THE PRINCIPLES OF
BIOLOGY
BY
HERBERT SPENCER
_IN TWO VOLUMES_
VOL. II
REVISED AND ENLARGED EDITION
1899
NEW YORK
D. APPLETON AND COMPANY
1900
COPYRIGHT, 1867, 1899,
BY D. APPLETON AND COMPANY.
PREFACE
TO THE REVISED AND ENLARGED EDITION OF VOL. II.
To the statements made in the preface to the first volume of this
revised edition, there must here be added a few having special
reference to this second volume.
One of them is that the revision has not been carried out in quite the
same way, but in a way somewhat less complete. When reviewing the first
volume a friendly critic, Prof. Lloyd Morgan, said:--
“But though the intellectual weight has also been augmented,
it is an open question whether it would not have been wiser
to leave intact a treatise, &c... relegating corrections and
additions to notes and appendices.”
I think that Prof. Morgan is right. Though at the close of the preface
to volume I, I wrote:--“in all sections not marked as new, the
essential ideas set forth are the same as they were in the original
edition of 1864,” yet the reader who has not read this statement,
or does not bear it in mind, will suppose that all or most of the
enunciated conceptions are of recent date, whereas only a small part of
them are. I have therefore decided to follow, in this second volume, a
course somewhat like that suggested by Prof. Morgan--somewhat like, I
say, because in sundry cases the amendments could not be satisfactorily
made by appended notes.
But there has been a further reason for this change of method. An
invalid who is nearly eighty cannot with prudence enter upon work which
will take long to complete. Hence I have thought it better to make the
needful alterations and additions in ways requiring relatively moderate
time and labour.
The additions made to this volume are less numerous and less important
than those made to the first volume. A new chapter ending Part V, on
“The Integration of the Organic World,” serves to round off the general
theory of Evolution in its application to living things. Beyond a new
section (§ 289_a_) and the various foot-notes, serving chiefly the
purpose of elucidation, there are notes of some significance appended
to Chapters I, III, IV, and V, in Part IV, Chapters V and VIII, in Part
V, and Chapters IX, X, and XII in Part VI. Moreover there are three
further appendices, D^2, F, and G, which have, I think, considerable
significance as serving to make clearer some of the views expressed in
the body of the work.
Turning from the additions to the revisions, I have to say that the
aid needed for bringing up to date the contents of this volume, has
been given me by the gentlemen who gave me like aid in revising the
first volume: omitting Prof. Perkin, within whose province none of the
contents of this volume fall. Plant-Morphology and Plant-Physiology
have been overseen by Mr. A. G. Tansley. Criticisms upon parts dealing
with Animal Morphology I owe to Mr. J. T. Cunningham and Prof. E. W.
MacBride. And the statements included under Animal Physiology have been
checked by Mr. W. B. Hardy.
For reasons like those named in the preface to the first volume, I
have not submitted the proofs of this revised second volume to these
gentlemen: a fact which it is needful to name, since one or other of
them might else be held responsible for some error which is not his but
mine. It is the more requisite to say this because while, in respect
of matters of fact, I have, save in one or two cases, accepted their
corrections as not to be questioned, I have not always done this in
respect of matters of inference, but in sundry places have adhered to
my own interpretations.
Perhaps I may be excused for expressing some satisfaction that I
have not been obliged to relinquish the views set forth in 1864–7.
The hypothesis of physiological units--or, as I would now call them,
constitutional units--has been adopted by several zoologists under
modified forms. So far as I am aware, the alleged general law of
organic symmetry has not called forth any manifestations of dissent.
The suggested theory of vertebrate structure appears to have become
current; and from the investigations of the late Prof. Cope, has
received verification. The conclusions drawn in Part VI on “The Laws
of Multiplication,” have not, I believe, been controverted. And though
only some works on botany have given currency to the doctrine set forth
in Appendix C, “On Circulation and the Formation of Wood in Plants,”
yet I have met with no attempt to disprove it. The only views contested
by certain of the gentlemen above named, are those concerning the
origin of the two great phænogamic types of plants, and the origin of
the annulose type of animals. I have not, however,--perhaps because
of natural bias--found myself compelled to surrender these views. My
reasons for adhering to them will be found in notes to the ends of
Chapters III and IV in Part IV, and in Appendix D^2.
On now finally leaving biological studies, it remains only to say that
I am glad I have survived long enough to give this work its finished
form.
BRIGHTON,
_October, 1899_.
PREFACE TO VOL. II.
The proof sheets of this volume, like those of the last volume, have
been looked through by Dr. Hooker and Prof. Huxley; and I have, as
before, to thank them for their valuable criticisms, and for the
trouble they have taken in checking the numerous statements of fact on
which the arguments proceed. The consciousness that their many duties
render time extremely precious to them, makes me feel how heavy is my
obligation.
Part IV., with which this volume commences, contains numerous figures.
Nearly one half of them are repetitions, mostly altered in scale and
simplified in execution, of figures, or parts of figures, contained
in the works of various Botanists and Zoologists. Among the authors
whom I have laid under contribution, I may name Berkeley, Carpenter,
Cuvier, Green, Harvey, Hooker, Huxley, Milne-Edwards, Ralfs, Smith.
The remaining figures, numbering 150, are from original sketches and
diagrams.
The successive instalments which compose this volume, were issued
to the Subscribers at the following dates:--No. 13 (pp. 1–80) in
January, 1865; No. 14 (pp. 81–160) in June, 1865; No. 15 (pp. 161–240)
in December, 1865; No. 16 (pp. 241–320) in June, 1866; No. 17 (pp.
321–400) in November, 1866; and No. 18 (pp. 401–566) in March, 1867.
LONDON, _March 23rd, 1867_.
CONTENTS OF VOL. II.
PART IV.--MORPHOLOGICAL DEVELOPMENT.
CHAP. PAGE
I.--THE PROBLEMS OF MORPHOLOGY 3
II.--THE MORPHOLOGICAL COMPOSITION OF PLANTS 17
III.--THE MORPHOLOGICAL COMPOSITION OF PLANTS--_Continued_ 37
IV.--THE MORPHOLOGICAL COMPOSITION OF ANIMALS 85
V.--THE MORPHOLOGICAL COMPOSITION OF ANIMALS--_Continued_ 111
VI.--MORPHOLOGICAL DIFFERENTIATION IN PLANTS 128
VII.--THE GENERAL SHAPES OF PLANTS 134
VIII.--THE SHAPES OF BRANCHES 145
IX.--THE SHAPES OF LEAVES 152
X.--THE SHAPES OF FLOWERS 161
XI.--THE SHAPES OF VEGETAL CELLS 175
XII.--CHANGES OF SHAPE OTHERWISE CAUSED 178
XIII.--MORPHOLOGICAL DIFFERENTIATION IN ANIMALS 183
XIV.--THE GENERAL SHAPES OF ANIMALS 186
XV.--THE SHAPES OF VERTEBRATE SKELETONS 209
XVI.--THE SHAPES OF ANIMAL CELLS 228
XVII.--SUMMARY OF MORPHOLOGICAL DEVELOPMENT 231
PART V.--PHYSIOLOGICAL DEVELOPMENT.
I.--THE PROBLEMS OF PHYSIOLOGY 239
II.--DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES
OF PLANTS 244
III.--DIFFERENTIATIONS AMONG THE OUTER TISSUES OF PLANTS 251
IV.--DIFFERENTIATIONS AMONG THE INNER TISSUES OF PLANTS 272
V.--PHYSIOLOGICAL INTEGRATION IN PLANTS 292
VI.--DIFFERENTIATIONS BETWEEN THE OUTER AND INNER TISSUES
OF ANIMALS 299
VII.--DIFFERENTIATIONS AMONG THE OUTER TISSUES OF ANIMALS 309
VIII.--DIFFERENTIATIONS AMONG THE INNER TISSUES OF ANIMALS 323
IX.--PHYSIOLOGICAL INTEGRATION IN ANIMALS 373
X.--SUMMARY OF PHYSIOLOGICAL DEVELOPMENT 384
X^A.--THE INTEGRATION OF THE ORGANIC WORLD 396
PART VI.--LAWS OF MULTIPLICATION.
I.--THE FACTORS 411
II.--_À PRIORI_ PRINCIPLE 417
III.--OBVERSE _À PRIORI_ PRINCIPLE 424
IV.--DIFFICULTIES OF INDUCTIVE VERIFICATION 432
V.--ANTAGONISM BETWEEN GROWTH AND ASEXUAL GENESIS 439
VI.--ANTAGONISM BETWEEN GROWTH AND SEXUAL GENESIS 448
VII.--THE ANTAGONISM BETWEEN DEVELOPMENT AND GENESIS,
ASEXUAL AND SEXUAL 461
VIII.--ANTAGONISM BETWEEN EXPENDITURE AND GENESIS 467
IX.--COINCIDENCE BETWEEN HIGH NUTRITION AND GENESIS 475
X.--SPECIALITIES OF THESE RELATIONS 486
XI.--INTERPRETATION AND QUALIFICATION 497
XII.--MULTIPLICATION OF THE HUMAN RACE 506
XIII.--HUMAN POPULATION IN THE FUTURE 522
APPENDICES.
A.--SUBSTITUTION OF AXIAL FOR FOLIAR ORGANS IN PLANTS 541
B.--A CRITICISM ON PROF. OWEN’S THEORY OF THE VERTEBRATE
SKELETON 548
C.--ON CIRCULATION AND THE FORMATION OF WOOD IN PLANTS 567
D.--ON THE ORIGIN OF THE VERTEBRATE TYPE 599
D^2.--THE ANNULOSE TYPE 602
E.--THE SHAPES AND ARRANGEMENTS OF FLOWERS 608
F.--PHYSIOLOGICAL (OR CONSTITUTIONAL) UNITS 612
G.--THE INHERITANCE OF FUNCTIONALLY-CAUSED MODIFICATIONS 618
PART IV.
MORPHOLOGICAL DEVELOPMENT.
CHAPTER I.
THE PROBLEMS OF MORPHOLOGY.
§ 175. The division of Morphology from Physiology, is one which may
be tolerably-well preserved so long as we do not carry our inquiries
beyond the empirical generalizations of their respective phenomena; but
it is one which becomes in great measure nominal, when the phenomena
are to be rationally interpreted. It would be possible, after analyzing
our Solar System, to set down certain general truths respecting the
sizes and distances of its primary and secondary members, omitting
all mention of their motions; and it would be possible to set down
certain other general truths respecting their motions, without
specifying their dimensions or positions, further than as greater
or less, nearer or more remote. But on seeking to account for these
general truths, arrived at by induction, we find ourselves obliged to
consider simultaneously the relative sizes and places of the masses,
and the relative amounts and directions of their motions. Similarly
with organisms. Though we may frame sundry comprehensive propositions
respecting the arrangements of their organs, considered as so many
inert parts; and though we may establish several wide conclusions
respecting the separate and combined actions of their organs, without
knowing anything definite respecting the forms and positions of these
organs; yet we cannot reach such a rationale of the facts as the
hypothesis of Evolution aims at, without contemplating structures and
functions in their mutual relations. Everywhere structures in great
measure determine functions; and everywhere functions are incessantly
modifying structures. In Nature the two are inseparable co-operators;
and Science can give no true interpretation of Nature without keeping
their co-operation constantly in view. An account of organic evolution,
in its more special aspects, must be essentially an account of the
interactions of structures and functions, as perpetually altered by
changes of conditions.
Hence, when treating apart Morphological Development and Physiological
Development, all we can do is to direct our attention mainly to the
one or to the other, as the case may be. In dealing with the facts of
structure, we must consider the facts of function only in such general
way as is needful to explain the facts of structure; and conversely
when dealing with the facts of function.
§ 176. The problems of Morphology fall into two distinct classes,
answering respectively to the two leading aspects of Evolution. In
things which evolve there go on two processes--increase of mass
and increase of structure. Increase of mass is primary, and in
simple evolution takes place almost alone. Increase of structure is
secondary, accompanying or following increase of mass with more or
less regularity, wherever evolution rises above that form which small
inorganic bodies, such as crystals, present to us. As the fundamental
antagonism between Dissolution and Evolution consists in this, that
while the one is an integration of motion and disintegration of matter,
the other is an integration of matter and disintegration of motion; and
as this integration of matter accompanying disintegration of motion,
is a necessary antecedent to the differentiation of the matter so
integrated; it follows that questions concerning the mode in which the
parts are united into a whole, must be dealt with before questions
concerning the mode in which these parts become modified.[1]
This is not obviously a morphological question. But an illustration or
two will make it manifest that fundamental differences may be produced
between aggregates by differences in the degrees of composition of the
increments: the ultimate units of the increments being the same. Thus
an accumulation of things of a given kind may be made by adding one
at a time. Or the things may be tied up into bundles of ten, and the
tens placed together. Or the tens may be united into hundreds, and a
pile of hundreds formed. Such unlikenesses in the structures of masses
are habitually seen in our mercantile transactions. Articles which
the consumer recognizes as single, the retailer keeps wrapped up in
dozens, the wholesaler sends in gross, and the manufacturer supplies
in packages of a hundred gross. That is, they severally increase
their stocks by units of simple, of compound, and of doubly-compound
kinds. Similarly result those differences of morphological composition
which we have first to consider. An organism consists of units.
These units may be aggregated into a mass by the addition of unit
to unit. Or they may be united into groups, and the groups joined
together. Or these groups of groups may be so combined as to form a
doubly-compound aggregate. Hence there arises respecting each organic
form the question--is its composition of the first, second, third,
or fourth order?--does it exhibit units of a singly-compounded kind
only, or are these consolidated into units of a doubly-compounded
kind, or a triply-compounded kind? And if it displays double or triple
composition, the homologies of its different parts become problems.
Under the disguises induced by the consolidation of primary, secondary,
and tertiary units, it has to be ascertained which answer to which, in
their degrees of composition.
Such questions are more intricate than they at first appear; since,
besides the obscurities caused by progressive integration, and those
due to accompanying modifications of form, further obscurities
result from the variable growths of units of the different orders.
Just as an army may be augmented by recruiting each company, without
increasing the number of companies; or may be augmented by making up
the full complement of companies in each regiment, while the number
of regiments remains the same; or may be augmented by putting more
regiments into each division, other things being unchanged; or may be
augmented by adding to the number of its divisions without altering
the components of each division; or may be augmented by two or three
of these processes at once; so, in organisms, increase of mass may
result from additions of units of the first order, or those of the
second order, or those of still higher orders; or it may be due to
simultaneous additions to units of several orders. And this last mode
of integration being the general mode, puts difficulties in the way of
analysis. Just as the structure of an army would be made less easy to
understand if companies often outgrew regiments, or regiments became
larger than brigades; so these questions of morphological composition
are complicated by the indeterminate sizes of the units of each
kind: relatively-simple units frequently becoming more bulky than
relatively-compound units.
§ 177. The morphological problems of the second class are those having
for their subject-matter the changes of shape which accompany changes
of aggregation. The most general questions respecting the structure
of an organism, having been answered when it is ascertained of what
units it is composed as a whole, and in its several parts; there come
the more special questions concerning its form--form in the ordinary
sense. After the contrasts caused by variations in the process of
integration, we have to consider the contrasts caused by variations
in the process of differentiation. To speak specifically--the shape
of the organism as a whole, irrespective of its composition, has to
be accounted for. Reasons have to be found for the unlikeness between
its general outlines and the general outlines of allied organisms. And
there have to be answered kindred inquiries respecting the proportions
of its component parts:--Why, among such of these as are homologous
with one another, have there arisen the differences that exist? And how
have there been produced the contrasts between them and the homologous
parts of organisms of the same type?
Very numerous are the heterogeneities of form presenting themselves
for interpretation under these heads. The ultimate morphological
units combined in any group, may be differentiated individually, or
collectively, or both: each of them may undergo changes of shape;
or some of them may be changed and others not; or the group may be
rendered multiform by the greater growth of some of its units than of
others. Similarly with the compound units arising by union of these
simple units. Aggregates of the second order may be made relatively
complex in form, by inequalities in the rates of multiplication of
their component units in diverse directions; and among a number of such
aggregates, numerous unlikenesses may be constituted by differences in
their degrees of growth, and by differences in their modes of growth.
Manifestly, at each higher stage of composition the possible sources of
divergence are multiplied still further.
That facts of this order can be accounted for in detail is not to
be expected--the data are wanting. All that we may hope to do is to
ascertain their general laws. How this is to be attempted we will now
consider.
§ 178. The task before us is to trace throughout these phenomena
the process of evolution; and to show how, as displayed in them, it
conforms to those first principles which evolution in general conforms
to. Two sets of factors have to be taken into account. Let us look at
them.
The factors of the first class are those which tend directly to
change an organic aggregate, in common with every other aggregate,
from that more simple form which is not in equilibrium with incident
forces, to that more complex form which is in equilibrium with them.
We have to mark how, in correspondence with the universal law that
the uniform lapses into the multiform, and the less multiform into
the more multiform, the parts of each organism are ever becoming
further differentiated; and we have to trace the varying relations
to incident forces by which further differentiations are entailed.
We have to observe, too, how each primary modification of structure,
induced by an altered distribution of forces, becomes a parent of
secondary modifications--how, through the necessary multiplication of
effects, change of form in one part brings about changes of form in
other parts. And then we have also to note the metamorphoses constantly
being induced by the process of segregation--by the gradual union
of like parts exposed to like forces, and the gradual separation of
like parts exposed to unlike forces. The factors of the second class
which we have to keep in view throughout our interpretations, are
the formative tendencies of organisms themselves--the proclivities
inherited by them from antecedent organisms, and which past processes
of evolution have bequeathed. We have seen it to be inferable from
various orders of facts (§§ 65, 84, 97–97_g_), that organisms are
built up of certain highly-complex molecules, which we distinguished as
physiological units [or constitutional units as they might otherwise
be called]--each kind of organism being built up of units peculiar to
itself. We recognized in these units, powers of arranging themselves
into the forms of the organisms to which they belong; analogous
to the powers which the molecules of inorganic substances have of
aggregating into specific crystalline forms. We have consequently
to regard this proclivity of the physiological units, as producing,
during the development of any organism, a combination of internal
forces that expend themselves in working out a structure in equilibrium
with the forces to which ancestral organisms were exposed; but not in
equilibrium with the forces to which the existing organism is exposed,
if the environment has been changed. Hence the problem in all cases is,
to ascertain the resultant of internal organizing forces, tending to
reproduce the ancestral form, and external modifying forces, tending
to cause deviations from that form. Moreover, we have to take into
account, not only the characters of immediately-preceding ancestors,
but also those of their ancestors, and ancestors of all degrees of
remoteness. Setting out with rudimentary types, we have to consider
how, in each successive stage of evolution, the structures acquired
during previous stages have been obscured by further integrations
and further differentiations; or, conversely, how the lineaments of
primitive organisms have all along continued to manifest themselves
under the superposed modifications.
§179. Two ways of carrying on the inquiry suggest themselves. We may
go through the several great groups of organisms, with the view of
reaching, by comparison of parts, certain general truths respecting
the homologies, the forms, and the relations of their parts; and then,
having dealt with the phenomena inductively, may retrace our steps with
the view of deductively interpreting the general truths reached. Or,
instead of thus separating the two investigations, we may carry them on
hand in hand--first establishing each general truth empirically, and
then proceeding to the rationale of it. This last method will, I think,
conduce to both brevity and clearness. Let us now thus deal with the
first class of morphological problems.
[NOTE.--In preparation for treating of morphological development,
sundry other general considerations should have been included in the
foregoing chapter when originally published. This seems the most
appropriate place for now naming them. Some were implicitly contained
in the first volume, but it will be well definitely to state these, as
well as the others not yet implied.
Interpretation of the forms of organisms and the forms of their parts,
must depend mainly on the conclusions previously drawn respecting
their phylogeny; and the drawing of such conclusions must be guided
by recognition of the various factors of Evolution, as well as by
recognition of certain extremely general results of Evolution and
certain concomitants of Evolution.
A primary one among these is that no existing species can exhibit
more than approximately the ancestral structure of any other existing
species. As all ancestors have disappeared, so, in a greater or less
degree, the traits, specific, generic, or ordinal, which distinguished
the earlier of them have disappeared. Setting out with the familiar
symbol, a tree, let us regard its peripheral twigs as representing
extant species; let us assume that the interior of the tree is filled
up with some supporting substance, leaving only the ends of the
living twigs projecting; and let us suppose the trunk, main branches,
secondary branches, tertiary branches, &c., have decayed away.
Then if we take these decayed parts to stand for the divergent and
re-divergent lines of evolution which are represented by fossils in the
Earth’s crust, it will be manifest, first, that no one of the living
superficial twigs (or species) exhibits the ancestral organization
whence any other of the living superficial twigs (or species) has been
developed; it will be manifest, second, that the generic structure
inherited by any existing species must be a structure out of which
came sundry allied species--the fork, as it were, at which adjacent
twigs diverged; and third, that the ancestor of an order must, in like
manner, be sought at some point deeper down in the symbolic tree--a
place of divergence of the sub-branches representing allied genera.
Similarly with the ancestral types of classes, still deeper down in
the tree or further back in time. So that phylogeny becomes more and
more speculative as its questions become more and more radical. And
the difficulty is made greater by the deficiency of palæontological
evidence.
One obvious corollary is that an ancestral type from which sundry
allied types now existing diverged, was, speaking generally, simpler
than these; since the divergent types became different by the
superposing of modifications, adding to their complexities. There is
a further reason for inferring that the least specialized member of
any group is more like the remote ancestor than any of the others; for
every adaptation stands in the way of subsequent re-adaptations: it
presents a greater amount of structure to be undone. To get some idea
of the ancestral type where no extant member of the group is manifestly
simpler than the rest, the method must be to take all its extant
members and, after letting their differences mutually cancel, observe
what remains common to them all.
But there are difficulties standing in the way of phylogeny, and
consequently of morphology, much greater than these. Returning to
our symbolic tree, it is clear that it would be far from easy to say
of any one twig which extinct sub-branch, branch, and main branch it
belonged to, even supposing that the growths of all parts had been
uniformly outwards. Immensely more perplexing, then, must be the
affiliation if various of the branches, sub-branches, &c., have sent
out backward-growing shoots which have come to the surface only after
prolonged retrograde courses, and if other branches have sent shoots
into regions occupied by alien branches--shoots bearing twigs which
come to the surface along with those to which they are but remotely
allied. The problems of origin and of structure which organisms
present, are met by both of the difficulties thus symbolized.
One of them arises from the prevalence of retrograde metamorphoses.
Throughout the animal world these are variously displayed by parasites,
multitudinous in their kinds; for most of them belong to types
much higher in organization. Changed habits and consequent changed
structures have so transferred them that only by study of their
embryonic stages can their kinships be made out. And these retrograde
metamorphoses, conspicuous among parasites, have, in the course of
evolution, affected some members of all groups; for in all groups the
struggle for existence has compelled some to adopt careers less trying
but less profitable.
Not only by forcing on many kinds of organisms simpler ways of living,
and consequent degeneracy, has the universal competition caused
obscuring transformations. It has done this also by tempting many other
kinds of organisms to adopt ways of life not simpler than before but
merely different. Pressure continually prompts every type to intrude on
other types’ spheres of activity; and so causes it to assume certain
structural characters of the types whose spheres it invades, masking
its previous characters. Modifications hence arising have, in the great
mass of cases, been superposed one on another time after time. The
aquatic animal becomes through several transitions a land-animal, and
then the land-animal through other transitions becomes now an aërial
animal like the bat and now an aquatic animal like the whale. Certain
kinds of birds furnish extreme illustrations. There was the change from
the fish to the water-breathing amphibian and then to the air-breathing
amphibian; thence to the reptile living on the Earth’s surface; thence
to the flying reptile and the bird; then came the diving birds, joining
with their aërial life a life passed partly in the water; and finally
came a type like the penguin, in which the power of flight has been
lost and the water has again become the almost exclusive medium, except
for breathing. Of course the mouldings and re-mouldings of structure
resulting from these successive unlike modes of life, in many cases put
great difficulties in the way of ascertaining which are the original
corresponding parts. Some parts have become abnormally large; others
have dwindled or disappeared; and the relative positions of parts
have often been greatly changed. A bat’s wing and a bird’s wing are
analogous organs, but their frameworks are but partially homologous.
While in the bird the terminal parts of the fore-limb do little towards
supporting the wing, in the bat the wing is mainly supported by
enormously-developed terminal parts.
The effects of the struggle to survive, which here prompts a simpler
life with resulting degeneracy and there a different life with
resulting new developments, are far from being the only causes of
morphological obscurations. Fulfilment of certain highly general
requirements gives certain common traits to plants of widely divergent
classes; and fulfilment of certain other highly general requirements
gives certain common traits to animals of widely divergent classes.
It was remarked in the first volume (§ 54_f_) that the cardinal
distinction between the characters of plants and animals arises from
the fact that while the chief food of plants is universally present the
food of animals is scattered. Here it has to be added that to utilize
the universally distributed food the ordinary plant needs the aid of
light, and has to acquire structures enabling it to get that aid;
while the ordinary animal, to utilize the scattered food, must acquire
the structures needful for locomotion. Let us contemplate separately
the traits hence resulting in the vegetal world and the traits hence
resulting in the animal world.
The familiar plantain meets the requirement by growing stiff leaves
enabling it to press down the competing grasses around which would
else shade it; but the great majority of ordinary plants meet the
requirement by raising themselves into the air. Hence the need for a
stem, and hence the fact that plants of widely unlike natures similarly
form stems which, in achieving strength enough to support the foliage
and resist the wind, acquire certain adaptive structures having a
general similarity. Here from the edge of a pool is a reed, and here
from the adjacent copse is a hemlock: the one having grown tall in
escaping the shade of its companions and the other in escaping the
shade of the surrounding brushwood. On being cut across each discloses
a tube, and each exhibits septa dividing this tube into chambers. In
either case by the tubular structure is gained the greatest strength
with the least material; but there is no morphological kinship between
the tubes nor between the septa. Still more marked is the simulation
of homology by analogy in another plant which the adjacent ditch may
furnish--the common Horsetail. In this, again, we see an elongated
vertical-growing part, raising the foliage into the air; and, as
before, this is tubular and divided by septa. A type utterly alien from
the other two has, by survival of the fittest, been similarly moulded
to meet mechanical needs.
Passing now to the obscurations in the animal world caused by
alterations favouring locomotion, we note first that the locomotive
power is at the outset very slight. Among many orders of _Protozoa_,
as also among many low types of _Metazoa_, vibratile cilia are the
most general agents of locomotion--necessarily feeble locomotion.
Regarded in the mass, the _Cœlenterata_, when not stationary like
the _Hydra_ or higher types in the hydroid stage, usually possess
only such small self-mobility as the slow rhythmical contractions of
their umbrella-disks effect, or else such as is effected by bands of
cilia or of vibratile plates, as in the _Beroe_. Even among these low
tpes of _Metazoa_, however, in which ordinarily the radial structure
is conspicuous, or but slightly obscured by an ovoid form as in the
_Ctenophora_, we find, in the _Cestus veneris_, extreme obscuration
caused by an elongation which facilitates movement through the water;
alike by the actions of its vibratile plates and by its undulations,
which simulate those of sundry higher animals.
And here we come upon the essential fact to be recognized. Elongation
favours locomotion in various ways that are severally taken advantage
of by different types of creatures. (1) To a given mass of moving
matter the resistance of the medium decreases along with decrease
in the area of its transverse section, and this implies increase of
length: a given force will move the lengthened mass along with greater
facility. (2) Reaching a certain point the elongated form enables an
animal to progress by undulations, as in the water fish do, and even
some cœlenterates and turbellarians do, and as on land snakes do:
lateral resistances serving in either case as fulcra. (3) Lengthening
of the body serves otherwise to aid locomotion in the creeping or
burrowing worm, which, utilizing the statical resistance of its hinder
part thrusts onwards its fore part, and then, holding fast its fore
part by the aid of minute _setæ_, draws the hinder part after it. But
elongation, doubly advantageous at first, while the body is itself the
chief instrument of locomotion, gradually loses its advantageousness
as special instruments of locomotion are developed. (4) This we
see in that locomotive action effected by limbs, which, many and
small in the lower _Arthropoda_ and becoming few and larger in the
higher, at length give great activity to a shortened and consolidated
body: a stage reached only through stages of decreasing elongation
accompanying increase of limb-power. (5) In the _Vertebrata_ locomotion
by undulations comes, along certain lines of evolution, to be replaced
by that limb locomotion which accompanies the rise from water-life
to land-life: the evolution of Amphibians exhibiting the transition.
(6) Further, we see among mammals that as limbs become efficient the
elongated body ceases to be itself instrumental in locomotion, but that
still some elongation remains a characteristic. (7) Finally, where
limb locomotion reaches its highest degree, as in birds, elongation
disappears.
These classes of familiar facts I have recalled to show that, in
the course of evolution, achievement by plants of the all-essential
elevation into the air and by animals of the all-essential power of
movement have developed this trait of elongation in various types;
and that in each kingdom acquisition of the common trait has had a
tendency now to obscure morphological equivalence, and now to give the
appearance of kinship where there is none. A further purpose has been
to prepare the way for a question hereafter to be discussed--whether,
in the various types of either kingdom, the elongation is effected in
the same ways or in different ways. We shall have to ask whether the
vertically-growing part is always, like that of _Lessonia_, a simple
individual, or whether, as possibly in Phænogams, it is a united series
of individuals; and similarly whether the elongated body is always
single, like that of a mollusc, or whether, as possibly in annulose
animals, it is a series of united individuals.]