
proclivities, in producing an aggregate of the specific structure, and
which are at the same time slowly modifiable by the re-actions of this
aggregate--we see why the multiplication of organisms proceeds in the
several ways, and with the various results, which naturalists have
observed.
Heredity, as shown not only in the repetition of the specific structure but
in the repetition of ancestral deviations from it, becomes a matter of
course; and it falls into unison with the fact that, in various inferior
organisms, lost parts can be replaced, and that, in still lower organisms,
a fragment can develop into a whole.
While an aggregate of physiological units continues to grow by the
assimilation of matter which it moulds into other units of like type; and
while it continues to undergo changes of structure; no equilibrium can be
arrived at between the whole and its parts. Under these conditions, then,
an un-differentiated portion of the aggregate--a group of physiological
units not bound up into a specialized tissue--will be able to arrange
itself into the structure peculiar to the species; and will so arrange
itself, if freed from controlling forces and placed in fit conditions of
nutrition and temperature. Hence the continuance of agamogenesis in
little-differentiated organisms, so long as assimilation continues to be
greatly in excess of expenditure.
But let growth be checked and development approach its completion--let the
units of the aggregate be severally exposed to an almost constant
distribution of forces; and they must begin to equilibrate themselves.
Arranged, as they will gradually be, into comparatively stable attitudes in
relation to one another, their mobility will diminish; and groups of them,
partially or wholly detached, will no longer readily re-arrange themselves
into the specific form. Agamogenesis will be no longer possible; or, if
possible, will be no longer easy.
When we remember that the force which keeps the Earth in its orbit is the
gravitation of each particle in the Earth towards every one of the group of
particles existing 92,000,000 of miles off; we cannot reasonably doubt that
each unit in an organism acts on all the other units, and is reacted on by
them: not by gravitation only but chiefly by other energies. When, too, we
learn that glass has its molecular constitution changed by light, and that
substances so rigid and stable as metals have their atoms re-arranged by
forces radiated in the dark from adjacent objects;[41] we are obliged to
conclude that the excessively-unstable units of which organisms are built,
must be sensitive in a transcendant degree to all the forces pervading the
organisms composed of them--must be tending ever to re-adjust, not only
their relative attitudes but their molecular structures, into equilibrium
with these forces. Hence, if aggregates of the same species are differently
conditioned, and re-act differently on their component units, their
component units will be rendered somewhat different; and they will become
the more different the more widely the re-actions of the aggregates upon
them differ, and the greater the number of generations through which these
different re-actions of the aggregates upon them are continued.
If, then, unlikenesses of function among individuals of the same species,
produce unlikenesses between the physiological units of one individual and
those of another, it becomes comprehensible that when groups of units
derived from two individuals are united, the group formed will be more
unstable than either of the groups was before their union. The mixed units
will be less able to resist those re-distributing forces which cause
evolution; and may thus have restored to them the capacity for development
which they had lost.
This view harmonizes with the conclusion, which we saw reason to draw, that
fertilization does not depend on any intrinsic peculiarities of sperm-cells
and germ-cells, but depends on their derivation from different individuals.
It explains the facts that nearly-related individuals are less likely to
have offspring than others, and that their offspring, when they have them,
are frequently feeble. And it gives us a key to the converse fact that the
crossing of varieties results in unusual vigour.
Bearing in mind that the slightly-different orders of physiological units
which an organism inherits from its parents, are subject to the same set of
forces, and that when the organism is fully developed this set of forces,
becoming constant, tends slowly to re-mould the two orders of units into
the same form; we see how it happens that self-fertilization becomes
impossible in the higher organisms, while it remains possible in the lower
organisms. In long-lived creatures which have tolerably-definite limits of
growth, this assimilation of the somewhat-unlike physiological units is
liable to go on to an appreciable extent; whereas in organisms which do not
continuously subject their component units to constant forces, there will
be much less of this assimilation. And where the assimilation is not
considerable, the segregation of mixed units may cause the sperm-cells and
germ-cells developed in the same individual, to be sufficiently different
to produce, by their union, fertile germs; and several generations of
self-fertilizing descendants may succeed one another, before the two orders
of units have had their unlikenesses so far diminished that they will no
longer do this. The same principles explain for us the variable results of
union between nearly-related organisms. According to the contrasts among
the physiological units they inherit from parents and ancestors; according
to the unlike proportions of the contrasted units which they severally
inherit; and according to the degrees of segregation of such units in
different sperm-cells and germ-cells; it may happen that two kindred
individuals will produce the ordinary number of offspring or will produce
none; or will at one time be fertile and at another not; or will at one
time have offspring of tolerable strength and at another time feeble
offspring.
To the like causes are also ascribable the phenomena of Variation. These
are unobtrusive while the tolerably-uniform conditions of a species
maintain tolerable uniformity among the physiological units of its members;
but they become obtrusive when differences of conditions, entailing
considerable functional differences, have entailed decided differences
among the physiological units, and when the different physiological units,
differently mingled in every individual, come to be variously segregated
and variously combined.
Did space permit, it might be shown that this hypothesis is a key to many
further facts--to the fact that mixed races are comparatively plastic under
new conditions; to the fact that pure races show predominant influences in
the offspring when crossed with mixed races; to the fact that while mixed
breeds are often of larger growth, pure breeds are the more hardy--have
functions less-easily thrown out of balance. But without further argument
it will, I think, be admitted that the power of this hypothesis to explain
so many phenomena, and to bring under a common bond phenomena which seem so
little allied, is strong evidence of its truth. And such evidence gains
greatly in strength on observing that this hypothesis brings the facts of
Genesis, Heredity, and Variation into harmony with first principles. We see
that these plastic physiological units, which we find ourselves obliged to
assume, are just such more integrated, more heterogeneous, more unstable,
and more multiform molecules, as would result from continuance of the steps
through which organic matter is reached. We see that the differentiations
of them assumed to occur in differently-conditioned aggregates, and the
equilibrations of them assumed to occur in aggregates which maintain
constant conditions, are but corollaries from those universal principles
implied by the persistence of force. We see that the maintenance of life in
the successive generations of a species, becomes a consequence of the
continual incidence of new forces on the species, to replace the forces
that are ever being rhythmically equilibrated in the propagation of the
species. And we thus see that these apparently-exceptional phenomena
displayed in the multiplication of organic beings, fall into their places
as results of the general laws of Evolution. We have, therefore, weighty
reasons for entertaining the hypothesis which affords us this
interpretation.
CHAPTER X^A.
GENESIS, HEREDITY, AND VARIATION
_CONCLUDED_.
§ 97a. Since the foregoing four chapters were written, thirty-four years
ago, the topics with which they deal have been widely discussed and many
views propounded. Ancient hypotheses have been abandoned, and other
hypotheses, referring tacitly or avowedly to the cell-doctrine, have been
set forth. Before proceeding it will be well to describe the chief among
these.
Most if not all of them proceed on the assumption, shown in § 66 to be
needful, that the structural characters of organisms are determined by the
special natures of units which are intermediate between the chemical units
and the morphological units--between the invisible molecules of
proteid-substances and the visible tissue-components called cells.
Four years after the first edition of this volume was published, appeared
Mr. Darwin's work, _The Variation of Animals and Plants under
Domestication_; and in this he set forth his doctrine of Pangenesis.
Referring to the doctrine of physiological units which the preceding
chapters work out, he at first expressed a doubt whether his own was or was
not the same, but finally concluded that it was different. He was right in
so concluding. Throughout my argument the implication everywhere is that
the physiological units are all of one kind; whereas Mr. Darwin regards his
component units, or "gemmules," as being of innumerable unlike kinds. He
supposes that every cell of every tissue gives off gemmules special to
itself, and capable of developing into similar cells. We may here, in
passing, note that this view implies a fundamental distinction between
unicellular organisms and the component cells of multicellular organisms,
which are otherwise homologous with them. For while in their essential
structures, their essential internal changes, and their essential processes
of division, the _Protozoa_ and the component units of the _Metazoa_ are
alike, the doctrine of Pangenesis implies that though the units when
separate do not give off invisible gemmules the grouped units do.
Much more recently have been enunciated the hypotheses of Prof. Weismann,
differing from the foregoing hypotheses in two respects. In the first place
it is assumed that the fragment of matter out of which each organism arises
consists of two portions--one of them, the germ-plasm, reserved within the
generative organ of the incipient individual, representing in its
components the structure of the species, and gives origin to the germs of
future individuals; and the other of them, similarly representative of the
specific structure, giving origin to the rest of the body, or soma, but
contains in its components none of those latent powers possessed by those
of the germ-plasm. In the second place the germ-plasm, in common with the
soma-plasm, consists of multitudinous kinds of units portioned out to
originate the various organs. Of these there are groups, sub-groups, and
sub-sub-groups. The largest of them, called "idants," are supposed each to
contain a number of "ids"; within each id there are numerous
"determinants"; and each determinant is made up of many "biophors"--the
smallest elements possessing vitality. Passing over details, the essential
assumption is that there exists a separate determinant for each part of the
organism capable of independent variation; and Prof. Weismann infers that
while there may be but one for the blood and but one for a considerable
area of skin (as a stripe of the zebra) there must be a determinant for
each scale on a butterfly's wing: the number on the four wings being over
two hundred thousand. And then each cluster of biophors composing a
determinant has to find its way to the place where there is to be formed
the part it represents.
Here it is needless to specify the modifications of these hypotheses
espoused by various biologists--all of them assuming that the structural
traits of each species are expressed in certain units intermediate between
morphological units and chemical units.
§ 97b. A true theory of heredity must be one which recognizes the relevant
phenomena displayed by all classes of organism. We cannot assume two kinds
of heredity, one for plants and another for animals. Hence a theory of
heredity may be first tested by observing whether it is equally applicable
to both kingdoms of living things. Genesis, heredity, and variation, as
seen in plants, are simpler and more accessible than as seen in animals.
Let us then note what these imply.
Already in § 77 I have illustrated the power which some plants possess of
developing new individuals from mere fragments of leaves and even from
detached scales. Striking as are the facts there instanced, they are
scarcely more significant than some which are familiar. The formation of
cauline buds, presently growing into shoots, shows us a kind of inheritance
which a true theory must explain. As described by Kerner, such buds arise
in Pimpernel, Toad-flax, etc., below the seed-leaves, even while yet there
are no axils in which buds usually grow; and in many plants they arise from
intermediate places on the stem: that is, without definite relations to
pre-existing structures. How fortuitous is their origin is shown when a
branch is induced to bud by keeping it wrapped round with a wet cloth. Even
still better proved is the absence of any relation between cauline buds and
normal germs by the frequent growth of them out of "callus"--the tissue
which spreads over wounds and the cut ends of branches. It is not easy to
reconcile these facts with Mr. Darwin's hypothesis of gemmules. We have to
assume that where a cauline bud emerges there are present in due
proportions gemmules of all the parts which will presently arise from
it--leaves, stipules, bracts, petals, stamens, anthers, etc. We have to
assume this though, at the time the bud originates, sundry of these organs,
as the parts of flowers, do not exist on the plant or tree. And we have to
assume that the gemmules of such parts are duly provided in a portion of
adventitious callus, far away from the normal places of fructification.
Moreover, the resulting shoot may or may not produce all the parts which
the gemmules represent; and when, perhaps after years, flowers are produced
on its side shoots, there must exist at each point the needful proportion
of the required gemmules; though there have been no cells continually
giving them off.
Still less does the hypothesis of Prof. Weismann harmonize with the
evidence as plants display it. Plant-embryogeny yields no sign of
separation between germ-plasm and soma-plasm; and, indeed, the absence of
such separation is admitted. After instancing cases among certain of the
lower animals, in which no differentiation of the two arises in the first
generation resulting from a fertilized ovum, Prof. Weismann continues:--
"The same is true as regards the higher plants, in which the first shoot
arising from the seed never contains germ-cells, or even cells which
subsequently become differentiated into germ cells. In all these
last-mentioned cases the germ-cells are not present in the first person
arising by embryogeny as special cells, but are only formed in much later
cell-generations from the offspring of certain cells of which this first
person was composed." (_Germ-Plasm_, p. 185.)
How this admission consists with the general theory it is difficult to
understand. The units of the soma-plasm are here recognized as having the
same generative powers as the units of the germ-plasm. In so far as one
organic kingdom and a considerable part of the other are concerned the
doctrine is relinquished. Relinquishment is, indeed, necessitated even by
the ordinary facts, and still more by the facts just instanced. Defence of
it involves the assertion that where buds arise, normal or cauline, there
exist in due proportion the various ids with their contained
determinants--that these are diffused throughout the growing part of the
soma; and this implies that the somatic tissue does not differ in
generative power from the germ-plasm.
The hypothesis of physiological units, then, remains outstanding. For
cauline buds imply that throughout the plant-tissue, where not unduly
differentiated, the local physiological units have a power of arranging
themselves into the structure of the species.
But this hypothesis, too, as it now stands, is inadequate. Under the form
thus far given to it, it fails to explain some accompanying facts. For if
the branch just instanced as producing a cauline bud be cut off and its end
stuck in the ground, or if it be bent down and a portion of it covered with
earth, there will grow from it rootlets and presently roots. The same
portion of tissue which otherwise would have produced a shoot with all its
appendages, constituting an individual, now produces only a special part of
an individual.
§ 97c. Certain kindred facts of animal development may now be considered.
Similar insufficiencies are disclosed.
The often-cited reproduction of a crab's lost claw or a lizard's tail, Mr.
Darwin thought explicable by his hypothesis of diffused gemmules,
representing all organs or their component cells. But though, after simple
amputation, regrowth of the proximate part of the tail is conceivable as
hence resulting, it is not easy to understand how the remoter part, the
components of which are now absent from the organism, can arise afresh from
gemmules no longer originated in due proportion. Prof. Weismann's
hypothesis, again, implies that there must exist at the place of
separation, a ready-provided supply of determinants, previously latent,
able to reproduce the missing tail in all its details--nay, even to do this
several times over: a strong supposition! The hypothesis of physiological
units, as set forth in preceding chapters, appears less incompetent:
reproduction of the lost part would seem to be a normal result of the
proclivity towards the form of the entire organism. But now what are we to
say when, instead of being cut off transversely, the tail is divided
longitudinally and each half becomes a complete tail? What are we to say
when, if these two tails are similarly dealt with, the halves again
complete themselves; and so until as many as sixteen tails have been
formed? Here the hypothesis of physiological units appears to fail utterly;
for the tendency it implies is to complete the specific form, by
reproducing a single tail only.
Various annulose animals display anomalies of development difficult to
explain on any hypothesis. We have creatures like the fresh-water _Nais_
which, though it has advanced structures, including a vascular system,
branchiæ, and a nervous cord ending with cephalic ganglia, nevertheless
shows us an ability like that of the _Hydra_ to reproduce the whole from a
small part: nearly forty pieces into which a _Nais_ was cut having
severally grown into complete animals. Again we have, in the order
_Polychætæ_, types like _Myrianida_, in which by longitudinal budding a
string of individuals, sometimes numbering even thirty, severally develop
certain segments into heads, while increasing their segments in number. In
yet other types there occurs not longitudinal gemmation only, but lateral
gemmation: a segment will send out sideways a bud which presently becomes a
complete worm. Once more, _Syllis ramosa_ is a species in which the
individual worms growing from lateral buds, while remaining attached to the
parent, themselves give origin to buds; and so produce a branched aggregate
of worms. How shall we explain the reparative and reproductive powers thus
exemplified? It seems undeniable that each portion has an ability to
produce, according to circumstances, the whole creature or a missing part
of the creature. When we read of Sir J. Dalyell that he "cut a _Dasychone_
into three pieces; the hindermost produced a head, the anterior piece
developed an anus, and the middle portion formed both a head and a tail" we
are not furnished with an explanation by the hypothesis of gemmules or by
the hypothesis of determinants; for we cannot arbitrarily assume that
wherever a missing organ has to be produced there exists the needful supply
of gemmules or of determinants representing that organ. The hypothesis that
physiological units have everywhere a proclivity towards the organic form
of the species, appears more congruous with the facts; but even this does
not cover the cases in which a new worm grows from a lateral bud. The
tendency to complete the individual structure might be expected rather to
restrain this breaking of the lines of complete structure.
Still less explicable in any way thus far proposed are certain remedial
actions seen in animals. An example of them was furnished in § 67, where
"false joints" were described--joints formed at places where the ends of a
broken bone, failing to unite, remain moveable one upon the other.
According to the character of the habitual motions there results a rudely
formed hinge-joint or a ball-and-socket joint, either having the various
constituent parts--periosteum, fibrous tissue, capsule, ligaments. Now Mr.
Darwin's hypothesis, contemplating only normal structures, fails to account
for this formation of an abnormal structure. Neither can we ascribe this
local development to determinants: there were no appropriate ones in the
germ-plasm, since no such structure was provided for. Nor does the
hypothesis of physiological units, as presented in preceding chapters,
yield an interpretation. These could have no other tendency than to restore
the normal form of the limb, and might be expected to oppose the genesis of
these new parts.
Thus we have to seek, if not another hypothesis, then some such
qualification of an existing hypothesis as will harmonize it with various
exceptional phenomena.
§ 97d. In Part II of the _Principles of Sociology_, published in 1876, will
be found elaborated in detail that analogy between individual organization
and social organization which was briefly sketched out in an essay on "The
Social Organism" published in 1860. In §§ 241-3 a parallel is drawn between
the developments of the sustaining systems of the two; and it is pointed
out how, in the one case as in the other, the components--here organic
units and there citizens--have their activities and arrangements mainly
settled by local conditions. One leading example is that the parts
constituting the alimentary canal, while jointly fitted to the nature of
the food, are severally adapted to the successive stages at which the food
arrives in its progress; and that in an analogous way the industries
carried on by peoples forming different parts of a society, are primarily
determined by the natures of things around--agriculture, pastoral and
arable, special manufactures and minings, ship-building and fishing: the
respective groups falling into fit combinations and becoming partially
modified to suit their work. The implication is that while the organization
of a society as a whole depends on the characters of its units, in such way
that by some types of men despotisms are always evolved while by other
types there are evolved forms of government partially free--forms which
repeat themselves in colonies--there is, on the other hand, in every case a
local power of developing appropriate structures. And it might have been
pointed out that similarly in types of creatures not showing much
consolidation, as the _Annelida_, many of the component divisions, largely
independent in their vitalities, are but little affected in their
structures by the entire aggregate.
My purpose at that time being the elucidation of sociological truths, it
did not concern me to carry further the biological half of this comparison.
Otherwise there might have been named the case in which a supernumerary
finger, beginning to bud out, completes itself as a local organ with bones,
muscles, skin, nail, etc., in defiance of central control: even repeating
itself when cut off. There might also have been instanced the above-named
formation of a false joint with its appurtenances. For the implication in
both cases is that a local group of units, determined by circumstances
towards a certain structure, coerces its individual units into that
structure.
Now let us contemplate the essential fact in the analogy. The men in an
Australian mining-camp, as M. Pierre Leroy Beaulieu points out, fall into
Anglo-Saxon usages different from those which would characterize a French
mining-camp. Emigrants to a far West settlement in America quickly
establish post-office, bank, hotel, newspaper, and other urban
institutions. We are thus shown that along with certain traits leading to a
general type of social organization, there go traits which independently
produce fit local organizations. Individuals are led into occupations and
official posts, often quite new to them, by the wants of those around--are
now influenced and now coerced into social arrangements which, as shown
perhaps by gambling saloons, by shootings at sight, and by lynchings, are
scarcely at all affected by the central government. Now the physiological
units in each species appear to have a similar combination of capacities.
Besides their general proclivity towards the specific organization, they
show us abilities to organize themselves locally; and these abilities are
in some cases displayed in defiance of the general control, as in the
supernumerary finger or the false joint. Apparently each physiological
unit, while having in a manner the whole organism as the structure which,
along with the rest, it tends to form, has also an aptitude to take part in
forming any local structure, and to assume its place in that structure
under the influence of adjacent physiological units.
A familiar fact supports this conclusion. Everyone has at hand, not
figuratively but literally, an illustration. Let him compare the veins on
the backs of his two hands, either with one another or with the veins on
another person's hands, and he will see that the branchings and
inosculations do not correspond: there is no fixed pattern. But on
progressing inwards from the extremities, the distribution of the veins
becomes settled--there is a pattern-arrangement common to all persons.
These facts imply a predominating control by adjacent parts where control
by the aggregate is less easy. A constant combination of forces which,
towards the centre, produces a typical structure, fails to do this at the
periphery where, during development, the play of forces is less settled.
This peripheral vascular structure, not having become fixed because one
arrangement is as good as another, is in each determined by the immediately
surrounding influences.
§ 97e. And now let us contemplate the verifications which recent
experiments have furnished--experiments made by Prof. G. Born of Breslau,
confirming results earlier reached by Vulpian and adding more striking
results of kindred nature. They leave no longer doubtful the large share
taken by local organizing power as distinguished from central organizing
power.
The independent vitality shown by separated portions of ventral skin from
frog-larvæ may be named as the first illustration. With their attached
yolk-cells these lived for days, and underwent such transformations as
proved some structural proclivity, though of course the product was
amorphous. Detached portions of tails of larvæ went on developing their
component parts in much the same ways as they would have done if remaining
attached. More striking still was the evidence furnished by experiments in
grafting. These proved that the undifferentiated rudiment of an organ will,
when cut off and joined to a non-homologous place in another individual,
develop itself as it would have done if left in its original place. In
brief, then, we may say that each part is in chief measure autogenous.
These strange facts presented by small aggregates of organic matter, which
are the seats of extremely complex forces, will seem less incomprehensible
if we observe what has taken place in a vast aggregate of inorganic matter
which is the seat of very simple forces--the Solar System. Transcendently
different as this is in all other respects, it is analogous in the respect
that, as factors of local structures, local influences predominate over the
influences of the aggregate. For while the members of the Solar System,
considered as a whole, are subordinate to the totality of its forces, the
arrangements in each part of it are produced almost wholly by the play of
forces in that part. Though the Sun affects the motions of the Moon, and
though during the evolution of the Earth-and-Moon system the Sun exercised
an influence, yet the relations of our world and its satellite in respect
of masses and motions were in the main locally determined. Still more
clearly was it thus with Jupiter and his satellites or Saturn with his
rings and satellites. Remembering that the ultimate units of matter of
which the Solar System is composed are of the same kinds, and that they act
on one another in conformity with the same laws, we see that, remote as the
case is from the one we are considering in all other respects, it is
similar in the respect that during organization the energies in each
locality work effects which are almost independent of the effects worked by
the general energies. In this vast aggregate, as in the minute aggregates
now in question, the parts are practically autogenous.
Having thus seen that in a way we have not hitherto recognized the same
general principles pervade inorganic and organic evolution, let us revert
to the case of super-organic evolution from which a parallel was drawn
above. As analogous to the germinal mass of units out of which a new
organism is to evolve, let us take an assemblage of colonists not yet
socially organized but placed in a fertile region--men derived from a
society (or rather a succession of societies) of long-established type, who
have in their adapted natures the proclivity towards that type. In passing
from its wholly unorganized state to an organized state, what will be the
first step? Clearly this assemblage, though it may have within the
constitutions of its units the potentialities of a specific structure, will
not develop forthwith the details of that structure. The inherited natures
of its units will first show themselves by separating into large groups
devoted to strongly-distinguished occupations. The great mass, dispersing
over promising lands, will make preparations for farming. Another
considerable portion, prompted by the general needs, will begin to form a
cluster of habitations and a trading centre. Yet a third group, recognizing
the demand for wood, alike for agricultural and building purposes, will
betake themselves to the adjacent forests. But in no case will the primary
assemblage, before these separations, settle the arrangements and actions
of each group: it will leave each group to settle them for itself. So, too,
after these divisions have arisen. The agricultural division will not as a
whole prescribe the doings of its members. Spontaneous segregation will
occur: some going to a pastoral region and some to a tract which promises
good crops. Nor within each of these bodies will the organization be
dictated by the whole. The pastoral group will separate itself into
clusters who tend sheep on the hills and clusters who feed cattle on the
plains. Meanwhile those who have gravitated towards urban occupations will
some of them make bricks or quarry stone, while others fall into classes
who build walls, classes who prepare fittings, classes who supply
furniture. Then along with completion of the houses will go occupation of
them by men who bake bread, who make clothing, who sell liquors, and so on.
Thus each great group will go on organizing itself irrespective of the
rest; the sub-groups within each will do the same; and so will the
sub-sub-groups. Quite independently of the people on the hills and the
plains and in the town, those in the forest will divide spontaneously into
parties who cut down trees, parties who trim and saw them, parties who
carry away the timbers; while every party forms for itself an organization
of "butty" or "boss," and those who work under him. Similarly with the
ultimate divisions--the separate families: the arrangements and
apportionments of duties in each are internally determined. Mark the fact
which here chiefly concerns us. This formation of a heterogeneous aggregate
with its variously adapted parts, which while influenced by the whole are
mainly self-formed, goes on among units of essentially the same natures,
inherited from units who belonged to similar societies. And now, carrying
this conception with us, we may dimly perceive how, in a developing embryo,
there may take place the formation, first of the great divisions--the
primary layers--then of the outlines of systems, then of component organs,
and so on continually with the minor structures contained in major
structures; and how each of these progressively smaller divisions develops
its own organization, irrespective of the changes going on throughout the
rest of the embryo. So that though all parts are composed of physiological
units of the same nature, yet everywhere, in virtue of local conditions and
the influence of its neighbours, each unit joins in forming the particular
structure appropriate to the place. Thus conceiving the matter, we may in a
vague way understand the strange facts of autogenous development disclosed
by the above named experiments.
§ 97f. "But how immeasurably complex must be the physiological units which
can behave thus!" will be remarked by the reader. "To be able to play all
parts, alike as members of the whole and as members of this or that organ,
they must have an unimaginable variety of potentialities in their natures.
Each must, indeed, be almost a microcosm within a microcosm."
Doubtless this is true. Still we have a _consensus_ of proofs that the
component units of organisms have constitutions of extremely involved
kinds. Contemplate the facts and their implications. (1) Here is some large
division of the animal kingdom--say the _Vertebrata_. The component units
of all its members have certain fundamental traits in common: all of them
have proclivities towards formation of a vertebral column. Leaving behind
the great assemblage of Fishes with its multitudinous types, each having
special units of composition, we pass to the _Amphibia_, in the units of
which there exist certain traits superposed upon the traits they have in
common with those of Fishes. Through unknown links we ascend to incipient
Mammalian types and then to developed Mammalian types, the units of which
must have further superposed traits. Additional traits distinguish the
units of each Mammalian order; and, again, those of every genus included in
it; while others severally characterize the units of each species.
Similarly with the varieties in each species, and the stirps in each
variety. Now the ability of any component unit to carry within itself the
traits of the sub-kingdom, class, order, genus, species, variety, and at
the same time to bear the traits of immediate ancestors, can exist only in
a something having multitudinous proximate elements arranged in innumerable
ways. (2) Again, these units must be at once in some respects fixed and in
other respects plastic. While their fundamental traits, expressing the
structure of the type, must be unchangeable, their superficial traits must
admit of modification without much difficulty; and the modified traits,
expressing variations in the parents and immediate ancestors, though
unstable, must be considered as capable of becoming stable in course of
time. (3) Once more we have to think of these physiological units (or
constitutional units as I would now re-name them) as having such natures
that while a minute modification, representing some small change of local
structure, is inoperative on the proclivities of the units throughout the
rest of the system, it becomes operative in the units which fall into the
locality where that change occurs.
But unimaginable as all this is, the facts may nevertheless in some way
answer to it. As before remarked, progressing science reveals complexity
within complexity--tissues made up of cells, cells containing nuclei and
cytoplasm, cytoplasm formed of a protoplasmic matrix containing granules;
and if now we conclude that the unit of protoplasm is itself an
inconceivably elaborate structure, we do but recognize the complexity as
going still deeper. Further, if we must assume that these component units
are in every part of the body acting on one another by extremely
complicated sets of forces (ethereal undulations emanating from each of the
constituent molecules) determining their relative positions and actions, we
are warranted by the discoveries which every day disclose more of the
marvellous properties of matter. When to such examples as were given in
§ 36e we add the example yielded by recent experiments, showing that even a
piece of bread, after subjection to pressure, exhibits diamagnetic
properties unlike those it previously exhibited, we cannot doubt that these
complex units composing living bodies are all of them seats of energies
diffused around, enabling them to act and re-act so as to modify one
another's states and positions. We are shown, too, that whatever be the
natures of the complex forces emanating from each, it will, as a matter of
course, happen that the power of each will be relatively great in its own
neighbourhood and become gradually smaller in parts increasingly remote:
making more comprehensible the autogenous character of each local
structure.
Whatever be their supposed natures we are compelled to ascribe extreme
complexity to these unknown somethings which have the power of organizing
themselves into a structure of this or that species. If gemmules be
alleged, then the ability of every organ and part of an organ to vary,
implies that the gemmules it gives off are severally capable of receiving
minute modifications of their ordinary structures: they must have many
parts admitting of innumerable relations. Supposing determinants be
assumed, then in addition to the complexity which each must have to express
in itself the structure of the part evolved from it, it must have the
further complexity implied by every superposed modification which causes a
variation of that part. And, as we have just seen, the hypothesis of
physiological units does not relieve us from the need for kindred
suppositions.
One more assumption seems necessary if we are to imagine how changes of
structure caused by changes of function can be transmitted. Reverting to
§ 54d, where an unceasing circulation of protoplasm throughout an organism
was inferred, we must conceive that the complex forces of which each
constitutional unit is the centre, and by which it acts on other units
while it is acted on by them, tend continually to remould each unit into
congruity with the structures around: superposing on it modifications
answering to the modifications which have arisen in those structures.
Whence is to be drawn the corollary that in course of time all the
circulating units,--physiological, or constitutional if we prefer so to
call them--visiting all parts of the organism, are severally made bearers
of traits expressing local modifications; and that those units which are
eventually gathered into sperm-cells and germ-cells also bear these
superposed traits.
If against all this it be urged that such a combination of structures and
forces and processes is inconceivably involved, then the reply is that so
astonishing a transformation as that which an unfolding organism displays
cannot possibly be effected by simple agencies.
§ 97g. But now let it be confessed that none of these hypotheses serves to
render the phenomena really intelligible; and that probably no hypothesis
which can be framed will do this. Many problems beyond those which
embryology presents have to be solved; and no solution is furnished.
What are we to say of the familiar fact that certain small organs which,
with the approach to maturity, become active, entail changes of structure
in remote parts--that after the testes have undergone certain final
developments, the hairs on the chin grow and the voice deepens? It has been
contended that certain concomitant modifications in the fluids throughout
the body may produce correlated sexual traits; and there is proof that in
many of the lower animals the period of sexual activity is accompanied by a
special bodily state--sometimes such that the flesh becomes unwholesome and
even poisonous. But a change of this kind can hardly account for a
structural change in the vocal organs in Man. No hypothesis of gemmules or
determinants or physiological units enables us to understand how removal of
the testes prevents those developments of the larynx and vocal cords which
take place if they remain.
The inadequacy of our explanations we at once see in presence of a
structure like a peacock's tail-feather. Mr. Darwin's hypothesis is that
all parts of every organ are continually giving off gemmules, which are
consequently everywhere present in their due proportions. But a completed
feather is an inanimate product and, once formed, can add to the
circulating fluids no gemmules representing all its parts. If we follow
Prof. Weismann we are led into an astounding supposition. He admits that
every variable part must have a special determinant, and that this results
in the assumption of over two hundred thousand for the four wings of a
butterfly. Let us ask what must happen in the case of a peacock's feather.
On looking at the eye near its end, we see that the minute processes on the
edge of each lateral thread must have been in some way exactly adjusted, in
colour and position, so as to fall into line with the processes on adjacent
threads: otherwise the symmetrical arrangement of coloured rings would be
impossible. Each of these processes, then, being an independent variable,
must have had its particular determinant. Now there are about 300 threads
on the shaft of a large feather, and each of them bears on the average
1,600 processes, making for the whole feather 480,000 of these processes.
For one feather alone there must have been 480,000 determinants, and for
the whole tail many millions. And these, along with the determinants for
the detailed parts of all the other feathers, and for the variable
components of all organs forming the body at large, must have been
contained in the microscopic head of a spermatozoon! Hardly a credible
supposition. Nor is it easy to see how we are helped by the hypothesis of
constitutional units. Take the feather in its budding state and ask how the
group of such units, alike in structure and perpetually multiplying while
the unfolding goes on, can be supposed by their mutual actions so to affect
one another as eventually to produce the symmetrically-adjusted processes
which constitute the terminal eye. Imagination, whatever licence may be
given, utterly fails us.
At last then we are obliged to admit that the actual organizing process
transcends conception. It is not enough to say that we cannot know it; we
must say that we cannot even conceive it. And this is just the conclusion
which might have been drawn before contemplating the facts. For if, as we
saw in the chapter on "The Dynamic Element in Life," it is impossible for
us to understand the nature of this element--if even the ordinary
manifestations of it which a living body yields from moment to moment are
at bottom incomprehensible; then, still more incomprehensible must be that
astonishing manifestation of it which we have in the initiation and
unfolding of a new organism.
Thus all we can do is to find some way of symbolizing the process so as to
enable us most conveniently to generalize its phenomena; and the only
reason for adopting the hypothesis of physiological units or constitutional
units is that it best serves this purpose.
CHAPTER XI.
CLASSIFICATION.
§ 98. That orderly arrangement of objects called Classification has two
purposes, which, though not absolutely distinct, are distinct in great
part. It may be employed to facilitate identification, or it may be
employed to organize our knowledge. If a librarian places his books in the
alphabetical succession of the author's names, he places them in such way
that any particular book may easily be found, but not in such way that
books of a given nature stand together. When, otherwise, he makes a
distribution of books according to their subjects, he neglects various
superficial similarities and distinctions, and groups them according to
certain primary and secondary and tertiary attributes, which severally
imply many other attributes--groups them so that any one volume being
inspected, the general characters of all the neighbouring volumes may be
inferred. He puts together in one great division all works on History; in
another all Biographical works; in another all works that treat of Science;
in another Voyages and Travels; and so on. Each of his great groups he
separates into sub-groups; as when he puts different kinds of Literature
under the heads of Fiction, Poetry, and the Drama. In some cases he makes
sub-sub-groups; as when, having divided his Scientific treatises into
abstract and concrete, putting in the one Logic and Mathematics and in the
other Physics, Astronomy, Geology, Chemistry, Physiology, &c.; he goes on
to sub-divide his books on Physics, into those which treat of Mechanical
Motion, those which treat of Heat, those which treat of Light, of
Electricity, of Magnetism.
Between these two modes of classification note the essential distinctions.
Arrangement according to any single conspicuous attribute is comparatively
easy, and is the first that suggests itself: a child may place books in the
order of their sizes, or according to the styles of their bindings. But
arrangement according to combinations of attributes which, though
fundamental, are not conspicuous, requires analysis; and does not suggest
itself till analysis has made some progress. Even when aided by the
information which the author gives on his title page, it requires
considerable knowledge to classify rightly an essay on Polarization; and in
the absence of a title page it requires much more knowledge. Again,
classification by a single attribute, which the objects possess in
different degrees, may be more or less serial, or linear. Books may be put
in the order of their dates, in single file; or if they are grouped as
works in one volume, works in two volumes, works in three volumes, &c., the
groups may be placed in an ascending succession. But groups severally
formed of things distinguished by some common attribute which implies many
other attributes, do not admit of serial arrangement. You cannot rationally
say either that Historical Works should come before Biographical Works, or
Biographical Works before Historical Works; nor of the sub-divisions of
creative Literature, into Fiction, Poetry, and the Drama, can you give a
good reason why any one should take precedence of the others.
Hence this grouping of the like and separation of the unlike which
constitutes Classification, can reach its complete form only by slow steps.
I have shown (_Essays_, Vol. II., pp. 145-7) that, other things equal, the
relations among phenomena are recognized in the order of their
conspicuousness; and that, other things equal, they are recognized in the
order of their simplicity. The first classifications are sure, therefore,
to be groupings of objects which resemble one another in external or
easily-perceived attributes, and attributes that are not of complex
characters. Those likenesses among things which are due to their possession
in common of simple obvious properties, may or may not coexist with further
likenesses among them. When geometrical figures are classed as curvilinear
and rectilinear, or when the rectilinear are divided into trilateral,
quadrilateral, &c., the distinctions made connote various other
distinctions with which they are necessarily bound up; but if liquids be
classed according to their visible characters--if water, alcohol, sulphuret
of carbon, &c., be grouped as colourless and transparent, we have things
placed together which are unlike in their essential natures. Thus, where
the objects classed have numerous attributes, the probabilities are that
the early classifications, based on simple and manifest attributes, unite
under the same head many objects that have no resemblance in the majority
of their attributes. As the knowledge of objects increases, it becomes
possible to make groups of which the members have more numerous properties
in common; and to ascertain what property, or combination of properties, is
most characteristic of each group. And the classification eventually
arrived at is of such kind that the objects in each group have more
attributes in common with one another than they have in common with any
excluded objects; one in which the groups of such groups are integrated on
the same principle; and one in which the degrees of differentiation and
integration are proportioned to the degrees of intrinsic unlikeness and
likeness. And this ultimate classification, while it serves to identify the
things completely, serves also to express the greatest amount of knowledge
concerning the things--enables us to predicate the greatest number of facts
about each thing; and by so doing implies the most precise correspondence
between our conceptions and the realities.
§ 99. Biological classifications illustrate well these phases through which
classifications in general pass. In early attempts to arrange organisms in
some systematic manner, we see at first a guidance by conspicuous and
simple characters, and a tendency towards arrangement in linear order. In
successively later attempts, we see more regard paid to combinations of
characters which are essential but often inconspicuous, and an abandonment
of a linear arrangement for an arrangement in divergent groups and
re-divergent sub-groups.
In the popular mind, plants are still classed under the heads of Trees,
Shrubs, and Herbs; and this serial classing according to the single
attribute of magnitude, swayed the earliest observers. They would have
thought it absurd to call a bamboo, thirty feet high, a kind of grass; and
would have been incredulous if told that the Hart's-tongue should be placed
in the same great division with the Tree-ferns. The zoological
classifications current before Natural History became a science, had
divisions similarly superficial and simple. Beasts, Birds, Fishes, and
Creeping-things are names of groups marked off from one another by
conspicuous differences of appearance and modes of life--creatures that
walk and run, creatures that fly, creatures that live in the water,
creatures that crawl. And these groups were thought of in the order of
their importance.
The first arrangements made by naturalists were based either on single
characters or on very simple combinations of characters; as that of
Clusius, and afterwards the more scientific system of Cesalpino,
recognizing the importance of inconspicuous structures. Describing
plant-classifications, Lindley says:--"Rivinus invented, in 1690, a system
depending upon the formation of the corolla; Kamel, in 1693, upon the fruit
alone; Magnol, in 1720, on the calyx and corolla; and finally, Linnæus, in
1731, on variations in the stamens and pistil." In this last system, which
has been for so long current as a means of identification (regarded by its
author as transitional), simple external attributes are still depended on;
and an arrangement, in great measure serial, is based on the degrees in
which these attributes are possessed. In 1703, some thirty years before the
time of Linnæus, our countryman Ray had sketched the outlines of a more
advanced system. He said that--
Plants are either
Flowerless, or
Flowering; and these are
Dicotyledones, or
Monocotyledones.
Among the minor groups which he placed under these general heads, "were
Fungi, Mosses, Ferns, Composites, Cichoraceæ, Umbellifers, Papilionaceous
plants, Conifers, Labiates, &c., under other names, but with limits not
very different from those now assigned to them." Being much in advance of
his age, Ray's ideas remained dormant until the time of Jussieu; by whom
they were developed into what has become known as the Natural System: a
system subsequently improved by De Candolle. Passing through various
modifications in the hands of successive botanists, the Natural System is
now represented by the following form, which is based upon the table of
contents prefixed to Vol. II. of Prof. Oliver's translation of the _Natural
History of Plants_, by Prof. Kerner. His first division, Myxothallophyta (=
Myxomycetes), I have ventured to omit. The territory it occupies is in
dispute between zoologists and botanists, and as I have included the group
in the zoological classification, agreeing that its traits are more animal
than vegetal, I cannot also include it in the botanical classification.
Here, linear arrangement has disappeared: there is a breaking up into
groups and sub-groups and sub-sub-groups, which do not admit of being
placed in serial order, but only in divergent and re-divergent order. Were
there space to exhibit the way in which the Alliances are subdivided into
Orders, and these into Genera, and these into Species, the same principle
of co-ordination would be still further manifested.
PHYLA. CLASSES. ALLIANCES.
SUB-PHYLA. SUB-CLASSES.
THALLOPHYTA
I. Schizophyta
2. Cyanophyceæ. Blue-green Algæ.
3. Schizomycetes.
II. Dinoflagellata
Peridineæ
4.
III. Bacillariales
5.
IV. Gamophyceæ
I. Chlorophyceæ
6. Protococcoideæ.
7. Siphoneæ.
8. Confervoideæ.
9. Conjugatæ.
10. Charales.
11. Phæophyceæ.
12. Dictyotales.
13. Florideæ, Red Seaweeds.
V. Fungi
I. Phycomycetes
14. Oomycetes.
15. Zygomycetes.
II. Mesomycetes
16.
17.
III. Mycomycetes
18.
19.
Additional group of Fungi, Lichenes.
ARCHEGONIATÆ
I. Bryophyta
20. Hepaticæ, Liverworts.
21. Musci, Mosses.
II. Pteridophyta
Vas. Cryptogams
22. Filices, Ferns.
23. Hydropterides, Rhizocarps.
24. Equisetales, Horse-tails.
25. Lycopodiales, Club-mosses.
PHANEROGAMIA (Flowering Plants.)
GYMNOSPERMÆ
I. Cycadales, Cycads
26.
II. Coniferæ
27.
III. Gnetales
28.
ANGIOSPERMÆ
I. Monocotyledons
29. Liliifloræ.
30. Scitamineæ.
31. Gynandræ.
32. Fluviales.
33. Spadicifloræ.
34. Glumifloræ.
II. Dicotyledons
I. Monochlamydæ
35. Centrospermæ.
36. Protiales.
37. Daphnales.
38. Santalales.
39. Rafflesiales.
40. Asarales.
41. Euphorbiales.
42. Podostemales.
43. Viridifloræ.
44. Amentales.
45. Balanophorales.
II. Monopetalæ
46. Caprifoliales.
47. Asterales.
48. Campanales.
49. Ericales.
50. Vaccinales.
51. Primulales.
52. Tubifloræ.
III. Polypetalæ
53. Ranales.
54. Parietales.
55. Malvales.
56. Discifloræ.
57. Crateranthæ.
58. Myrtales.
59. Melastomales.
60. Lythrales.
61. Hygrobiæ.
62. Passifloræ.
63. Pepones.
64. Cactales.
65. Ficoidales.
66. Umbellales.
On studying the definitions of these primary, secondary, and tertiary
classes, it will be found that the largest are marked off from one another
by some attribute which connotes sundry other attributes; that each of the
smaller classes comprehended in one of these largest classes, is marked off
in a similar way from the other smaller classes bound up with it; and that
so, each successively smaller class has an increased number of co-existing
attributes.
§ 100. Zoological classification has had a parallel history. The first
attempt which we need notice, to arrange animals in such a way as to
display their affinities, is that of Linnæus. He grouped them thus:[42]--
CL. 1. MAMMALIA. _Ord._ Primates, Bruta, Feræ, Glires, Pecora, Belluæ,
Cete.
CL. 2. AVES. _Ord._ Accipitres, Picæ, Anseres, Grallæ, Gallinæ, Passeres.
CL. 3. AMPHIBIA. _Ord._ Reptiles, Serpentes, Nantes.
CL. 4. PISCES. _Ord._ Apodes, Jugulares, Thoracici, Abdominales.
CL. 5. INSECTA. _Ord._ Coleoptera, Hemiptera, Lepidoptera, Neuroptera,
Diptera, Aptera.
CL. 6. VERMES. _Ord._ Intestina, Mollusca, Testacea, Lithophyta,
Zoophyta.
This arrangement of classes is obviously based on apparent gradations of
rank; and the placing of the orders similarly betrays an endeavour to make
successions, beginning with the most superior forms and ending with the
most inferior forms. While the general and vague idea of perfection
determines the leading character of the classification, its detailed
groupings are determined by the most conspicuous external attributes. Not
only Linnæus but his opponents, who proposed other systems, were "under the
impression that animals were to be arranged together into classes, orders,
genera, and species, according to their more or less close external
resemblance." This conception survived until the time of Cuvier.
"Naturalists," says Agassiz, "were bent upon establishing one continual
uniform series to embrace all animals, between the links of which it was
supposed there were no unequal intervals. The watchword of their school
was: _Natura non facit saltum_. They called their system _la chaine des
êtres_."
The classification of Cuvier, based on internal organization instead of
external appearance, was a great advance. He asserted that there are four
principal forms, or four general plans, on which animals are constructed;
and, in pursuance of this assertion, he drew out the following scheme.
First Branch. ANIMALIA VERTEBRATA.
Cl. 1. Mammalia.
Cl. 2. Birds.
Cl. 3. Reptilia.
Cl. 4. Fishes.
Second Branch. ANIMALIA MOLLUSCA.
Cl. 1. Cephalapoda.
Cl. 2. Pteropoda.
Cl. 3. Gasteropoda.
Cl. 4. Acephala.
Cl. 5. Brachiopoda.
Cl. 6. Cirrhopoda.
Third Branch. ANIMALIA ARTICULATA.
Cl. 1. Annelides.
Cl. 2. Crustacea.
Cl. 3. Arachnides.
Cl. 4. Insects.
Fourth Branch. ANIMALIA RADIATA.
Cl. 1. Echinoderms.
Cl. 2. Intestinal Worms.
Cl. 3. Acalephæ.
Cl. 4. Polypi.
Cl. 5. Infusoria.
But though Cuvier emancipated himself from the conception of a serial
progression throughout the Animal Kingdom, sundry of his contemporaries and
successors remained fettered by the old error. Less regardful of the
differently-combined sets of attributes distinguishing the different
sub-kingdoms, and swayed by the belief in a progressive development which
was erroneously supposed to imply a linear arrangement of animals, they
persisted in thrusting organic forms into a quite unnatural order. The
following classification of Lamarck illustrates this.
INVERTEBRATA.
I. APATHETIC ANIMALS. }
}
Cl. 1. Infusoria. } Do not feel, and move only by their
Cl. 2. Polypi. } excited irritability. No brain, no
Cl. 3. Radiaria. } elongated medullary mass; no senses;
Cl. 4. Tunicata. } forms varied; rarely articulations.
Cl. 5. Vermes. }
II. SENSITIVE ANIMALS. } Feel, but obtain from their sensations
} only perceptions of objects, a
Cl. 6. Insects. } sort of simple ideas, which they are
Cl. 7. Arachnids. } unable to combine to obtain complex
Cl. 8. Crustacea. } ones. No vertebral column; a brain
Cl. 9. Annelids. } and mostly an elongated medullary
Cl. 10. Cirripeds. } mass; some distinct senses; muscles
Cl. 11. Conchifera. } attached under the skin; form symmetrical,
Cl. 12. Mollusks. } the parts being in pairs.
VERTEBRATA.
{ Feel; acquire preservable ideas;
III. INTELLIGENT ANIMALS. { perform with them operations by which
{ they obtain others; are intelligent in
Cl. 13. Fishes. { different degrees. A vertebral column;
Cl. 14. Reptiles. { a brain and a spinal marrow; distinct
Cl. 15. Birds. { senses; the muscles attached to the
Cl. 16. Mammalia. { internal skeleton; form symmetrical,
{ the parts being in pairs.
Passing over sundry classifications in which the serial arrangement
dictated by the notion of ascending complexity, is variously modified by
the recognition of conspicuous anatomical facts, we come to classifications
which recognize another order of facts--those of development. The
embryological inquiries of Von Baer led him to arrange animals as
follows:--
I. Peripheric Type. (RADIATA.) _Evolutio radiata._ The development
proceeds from a centre, producing identical parts in a radiating order.
II. Massive Type. (MOLLUSCA.) _Evolutio contorta._ The development
produces identical parts curved around a conical or other space.
III. Longitudinal Type. (ARTICULATA.) _Evolutio gemina._ The development
produces identical parts arising on both sides of an axis, and closing up
along a line opposite the axis.
IV. Doubly Symmetrical Type. (VERTEBRATA.) _Evolutio bigemina._ The
development produces identical parts arising on both sides of an axis,
growing upwards and downwards, and shutting up along two lines, so that
the inner layer of the germ is inclosed below, and the upper layer above.
The embryos of these animals have a dorsal cord, dorsal plates, and
ventral plates, a nervous tube and branchial fissures.
Recognizing these fundamental differences in the modes of development, as
answering to fundamental divisions in the animal kingdom, Von Baer shows
(among the _Vertebrata_ at least) how the minor differences which arise at
successively later embryonic stages, correspond with the minor divisions.
Like the modern classification of plants, the modern classification of
animals shows us the assumed linear order completely broken up. In his
lectures at the Royal Institution, in 1857, Prof. Huxley expressed the
relations existing among the several great groups of the animal kingdom, by
placing them at the ends of four or five radii, diverging from a centre.
The diagram I cannot obtain; but in the published reports of his lectures
at the School of Mines the groups were arranged as on the following page.
What remnant there may seem to be of linear succession in some of the
sub-groups contained in it, is merely an accident of typographical
convenience. Each of them is to be regarded simply as a cluster. And if
Prof. Huxley had further developed the arrangement, by dispersing the
sub-groups and sub-sub-groups on the same principle, there would result an
arrangement perhaps not much unlike that shown on the page succeeding this.
VERTEBRATA
(_Abranchiata_)
Mammalia
Aves
Reptilia
(_Branchiata_)
Amphibia
Pisces.
MOLLUSCA ANNULOSA
Cephalopoda Heteropoda } _Articulata._
Gasteropoda-dioecia } Insecta Arachnida
} Myriapoda Crustacea
{ Pulmonata Gasteropoda-monoecia
{ Pteropoda _Annuloida._
Lamellibranchiata Annellata Scoleidæ
Echinodermata Trematoda
Rotifera Tæniadæ
Turbellaria
Nematoidea
COELENTERATA
Hydrozoa Actinozoa.
PROTOZOA
Infusoria Spongiadæ Gregarinidæ
_Noctilucidæ_ Foraminifera _Thallassicollidæ_
In the woodcut, the dots represent orders, the names of which it is
impracticable to insert. If it be supposed that when magnified, each of
these dots resolves itself into a cluster of clusters, representing genera
and species, an approximate idea will be formed of the relations among the
successively-subordinate groups constituting the animal kingdom. Besides
the subordination of groups and their general distribution, some other
facts are indicated. By the distances of the great divisions from the
general centre, are rudely symbolized their respective degrees of
divergence from the form of simple, undifferentiated organic matter; which
we may regard as their common source. Within each group, the remoteness
from the local centre represents, in a rough way, the degree of departure
from the general plan of the group. And the distribution of the sub-groups
within each group, is in most cases such that those which come nearest to
neighbouring groups, are those which show the nearest resemblances to
them--in their analogies though not in their homologies. No such scheme,
however, can give a correct conception. Even supposing the above diagram
expressed the relations of animals to one another as truly as they can be
expressed on a plane surface (which of course it does not), it would still
be inadequate. Such relations cannot be represented in space of two
dimensions, but only in space of three dimensions.
_Mammalia_
_Aves_
_Reptilia_
VERTEBRATA
\
_Amphibia_\ _Pisces_
\
\ _Arachnida_
\ _Insecta_
\ _Crustacea_
\
\ Articulata
\ |
\ | _Myriapoda_
\ |
\ ANNULOSA
\ |
\ |_Annelida_
\ _Scolecida_
\ |
\ Annuloida
\ | /
\ _Echinodermata_
\ | /
_Pteropoda_ _Cephalopoda_\ | |
_Gasteropoda dioecia_ | |
_Gasteropoda monoecia_ _Pulmonata_ | |
\ | |
MOLLUSCA------------- \ | |
\ \ | |
_Lamellibranchiata_ \ \ | |
\ \ | |
_Brachiopoda_ \ \ | |
\ \ _Gregarinida_
Molluscoida------------
_Rhizopoda_ \
_Ascidioida_ _Polyzoa_ / \
/ PROTOZOA
/ _Spongida_ _Infusoria_
_Hydrozoa_ /
/
COELENTERATA
_Actinozoa_
§ 100a. Two motives have prompted me to include in its original form the
foregoing sketch: the one being that in conformity with the course
previously pursued, of giving the successive forms of classifications, it
seems desirable to give this form which was approved thirty-odd years ago;
and the other being that the explanatory comments remain now as applicable
as they were then. Replacement of the diagram by one expressing the
relations of classes as they are now conceived, is by no means an easy
task; for the conceptions formed of them are unsettled. Concerning the
present attitude of zoologists, Prof. MacBride writes:--
"They all recognize a certain number of phyla. Each phylum includes a
group of animals about whose relation to each other no one entertains a
doubt. Each zoologist, however, has his own idea as to the relationship
which the various phyla bear to each other.
"The phyla recognized at present are:--
(1) Protozoa.
(2) Porifera (Sponges).
(3) Coelenterata.
(4) Echinodermata.
{ Cestodes.
(5) Platyhelminthes { Trematodes.
{ Turbellaria.
(6) Nemertea.
(7) Nematoda.
(8) Acanthocephala (Echinorhyncus).
(9) Chætognatha (Sagitta).
(10) Rotifera.
(11) Annelida (Includes Leeches and Gephyrea, Chætifera).
(12) Gephyrea, Achæta.
{ Tracheata (Peripatus, Myriapods, Insects).
(13) Arthropods { Arachnids.
{ Crustacea.
{ Pycnogonida.
(14) Mollusca.
(15) Polyzoa (Including Phoronis).
(16) Brachiopoda.
(17) Chordata (Includes Balanoglossus and Tunicates. Some
continental zoologists do not admit Balanoglossus)."
[This last phylum of course includes the _Vertebrata_.]
Though under present conditions, as above implied, it would be absurd to
attempt a definite scheme of relationships, yet it has seemed to me that
the adumbration of a scheme, presenting in a vague way such relationships
as are generally agreed upon and leaving others indeterminate, may be
ventured; and that a general impression hence resulting may be useful. On
the adjacent page I have tried to make a tentative arrangement of this
kind.
At the bottom of the table I have placed together, under the name "Compound
_Protozoa_," those kinds of aggregated _Protozoa_ which show no
differentiations among the members of groups, and are thus distinguished
from _Metazoa_; and I have further marked the distinction by their
position, which implies that from them no evolution of higher types has
taken place. Respecting the naming of the sub-kingdoms, phyla, classes,
orders, &c., I have not maintained entire consistency. The relative values
of groups cannot be typographically expressed in a small space with a
limited variety of letters. The sizes of the letters mark the
classificatory ranks, and by the thickness I have rudely indicated their
zoological importance. In fixing the order of subordination of groups I
have been aided by the table of contents prefixed to Mr. Adam Sedgwick's
_Student's Text Book of Zoology_ and have also made use of Prof. Ray
Lankester's classifications of several sub-kingdoms.
_Placental_-----+
| _Aves_
_Mammalia_----+ | _Arachnida_
| | |
_Implacental_---+ | _Insecta_ | _Crustacea_
| | | | |
VERTEBRATA | | | | |
| | | | |
_Reptilia_ _Chilopoda_
| |
_Amphibia_ ARTHROPODA |
| |
| _Diplopoda_
_Pisces_ |
| _Chætopoda_
_Cephalochorda_ | |
CHORDATA ANNELIDA
_Urochorda_ | | _Echiuroidea_
_(Tunicata)_ | | _Hirudinea_
+---------+ _Archiannelida_
| |
BRACHIOPODA | | ROTIFERA
_Dibranchiata_ | | | |
_Cephalopoda_ | | +----+
_Tetrabranchiata_ | | | _Crinoidea_
MOLLUSCA ---------------+ | | | +------ ECHINODERMATA
_Scaphopoda_ | | | | | _Asteroidea_
_Solenogastres_ | | | | | _Echinoidea_
_Gasteropoda_ | | | | | _Holothuroidea_
_Lammellibranchiata_----+ | | | | _Enteropneusta_
| | | | |
| | | | | _Acanthocephala_
POLYZOA --------------------+ | | | | | +--- NEMATHELMINTHES
| | | | | | | _Nematomorpha_
| | | | | | | _Nematoda_
| | | | | | |
_Zoantharia_ Ctenophora
_Rugosa_ _Acalephos_
_Alcyonaria_ COELENTERATA _Hydromedusae_
Actinozoa Hydrozoa |
| | |-------- NEMERTEA
| | |
| +----------+ _Turbellaria_
----------------|
|
PROTOZOA PLATYHELMINTHES
_Corticata_ _Trematoda_
_Gymnomyxa_ _Cestoda_
| |
| |
Myxomycetes | _Triaronia_
| _Vobrocina_ _Calcarea_
_Foraminifera_ | PORIFERA
Compound Protozoa _Demospongiae_
_Radiolaria_
Let me again emphasize the fact that the relationships of these diverging
and re-diverging groups cannot be expressed on a flat surface. If we
imagine a laurel-bush to be squashed flat by a horizontal plane descending
upon it, we shall see that sundry of the upper branches and twigs which
were previously close together will become remote, and that the relative
positions of parts can remain partially true only with the minor branches.
The reader must therefore expect to find some of the zoological divisions
which in the order of nature are near one another, shown in the table as
quite distant.
§ 101. While the classifications of botanists and zoologists have become
more and more natural in their arrangements, there has grown up a certain
artificiality in their abstract nomenclature. When aggregating the smallest
groups into larger groups and these into groups still larger, they have
adopted certain general terms expressive of the successively more
comprehensive divisions; and the habitual use of these terms, needful for
purposes of convenience, has led to the tacit assumption that they answer
to actualities in Nature. It has been taken for granted that species,
genera, orders, and classes, are assemblages of definite values--that every
genus is the equivalent of every other genus in respect of its degree of
distinctness; and that orders are separated by lines of demarcation which
are as broad in one place as another. Though this conviction is not a
formulated one, the disputes continually occurring among naturalists on the
questions, whether such and such organisms are specifically or generically
distinct, and whether this or that peculiarity is or is not of ordinal
importance, imply that the conviction is entertained even where not avowed.
Yet that differences of opinion like these arise and remain unsettled,
except when they end in the establishment of sub-species, sub-genera,
sub-orders, and sub-classes, sufficiently shows that the conviction is
ill-based. And this is equally shown by the impossibility of obtaining any
definition of the degree of difference which warrants each further
elevation in the hierarchy of classes.
It is, indeed, a wholly gratuitous assumption that organisms admit of being
placed in groups of equivalent values; and that these may be united into
larger groups which are also of equivalent values; and so on. There is no
_à priori_ reason for expecting this; and there is no _à posteriori_
evidence implying it, save that which begs the question--that which asserts
one distinction to be generic and another to be ordinal, because it is
assumed that such distinctions must be either generic or ordinal. The
endeavour to thrust plants and animals into these definite partitions is of
the same nature as the endeavour to thrust them into linear series. Not
that it does violence to the facts in anything like the same degree; but
still, it does violence to the facts. Doubtless the making of divisions and
sub-divisions, is extremely useful; or rather, it is necessary. Doubtless,
too, in reducing the facts to something like order they must be partially
distorted. So long as the distorted form is not mistaken for the actual
form, no harm results. But it is needful for us to remember that while our
successively subordinate groups have a certain general correspondence with
the realities, they tacitly ascribe to the realities a regularity which
does not exist.
§ 102. A general truth of much significance is exhibited in these
classifications. On observing the natures of the attributes which are
common to the members of any group of the first, second, third, or fourth
rank, we see that groups of the widest generality are based on characters
of the greatest importance, physiologically considered; and that the
characters of the successively-subordinate groups, are characters of
successively-subordinate importance. The structural peculiarity in which
all members of one sub-kingdom differ from all members of another
sub-kingdom, is a peculiarity that affects the vital actions more
profoundly than does the structural peculiarity which distinguishes all
members of one class from all members of another class. Let us look at a
few cases.
We saw (§ 56), that the broadest division among the functions is the
division into "the _accumulation of energy_ (latent in food); the
_expenditure of energy_ (latent in the tissues and certain matters absorbed
by them); and the _transfer of energy_ (latent in the prepared nutriment or
blood) from the parts which accumulate to the parts which expend." Now in
the lowest animals, united under the general name _Protozoa_, there is
either no separation of the parts performing these functions or very
indistinct separation: in the _Rhizopoda_, all parts are alike accumulators
of energy, expenders of energy and transferers of energy; and though in the
higher members of the group, the _Infusoria_, there are some
specializations corresponding to these functions, yet there are no distinct
tissues appropriated to them. Similarly when we pass from simple types to
compound types--from _Protozoa_ to _Metazoa_. The animals known as
_Coelenterata_ are characterized in common by the possession of a part
which accumulates energy more or less marked off from the part which does
not accumulate energy, but only expends it; and the _Hydrozoa_ and
_Actinozoa_, which are sub-divisions of the _Coelenterata_, are contrasted
in this, that in the second these parts are much more differentiated from
one another, as well as more complicated. Besides a completer
differentiation of the organs respectively devoted to the accumulation of
energy and the expenditure of energy, animals next above the _Coelenterata_
possess rude appliances for the transfer of energy: the peri-visceral sac,
or closed cavity between the intestine and the walls of the body, serves as
a reservoir of absorbed nutriment, from which the surrounding tissues take
up the materials they need. And then out of this sac originates a more
efficient appliance for the transfer of energy: the more highly-organized
animals, belonging to whichever sub-kingdom, all of them possess
definitely-constructed channels for distributing the matters containing
energy. In all of them, too, the function of expenditure is divided between
a directive apparatus and an executive apparatus--a nervous system and a
muscular system. But these higher sub-kingdoms are clearly separated from
one another by differences in the relative positions of their component
sets of organs. The habitual attitudes of annulose and molluscous
creatures, is such that the neural centres are below the alimentary canal
and the hæmal centres above. And while by these traits the annulose and
molluscous types are separated from the vertebrate, they are separated from
each other by this, that in the one the body is "composed of successive
segments, usually provided with limbs," but in the other, the body is not
segmented, "and no true articulated limbs are ever developed."
The sub-kingdoms being thus distinguished from one another, by the presence
or absence of specialized parts devoted to fundamental functions, or else
by differences in the distributions of such parts, we find, on descending
to the classes, that these are distinguished from one another, either by
modifications in the structures of fundamental parts, or by the presence or
absence of subsidiary parts, or by both. Fishes and _Amphibia_ are unlike
higher vertebrates in possessing branchiæ, either throughout life or early
in life. And every higher vertebrate, besides having lungs, is
characterized by having, during development, an amnion and an allantois.
Mammals, again, are marked off from Birds and Reptiles by the presence of
mammæ, as well as by the form of the occipital condyles. Among Mammals, the
next division is based on the presence or absence of a placenta. And
divisions of the _Placentalia_ are mainly determined by the characters of
the organs of external action.
Thus, without multiplying illustrations and without descending to genera
and species, we see that, speaking generally, the successively smaller
groups are distinguished from one another by traits of successively less
importance, physiologically considered. The attributes possessed in common
by the largest assemblages of organisms, are few in number but
all-essential in kind. Each secondary assemblage, included in one of the
primary assemblages, is characterized by further common attributes that
influence the functions less profoundly. And so on with each lower grade.
§ 103. What interpretation is to be put on these truths of classification?
We find that organic forms admit of an arrangement everywhere indicating
the fact, that along with certain attributes, certain other attributes,
which are not directly connected with them, always exist. How are we to
account for this fact? And how are we to account for the fact that the
attributes possessed in common by the largest assemblages of forms, are the
most vitally-important attributes?
No one can believe that combinations of this kind have arisen fortuitously.
Even supposing fortuitous combinations of attributes might produce
organisms that would work, we should still be without a clue to this
special mode of combination. The chances would be infinity to one against
organisms which possessed in common certain fundamental attributes, having
also in common numerous non-essential attributes.
Nor, again, can any one allege that such combinations are necessary, in the
sense that all other combinations are impracticable. There is not, in the
nature of things, a reason why creatures covered with feathers should
always have beaks: jaws carrying teeth would, in many cases, have served
them equally well or better. The most general characteristic of an entire
sub-kingdom, equal in extent to the _Vertebrata_, might have been the
possession of nictitating membranes; while the internal organizations
throughout this sub-kingdom might have been on many different plans.
If, as an alternative, this peculiar subordination of traits which organic
forms display be ascribed to design, other difficulties suggest themselves.
To suppose that a certain plan of organization was fixed on by a Creator
for each vast and varied group, the members of which were to have many
different modes of life, and that he bound himself to adhere rigidly to
this plan, even in the most aberrant forms of the group where some other
plan would have been more appropriate, is to ascribe a very strange motive.
When we discover that the possession of seven cervical vertebræ is a
general characteristic of mammals, whether the neck be immensely long as in
the giraffe, or quite rudimentary as in the whale, shall we say that
though, for the whale's neck, one vertebra would have been equally good,
and though, for the giraffe's neck, a dozen would probably have been better
than seven, yet seven was the number adhered to in both cases, because
seven was fixed upon for the mammalian type? And then, when it turns out
that this possession of seven cervical vertebræ is not an
absolutely-universal characteristic of mammals (there is one which has
eight), shall we conclude that while, in a host of cases, there was a
needless adherence to a plan for the sake of consistency, there was yet, in
some cases, an inconsistent abandonment of the plan? I think we may
properly refuse to draw any such conclusion.
What, then, is the meaning of these peculiar relations of organic forms?
The answer to this question must be postponed. Having here contemplated the
problem as presented in these wide inductions which naturalists have
reached; and having seen what proposed solutions of it are inadmissible; we
shall see, in the next division of this work, what is the only possible
solution.
CHAPTER XII.
DISTRIBUTION.
§ 104. There is a distribution of organisms in Space, and there is a
distribution of organisms in Time. Looking first at their distribution in
Space, we observe in it two different classes of facts. On the one hand,
the plants and animals of each species have their habitats limited by
external conditions: they are necessarily restricted to spaces in which
their vital actions can be performed. On the other hand, the existence of
certain conditions does not determine the presence of organisms that are
fit for them. There are many spaces perfectly adapted for life of a high
order in which only life of a much lower order is found.
While, in the inevitable restriction of organisms to environments with
which their natures correspond we find a _negative_ cause of distribution,
there remains to be found that _positive_ cause whence results the presence
of organisms in some places appropriate to them and their absence from
other places equally appropriate or more appropriate. Let us consider the
phenomena as thus classed.
§ 105. Facts which illustrate the limiting influence of surrounding
conditions are abundant, and familiar to all readers. It will be needful,
however, here to cite a few typical ones of each order.
The confinement of different kinds of plants and different kinds of
animals, to the media for which they are severally adapted, is the broadest
fact of distribution. We have extensive groups of plants that are
respectively sub-aerial and sub-aqueous; and of the sub-aqueous some are
exclusively marine, while others exist only in rivers and lakes. Among
animals we similarly find some classes confined to the air and others to
the water; and of the water-breathers some are restricted to salt water and
others to fresh water. Less conspicuous is the fact that within each of
these contrasted media there are further widespread limitations. In the
sea, certain organisms exist only between certain depths, and others only
between other depths--the limpet and the mussel within the littoral zone,
and numerous kinds at the bottom of the ocean; and on the land, there are
Floras and Faunas peculiar to low regions and others peculiar to high
regions. Next we have the familiar geographical limitations made by
climate. There are temperatures which restrict each kind of organism
between certain isothermal lines, and hygrometric states which prevent the
spread of each kind of organism beyond areas having a certain humidity or a
certain dryness. Besides such general limitations we find much more special
limitations. Some minute vegetal forms occur only in snow. Hot springs have
their peculiar _Infusoria_. The habitats of certain Fungi are mines or
other dark places. And there are creatures unknown beyond the water
contained in particular caves. After these limits to distribution imposed
by physical conditions, come limits imposed by the presence or absence of
other organisms. Obviously, graminivorous animals are confined within
tracts which produce plants fit for them to feed on. The great carnivores
cannot exist out of regions where there are creatures large enough and
numerous enough to serve for prey. The needs of the sloth limit it to
certain forest-covered spaces; and there can be no insectivorous bats where
there are no night-flying insects. To these dependences of the
relatively-superior organisms on the relatively-inferior organisms which
they consume, must be added certain reciprocal dependences of the inferior
on the superior. Mr. Darwin's inquiries have shown how generally the
fertilization of plants is due to the agency of insects, and how certain
plants, being fertilizable only by insects of certain structures, are
limited to regions inhabited by insects of such structures. Conversely, the
spread of organisms is often bounded by the presence of particular
organisms beyond the bounds--either competing organisms or organisms
directly inimical. A plant fit for some territory adjacent to its own,
fails to overrun it because the territory is pre-occupied by some plant
which is its superior, either in fertility or power of resisting
destructive agencies; or else fails because there lives in the territory
some mammal which browses on its foliage or bird which devours nearly all
its seeds. Similarly, an area in which animals of a particular species
might thrive, is not colonized by them because they are not fleet enough to
escape some beast of prey inhabiting this area, or because the area is
infested by some insect which destroys them, as the tsetse destroys the
cattle in parts of Africa. Yet another more special series of limitations
accompanies parasitism. There are parasitic plants that flourish only on
trees of some few species, and others that have particular animals for
their habitats--as the fungus which is fatal to the silk-worm, or that
which so strangely grows out of a New Zealand caterpillar. Of
animal-parasites various kinds lead lives involving specialities of
distribution. We have kinds which use other creatures for purposes of
locomotion, as the _Chelonobia_ uses the turtle, and as a certain _Actinia_
uses the shell inhabited by a hermit-crab. We have the parasitism in which
one creature habitually accompanies another to share its prey, like the
annelid which takes up its abode in a hermit-crab's shell, and snatches
from the hermit-crab the morsels of food it is eating. We have again the
commoner parasitism of the _Epizoa_--animals which attach themselves to the
surfaces of other animals, and feed on their juices or on their secretions.
And once more, we have the equally common parasitism of the
_Entozoa_--creatures which live within other creatures. Besides being
restricted to the bodies of the organisms it infests, each species has
usually still narrower limits of distribution; in some cases the infested
organisms furnish fit habitats for the parasites only in certain regions,
and in other cases only when in certain constitutional states. There are
more indirect modes in which the distributions of organisms affect one
another. Plants of some kinds are eaten by animals only in the absence of
kinds that are preferred to them; and hence the prosperity of such plants
partly depends on the presence of the preferred plants. Mr. Bates has shown
that some South American butterflies thrive in regions where insectivorous
birds would destroy them, did they not closely resemble butterflies of
another genus which are disliked by those birds. And Mr. Darwin gives cases
of dependence still more remote and involved.
Such are the chief negative causes of distribution--the inorganic and
organic agencies that set bounds to the spaces which organisms of each
species inhabit. Fully to understand their actions we must contemplate them
as working not separately but in concert. We have to regard the physical
influences, varying from year to year, as now producing an extension or
restriction of the habitat in this direction and now in that, and as
producing secondary extensions and restrictions by their effects on other
kinds of organisms. We have to regard the distribution of each species as
affected not only by causes which favour multiplication of prey or of
enemies within its own area, but also by causes which produce such results
in neighbouring areas. We have to conceive the forces by which the limit is
maintained, as including all meteorologic influences, united with the
influences, direct or remote, of numerous co-existing species.
One general truth, indicated by sundry of the above illustrations, calls
for special notice--the truth that all kinds of organisms intrude on one
another's spheres of existence. Of the ways in which they do this the
commonest is invasion of territory. That tendency which we see in the human
races, to overrun and occupy one another's lands, as well as the lands
inhabited by inferior creatures, is a tendency exhibited by all classes of
organisms in various ways. Among them, as among mankind, there are
permanent conquests, temporary occupations, and occasional raids. Every
spring an inroad is made into the area which our own birds occupy, by birds
from the South; and every winter the fieldfares of the North come to share
the hips and haws of our hedges, and thus entail on our native birds some
mortality. Besides these regularly-recurring incursions there are irregular
ones; as of locusts into countries not usually visited by them, or of
certain rodents which from time to time swarm into areas adjacent to their
own. Every now and then an incursion ends in permanent settlement--perhaps
in conquest over indigenous species. Within these few years an American
water-weed has taken possession of our ponds and rivers, and to some extent
supplanted native water-weeds. Of animals may be named a small kind of red
ant, having habits allied to those of tropical ants, which has of late
overrun many houses in London. The rat, which must have taken to infesting
ships within these few centuries, furnishes a good illustration of the
readiness of animals to occupy new places that are available. And the way
in which vessels visiting India are cleared of the European cockroach by
the kindred _Blatta orientalis_, shows us how these successful invasions
last only until there come more powerful invaders. Animals encroach on one
another's spheres of existence in further ways than by trespassing on one
another's areas: they adopt one another's modes of life. There are cases in
which this usurpation of habits is slight and temporary; and there are
cases where it is marked and permanent. Grey crows often join gulls in
picking up food between tide-marks; and gulls may occasionally be seen many
miles inland, feeding in ploughed fields and on moors. Mr. Darwin has
watched a fly-catcher catching fish. He says that the greater titmouse
sometimes adopts the practices of the shrike, and sometimes of the
nuthatch, and that some South American woodpeckers are frugivorous while
others chase insects on the wing. Of habitual intrusions on the occupations
of other creatures, one case is furnished by the sea-eagle, which, besides
hunting the surface of the land for prey, like the rest of the hawk-tribe,
often swoops down upon fish. And Mr. Darwin names a species of petrel that
has taken to diving, and has a considerably modified organization. The
last cases introduce a still more remarkable class of facts of kindred
meaning. This intrusion of organisms on one another's modes of life goes to
the extent of intruding on one another's media. The great mass of flowering
plants are terrestrial, and (aside from other needs) are required to be so
by their process of fructification. But there are some which live in the
water, and protrude their flowers above the surface. Nay, there is a still
more striking instance. At the sea-side may be found an alga a hundred
yards inland, and a phænogam rooted in salt water. Among animals these
interchanges of media are numerous. Nearly all coleopterous insects are
terrestrial; but the water-beetle, which like the rest of its order is an
air-breather, has aquatic habits. Water appears to be an extremely unfit
medium for a fly; and yet Mr. [now Sir John] Lubbock has discovered more
than one species of fly living beneath the surface of the water and coming
up occasionally for air. Birds, as a class, are specially fitted for an
aerial existence; but certain tribes of them have taken to an aquatic
existence--swimming on the surface of the water and making continual
incursions beneath it, and some kinds have wholly lost the power of flight.
Among mammals, too, which have limbs and lungs implying an organization for
terrestrial life, may be named kinds living more or less in the water and
are more or less adapted to it. We have water-rats and otters which unite
the two kinds of life, and show but little modification; hippopotami
passing the greater part of their time in the water, and somewhat more
fitted to it; seals living almost exclusively in the sea, and having the
mammalian form greatly obscured; whales wholly confined to the sea, and
having so little the aspect of mammals as to be mistaken for fish.
Conversely, sundry inhabitants of the water make excursions on the land.
Eels migrate at night from one pool to another. There are fish with
specially-modified gills and fin-rays serving as stilts, which, when the
rivers they inhabit are partially dried-up, travel in search of better
quarters. And while some kinds of crabs do not make land-excursions beyond
high-water mark, other kinds pursue lives almost wholly terrestrial.
Guided by these two classes of facts, we must regard the bounds to each
species' sphere of existence as determined by the balancing of two
antagonist sets of forces. The tendency which every species has to intrude
on other areas, other modes of life, and other media, is restrained by the
direct and indirect resistance of conditions, organic and inorganic. And
these expansive and repressive energies, varying continually in their
respective intensities, rhythmically equilibrate each other--maintain a
limit that perpetually oscillates from side to side of a certain mean.
§ 106. As implied at the outset, the character of a region, when
unfavourable to any species, sufficiently accounts for the absence of this
species; and thus its absence is not inconsistent with the hypothesis that
each species was originally placed in the regions most favourable to it.
But the absence of a species from regions that _are_ favourable to it
cannot be thus accounted for. Were plants and animals localized wholly with
reference to the fitness of their constitutions to surrounding conditions,
we might expect Floras to be similar, and Faunas to be similar, where the
conditions are similar; and we might expect dissimilarities among Floras
and among Faunas, proportionate to the dissimilarities of their conditions.
But we do not find such anticipations verified.
Mr. Darwin says that "in the Southern hemisphere, if we compare large
tracts of land in Australia, South Africa, and western South America,
between latitudes 25° and 35°, we shall find parts extremely similar in all
their conditions, yet it would not be possible to point out three faunas
and floras more utterly dissimilar. Or again we may compare the productions
of South America south of lat. 35° with those north of 25°, which
consequently inhabit a considerably different climate, and they will be
found incomparably more closely related to each other, than they are to the
productions of Australia or Africa under nearly the same climate." Still
more striking are the contrasts which Mr. Darwin points out between
adjacent areas that are totally cut off from each other. "No two marine
faunas are more distinct, with hardly a fish, shell, or crab in common,
than those of the eastern and western shores of South and Central America;
yet these great faunas are separated only by the narrow, but impassable,
isthmus of Panama." On opposite sides of high mountain-chains, also, there
are marked differences in the organic forms--differences not so marked as
where the barriers are absolutely impassable, but much more marked than are
necessitated by unlikenesses of physical conditions.
Not less suggestive is the converse fact that wide geographical areas which
offer decided geologic and meteorologic contrasts, are peopled by
nearly-allied groups of organisms, if there are no barriers to migration.
"The naturalist in travelling, for instance, from north to south never
fails to be struck by the manner in which successive groups of beings,
specifically distinct, yet clearly related, replace each other. He hears
from closely allied, yet distinct kinds of birds, notes nearly similar, and
sees their nests similarly constructed, but not quite alike, with eggs
coloured in nearly the same manner. The plains near the Straits of Magellan
are inhabited by one species of Rhea (American Ostrich), and northward the
plains of La Plata by another species of the same genus; and not by a true
ostrich or emu, like those found in Africa and Australia under the same
latitude. On these same plains of La Plata, we see the agouti and bizcacha,
animals having nearly the same habits as our hares and rabbits and
belonging to the same order of Rodents, but they plainly display an
American type of structure. We ascend the lofty peaks of the Cordillera and
we find an alpine species of bizcacha; we look to the waters, and we do not
find the beaver or musk-rat, but the coypu and capybara, rodents of the
American type. Innumerable other instances could be given. If we look to
the islands off the American shore, however much they may differ in
geological structure, the inhabitants, though they may be all peculiar
species, are essentially American."
What is the generalization implied by these two groups of facts? On the one
hand, we have similarly-conditioned, and sometimes nearly-adjacent, areas,
occupied by quite different Faunas. On the one hand, we have areas remote
from one another in latitude, and contrasted in soil as well as climate,
occupied by closely-allied Faunas. Clearly then, as like organisms are not
universally, or even generally, found in like habitats, nor very unlike
organisms in very unlike habitats, there is no manifest pre-determined
adaptation of the organisms to the habitats. The organisms do no occur in
such and such places solely because they are either specially fit for those
places, or more fit for them than all other organisms.
The induction under which these facts come, and which unites them with
various other facts, is a totally-different one. When we see that the
similar areas peopled by dissimilar forms, are those between which there
are impassable barriers; while the dissimilar areas peopled by similar
forms, are those between which there are no such barriers; we are at once
reminded of the general truth exemplified in the last section--the truth
that each species of organism tends ever to expand its sphere of
existence--to intrude on other areas, other modes of life, other media. And
we are shown that through these perpetually-recurring attempts to thrust
itself into every accessible habitat, each species spreads until it reaches
limits which are for the time insurmountable.
§ 107. We pass now to the distribution of organic forms in Time. Geological
inquiry has established the truth that during a Past of immeasurable
duration, plants and animals have existed on the Earth. In all countries
their buried remains are found in greater or less abundance. From
comparatively small areas multitudinous different types have been exhumed.
Every exploration of new areas, and every closer inspection of areas
already explored, brings more types to light. And beyond question, an
exhaustive examination of all exposed strata, and of all strata now covered
by the sea, would disclose types immensely out-numbering those at present
known. Further, geologists agree that even had we before us every kind of
fossil which exists, we should still have nothing like a complete index to
the past inhabitants of our globe. Many sedimentary deposits have been so
altered by the heat of adjacent molten matter, as greatly to obscure the
organic remains contained in them. The extensive formations once called
"transition," and now re-named "metamorphic," are acknowledged to be
formations of sedimentary origin, from which all traces of such fossils as
they probably included have been obliterated by igneous action. And the
accepted conclusion is that igneous rock has everywhere resulted from the
melting-up of beds of detritus originally deposited by water. How long the
reactions of the Earth's molten nucleus on its cooling crust, have been
thus destroying the records of Life, it is impossible to say; but there are
strong reasons for believing that the records which remain bear but a small
ratio to the records which have been destroyed. Thus we have but extremely
imperfect data for conclusions respecting the distribution of organic forms
in Time. Some few generalizations, however, may be regarded as established.
One is that the plants and animals now existing mostly differ from the
plants and animals which have existed. Though there are species common to
our present Fauna and to past Faunas, yet the _facies_ of our present Fauna
differs, more or less, from the _facies_ of each past Fauna. On carrying
out the comparison, we find that past Faunas differ from one another, and
that the differences between them are proportionate to their degrees of
remoteness from one another in Time, as measured by their relative
positions in the sedimentary series. So that if we take the assemblage of
organic forms living now, and compare it with the successive assemblages of
organic forms which have lived in successive geologic epochs, we find that
the farther we go back into the past, the greater does the unlikeness
become. The number of species and genera common to the compared
assemblages, becomes smaller and smaller; and the assemblages differ more
and more in their general characters. Though a species of brachiopod now
extant is almost identical with a species found in Silurian strata, though
between the Silurian Fauna and our own there are sundry common genera of
molluscs, yet it is undeniable that there is a proportion between lapse of
time and divergence of organic forms.
This divergence is comparatively slow and continuous where there is
continuity in the geological formations, but is sudden, and comparatively
wide, wherever there occurs a great break in the succession of strata. The
contrasts which thus arise, gradually or all at once, in formations that
are continuous or discontinuous, are of two kinds. Faunas of different eras
are distinguished partly by the absence from the one of type's present in
the other, and partly by the unlikenesses between the types common to both.
Such contrasts between Faunas as are due to the appearance or disappearance
of types, are of secondary significance: they possibly, or probably, do not
imply anything more than migrations or extinctions. The most significant
contrasts are those between successive groups of organisms of the same
type. And among such, as above said, the differences are, speaking
generally, small and continuous where a series of conformable strata gives
proof of continued existence of the type in the locality; while they are
comparatively large and abrupt where the adjacent formations are shown to
have been separated by long intervals.
Another general fact, referred to by Mr. Darwin as one which palæontology
has made tolerably certain, is that forms and groups of forms which have
once disappeared from the Earth, do not reappear. Passing over the few
species which have continued throughout the whole period geologically
recorded, it may be said that each species after arising, spreading for an
era, and continuing abundant for an era, eventually declines and becomes
extinct; and that similarly, each genus during a longer period increases in
the number of its species, and during a longer period dwindles and at last
dies out. After making its exit neither species nor genus ever re-enters.
The like is true even of those larger groups called orders. Four types of
reptiles which were once abundant have not been found in modern formations,
and do not at present exist. Though nothing less than an exhaustive
examination of all strata, can prove conclusively that a type of
organization when once lost is never reproduced, yet so many facts point to
this inference that its truth can scarcely be doubted.
To frame a conception of the total amount and general direction of the
change in organic forms during the time measured by our sedimentary series,
is at present impossible--the data are insufficient. The immense contrast
between the few and low forms of the earliest-known Fauna, and the many and
high forms of our existing Fauna, has been commonly supposed to prove, not
only great change but great progress. Nevertheless, this appearance of
progress may be, and probably is, mainly illusive. Wider knowledge has
shown that remains of comparatively well-organized creatures really existed
in strata long supposed to be devoid of them, and that where they are
absent, the nature of the strata often explains their absence, without
assuming that they did not exist when these strata were formed. It is a
tenable hypothesis that the successively-higher types fossilized in our
successively-later deposits, indicate nothing more than successive
migrations from pre-existing continents to continents that were step by
step emerging from the ocean--migrations which necessarily began with the
inferior orders of organisms, and included the successively-superior orders
as the new lands became more accessible to them and better fitted for
them.[43]
While the evidence usually supposed to prove progression is thus
untrustworthy, there is trustworthy evidence that there has been, in many
cases, little or no progression. Though the orders which have existed from
palæozoic and mesozoic times down to the present day, are almost
universally changed, yet a comparison of ancient and modern members of
these orders shows that the total amount of change is not relatively great,
and that it is not manifestly towards a higher organization. Though nearly
all the living forms which have prototypes in early formations differ from
these prototypes specially, and in most cases generically, yet ordinal
peculiarities are, in numerous cases, maintained from the earliest times
geologically recorded, down to our own time; and we have no visible
evidence of superiority in the existing genera of these orders. In his
lecture "On the Persistent Types of Animal Life," Prof. Huxley enumerated
many cases. On the authority of Dr. Hooker he stated "that there are
Carboniferous plants which appear to be generically identical with some now
living: that the cone of the Oolitic _Araucaria_ is hardly distinguishable
from that of an existing species; that a true _Pinus_ appears in the
Purbecks and a _Juglans_ in the chalk." Among animals he named palæozoic
and mesozoic corals which are very like certain extant corals; genera of
Silurian molluscs that answer to existing genera; insects and arachnids in
the coal-formations that are not more than generically distinct from some
of our own insects and arachnids. He instanced "the Devonian and
Carboniferous _Pleuracanthus_, which differs no more from existing sharks
than these do from one another;" early mesozoic reptiles "identical in the
essential characters of their organization with those now living;" and
Triassic mammals which did not differ "nearly so much from some of those
which now live, as these differ from one another." Continuing the argument
in his "Anniversary Address to the Geological Society" in 1862, Prof.
Huxley gave many cases in which the changes that have taken place, are not
changes towards a more specialized or higher organization--asking "in what
sense are the Liassic Chelonia inferior to those which now exist? How are
the Cretaceous Ichthyosauria, Plesiosauria, or Pterosauria less embryonic
or more differentiated species than those of the Lias?" While, however,
contending that in most instances "positive evidence fails to demonstrate
any sort of progressive modification towards a less embryonic or less
generalized type in a great many groups of animals of long-continued
geological existence," Prof. Huxley added that there are other groups,
"co-existing with them under the same conditions, in which more or less
distinct indications of such a process seem to be traceable." And in
illustration of this, he named that better development of the vertebræ
which characterizes some of the more modern fishes and reptiles, when
compared with ancient fishes and reptiles of the same orders; and the
"regularity and evenness of the dentition of the _Anoplotherium_ as
contrasting with that of existing Artiodactyles."[44]
The facts thus summed up do not show that higher forms have not arisen in
the course of geologic time, any more than the facts commonly cited prove
that higher forms have arisen; nor are they regarded by Professor Huxley as
showing this. Were those which have survived from palæozoic and mesozoic
days down to our own day, the only types; and did the modifications, rarely
of more than generic value, which these types have undergone, give no
better evidences of increased complexity than are actually given by them;
then it would be inferable that there has been no appreciable advance. But
there now exist, and have existed during the more recent geologic epochs,
various types which are not known to have existed in earlier epochs--some
of them widely unlike these persistent types and some of them nearly allied
to these persistent types. As yet, we know nothing about the origins of
these new types. But it is possible that causes like those which have
produced generic differences in the persistent types, have, in some or many
cases, produced modifications great enough to constitute ordinal
differences. If structural contrasts not exceeding certain moderate limits
are held to mark only generic distinctions; and if organisms displaying
larger contrasts are regarded as ordinally or typically distinct; it is
obvious that the persistence of a given type through a long geologic period
without apparently undergoing deviations of more than generic value, by no
means disproves the occurrence of far greater deviations in other cases;
since the forms resulting from such far greater deviations, being regarded
as typically distinct forms, will not be taken as evidence of great change
in an original type. That which Prof. Huxley's argument proves, and that
only which he considers it to prove, is that organisms have no innate
tendencies to assume higher forms; and that "any admissible hypothesis of
progressive modification, must be compatible with persistence without
progression through indefinite periods."
One very significant fact must be added concerning the relation between
distribution in Time and distribution in Space. I quote it from Mr.
Darwin:--"Mr. Clift many years ago showed that the fossil mammals from the
Australian caves were closely allied to the living marsupials of that
continent. In South America a similar relationship is manifest, even to an
uneducated eye, in the gigantic pieces of armour like those of the
armadillo, found in several parts of La Plata; and Professor Owen has shown
in the most striking manner that most of the fossil mammals, buried there
in such numbers, are related to the South American types. This relationship
is even more clearly seen in the wonderland collection of fossil bones made
by MM. Lund and Clausen in the caves of Brazil. I was so much impressed
with these facts that I strongly insisted, in 1839 and 1845, on this 'law
of the succession of types,'--on 'this wonderful relationship in the same
continent between the dead and the living.' Professor Owen has subsequently
extended the same generalization to the Mammals of the Old World. We see
the same law in this author's restorations of the extinct and gigantic
birds of New Zealand. We see it also in the birds of the caves of Brazil.
Mr. Woodward has shown that the same law holds good with sea-shells, but
from the wide distribution of most genera of molluscs, it is not well
displayed by them. Other cases could be added, as the relation between the
extinct and living landshells of Madeira, and between the extinct and
living brackish-water shells of the Aralo-Caspian Sea."
The general results, then, are these. Our knowledge of distribution in
Time, being derived wholly from the evidence afforded by fossils, is
limited to that geologic time of which some records remain--cannot extend
to those remoter times the records of which have been obliterated. From
these remaining records, which probably form but a small fraction of the
whole, the general facts deducible are these:--That such organic types as
have lived through successive epochs, have almost universally undergone
modifications of specific and generic values--modifications which have
commonly been great in proportion as the period has been long. That besides
the types which have persisted from ancient eras down to our own era, other
types have from time to time made their appearance in the ascending series
of strata--types of which some are lower and some higher than the types
previously recorded; but whence these new types came, and whether any of
them arose by divergence from the previously-recorded types, the evidence
does not yet enable us to say. That in the course of long geologic epochs
nearly all species, most genera, and a few orders, have become extinct; and
that a species, genus, or order, which has once disappeared from the Earth
never reappears. And, lastly, that the Fauna now occupying each separate
area of the Earth's surface is very nearly allied to the Fauna which
existed on that area during recent geologic times.
§ 108. Omitting sundry minor generalizations, the exposition of which would
involve too much detail, what is to be said of these major generalizations?
The distribution in Space cannot be said to imply that organisms have been
designed for their particular habitats and placed in them; since, besides
the habitat in which each kind of organism is found there are commonly
other habitats, as good or better for it, from which it is absent--habitats
to which it is so much better fitted than organisms now occupying them,
that it extrudes these organisms when allowed the opportunity. Neither can
we suppose that the purpose has been to establish varieties of Floras and
Faunas; since, if so, why are the Floras and Faunas but little divergent in
widely-sundered areas between which migration is possible, while they are
markedly divergent in adjacent areas between which migration is impossible?
Passing to distributions in Time, there arise the questions--why during
nearly the whole of that vast period geologically recorded have there
existed none of those highest organic forms which have now overrun the
Earth?--how is it that we find no traces of a creature endowed with large
capacities for knowledge and happiness? The answer that the Earth was not,
in remote times, a fit habitation for such a creature, besides being
unwarranted by the evidence, suggests the equally awkward question--why
during untold millions of years did the Earth remain fit only for inferior
creatures? What, again, is the meaning of extinction of types? To conclude
that the saurian type was replaced by other types at the beginning of the
tertiary period, because it was not adapted to the conditions which then
arose, is to conclude that it could not be modified into fitness for the
conditions; and this conclusion is at variance with the hypothesis that
creative skill is shown in the multiform adaptations of one type to many
ends.
What interpretations may rationally be put on these and other general facts
of distribution in Space and Time, will be seen in the next division of
this work.
PART III.
THE EVOLUTION OF LIFE.
CHAPTER I.
PRELIMINARY.
§ 109. In the foregoing Part, we have contemplated the most important of
the generalizations to which biologists have been led by observation of
organisms; as well as some others which contemplation of the facts has
suggested to me. These Inductions of Biology have also been severally
glanced at on their deductive sides; for the purpose of noting the harmony
existing between them and those primordial truths set forth in _First
Principles_. Having thus studied the leading phenomena of life separately,
we are prepared for studying them as an aggregate, with the view of
arriving at the most general interpretation of them.
There is an _ensemble_ of vital phenomena presented by each organism in the
course of its growth, development, and decay; and there is an _ensemble_ of
vital phenomena presented by the organic world as a whole. Neither of these
can be properly dealt with apart from the other. But the last of them may
be separately treated more conveniently than the first. What interpretation
we put on the facts of structure and function in each living body, depends
entirely on our conception of the mode in which living bodies in general
have originated. To form some conclusion respecting this mode--a
provisional if not a permanent conclusion--must therefore be our first
step.
We have to choose between two hypotheses--the hypothesis of Special
Creation and the hypothesis of Evolution. Either the multitudinous kinds of
organisms which now exist, and the far more multitudinous kinds which have
existed during past geologic eras, have been from time to time separately
made; or they have arisen by insensible steps, through actions such as we
see habitually going on. Both hypotheses imply a Cause. The last, certainly
as much as the first, recognizes this Cause as inscrutable. The point at
issue is, how this inscrutable Cause has worked in the production of living
forms. This point, if it is to be decided at all, is to be decided only by
examination of evidence. Let us inquire which of these antagonist
hypotheses is most congruous with established facts.