Ask a plumber who is repairing your pump, how the water is raised in it,
and he replies--"By suction." Recalling the ability which he has to suck
up water into his mouth through a tube, he is certain that he
understands the pump's action. To inquire what he means by suction,
seems to him absurd. He says you know as well as he does, what he means;
and he cannot see that there is any need for asking how it happens that
the water rises in the tube when he strains his mouth in a particular
way. To the question why the pump, acting by suction, will not make the
water rise above 32 feet, and practically not so much, he can give no
answer; but this does not shake his confidence in his explanation.
On the other hand an inquirer who insists on knowing what suction is,
may obtain from the physicist answers which give him clear ideas, not
only about it but about many other things. He learns that on ourselves
and all things around, there is an atmospheric pressure amounting to
about 15 pounds on the square inch: 15 pounds being the average weight
of a column of air having a square inch for its base and extending
upwards from the sea-level to the limit of the Earth's atmosphere. He is
made to observe that when he puts one end of a tube into water and the
other end into his mouth, and then draws back his tongue, so leaving a
vacant space, two things happen. One is that the pressure of air outside
his cheeks, no longer balanced by an equal pressure of air inside,
thrusts his cheeks inwards; and the other is that the pressure of air on
the surface of the water, no longer balanced by an equal pressure of air
within the tube and his mouth (into which part of the air from the tube
has gone) the water is forced up the tube in consequence of the unequal
pressure. Once understanding thus the nature of the so-called suction,
he sees how it happens that when the plunger of the pump is raised and
relieves from atmospheric pressure the water below it, the atmospheric
pressure on the water in the well, not being balanced by that on the
water in the tube, forces the water higher up the tube, so that it
follows the plunger. And now he sees why the water cannot be raised
beyond the theoretic limit of 32 feet: a limit made much lower in
practice by imperfections in the apparatus. For if, simplifying the
conception, he supposes the tube of the pump to be a square inch in
section, then the atmospheric pressure of 15 pounds per square inch on
the water in the well, can raise the water in the tube to such height
only that the entire column of it weighs 15 pounds. Having been thus
enlightened about the pump's action, the action of a barometer becomes
intelligible. He perceives how, under the conditions established, the
weight of the column of mercury balances that of an atmospheric column
of equal diameter; and how, as the weight of the atmospheric column
varies, there is a corresponding variation in the weight of the
mercurial column,--shown by change of height. Moreover, having
previously supposed that he understood the ascent of a balloon when he
ascribed it to relative lightness, he now sees that he did not truly
understand it. For he did not recognize it as a result of that upward
pressure caused by the difference between the weight of the mass formed
by the gas in the balloon _plus_ the cylindrical column of air extending
above it to the limit of the atmosphere, and the weight of a similar
cylindrical column of air extending down to the under surface of the
balloon: this difference of weight causing an equivalent upward pressure
on the under surface.
Why do I introduce these familiar truths so entirely irrelevant to my
subject? I do it to show, in the first place, the contrast between a
vague conception of a cause and a distinct conception of it; or rather,
the contrast between that conception of a cause which results when it is
simply classed with some other or others which familiarity makes us
think we understand, and that conception of a cause which results when
it is represented in terms of definite physical forces admitting of
measurement. And I do it to show, in the second place, that when we
insist on resolving a verbally-intelligible cause into its actual
factors, we get not only a clear solution of the problem before us, but
we find that the way is opened to solutions of sundry other problems.
While we rest satisfied with unanalyzed causes, we may be sure both that
we do not rightly comprehend the production of the particular effects
ascribed to them, and that we overlook other effects which would be
revealed to us by contemplation of the causes as analyzed. Especially
must this be so where the causation is complex. Hence we may infer that
the phenomena presented by the development of species, are not likely to
be truly conceived unless we keep in view the concrete agencies at work.
Let us look closely at the facts to be dealt with.
* * * * *
The growth of a thing is effected by the joint operation of certain
forces on certain materials; and when it dwindles, there is either a
lack of some materials, or the forces co-operate in a way different from
that which produces growth. If a structure has varied, the implication
is that the processes which built it up were made unlike the parallel
processes in other cases, by the greater or less amount of some one or
more of the matters or actions concerned. Where there is unusual
fertility, the play of vital activities is thereby shown to have
deviated from the ordinary play of vital activities; and conversely, if
there is infertility. If the germs, or ova, or seed, or offspring
partially developed, survive more or survive less, it is either because
their molar or molecular structures are unlike the average ones, or
because they are affected in unlike ways by surrounding agencies. When
life is prolonged, the fact implies that the combination of actions,
visible and invisible, constituting life, retains its equilibrium longer
than usual in presence of environing forces which tend to destroy its
equilibrium. That is to say, growth, variation, survival, death, if they
are to be reduced to the forms in which physical science can recognize
them, must be expressed as effects of agencies definitely
conceived--mechanical forces, light, heat, chemical affinity, &c.
This general conclusion brings with it the thought that the phrases
employed in discussing organic evolution, though convenient and indeed
needful, are liable to mislead us by veiling the actual agencies. That
which really goes on in every organism is the working together of
component parts in ways conducing to the continuance of their combined
actions, in presence of things and actions outside; some of which tend
to subserve, and others to destroy, the combination. The matters and
forces in these two groups, are the sole causes properly so called. The
words "natural selection," do not express a cause in the physical sense.
They express a mode of co-operation among causes--or rather, to speak
strictly, they express an effect of this mode of co-operation. The idea
they convey seems perfectly intelligible. Natural selection having been
compared with artificial selection, and the analogy pointed out, there
apparently remains no indefiniteness: the inconvenience being, however,
that the definiteness is of a wrong kind. The tacitly implied Nature
which selects, is not an embodied agency analogous to the man who
selects artificially; and the selection is not the picking out of an
individual fixed on, but the overthrowing of many individuals by
agencies which one successfully resists, and hence continues to live and
multiply. Mr. Darwin was conscious of these misleading implications. In
the introduction to his _Animals and Plants under Domestication_ (p. 6)
he says:--
"For brevity sake I sometimes speak of natural selection as an
intelligent power; ... I have, also, often personified the word
Nature; for I have found it difficult to avoid this ambiguity; but
I mean by nature only the aggregate action and product of many
natural laws,--and by laws only the ascertained sequence of
events."
But while he thus clearly saw, and distinctly asserted, that the factors
of organic evolution are the concrete actions, inner and outer, to which
every organism is subject, Mr. Darwin, by habitually using the
convenient figure of speech, was, I think, prevented from recognizing so
fully as he would otherwise have done, certain fundamental consequences
of these actions.
Though it does not personalize the cause, and does not assimilate its
mode of working to a human mode of working, kindred objections may be
urged against the expression to which I was led when seeking to present
the phenomena in literal terms rather than metaphorical terms--the
survival of the fittest;[42] for in a vague way the first word, and in a
clear way the second word, calls up an anthropocentric idea. The
thought of survival inevitably suggests the human view of certain sets
of phenomena, rather than that character which they have simply as
groups of changes. If, asking what we really know of a plant, we exclude
all the ideas associated with the words life and death, we find that the
sole facts known to us are that there go on in the plant certain
inter-dependent processes, in presence of certain aiding and hindering
influences outside of it; and that in some cases a difference of
structure or a favourable set of circumstances, allows these
inter-dependent processes to go on for longer periods than in other
cases. Again, in the working together of those many actions, internal
and external, which determine the lives or deaths of organisms, we see
nothing to which the words fitness and unfitness are applicable in the
physical sense. If a key fits a lock, or a glove a hand, the relation of
the things to one another is presentable to the perceptions. No approach
to fitness of this kind is made by an organism which continues to live
under certain conditions. Neither the organic structures themselves, nor
their individual movements, nor those combined movements of certain
among them which constitute conduct, are related in any analogous way to
the things and actions in the environment. Evidently the word fittest,
as thus used, is a figure of speech; suggesting the fact that amid
surrounding actions, an organism characterized by the word has either a
greater ability than others of its kind to maintain the equilibrium of
its vital activities, or else has so much greater a power of
multiplication that though not longer lived than they, it continues to
live in posterity more persistently. And indeed, as we here see, the
word fittest has to cover cases in which there may be less ability than
usual to survive individually, but in which the defect is more than made
good by higher degrees of fertility.
I have elaborated this criticism with the intention of emphasizing the
need for studying the changes which have gone on, and are ever going
on, in organic bodies, from an exclusively physical point of view. On
contemplating the facts from this point of view, we become aware that,
besides those special effects of the co-operating forces which eventuate
in the longer survival of one individual than of others, and in the
consequent increase through generations, of some trait which furthered
its survival, many other effects are being wrought on each and all of
the individuals. Bodies of every class and quality, inorganic as well as
organic, are from instant to instant subject to the influences in their
environments; are from instant to instant being changed by these in ways
that are mostly inconspicuous; and are in course of time changed by them
in conspicuous ways. Living things in common with dead things, are, I
say, being thus perpetually acted upon and modified; and the changes
hence resulting, constitute an all-important part of those undergone in
the course of organic evolution. I do not mean to imply that changes of
this class pass entirely unrecognized; for, as we shall see, Mr. Darwin
takes cognizance of certain secondary and special ones. But the effects
which are not taken into account, are those primary and universal
effects which give certain fundamental characters to all organisms.
Contemplation of an analogy will best prepare the way for appreciation
of them, and of the relation they bear to those which at present
monopolize attention.
An observant rambler along shores, will, here and there, note places
where the sea has deposited things more or less similar, and separated
them from dissimilar things--will see shingle parted from sand; larger
stones sorted from smaller stones; and will occasionally discover
deposits of shells more or less worn by being rolled about. Sometimes
the pebbles or boulders composing the shingle at one end of a bay, he
will find much larger than those at the other: intermediate sizes,
having small average differences, occupying the space between the
extremes. An example occurs, if I remember rightly, some mile or two to
the west of Tenby; but the most remarkable and well-known example is
that afforded by the Chesil bank. Here, along a shore some sixteen miles
long, there is a gradual increase in the sizes of the stones; which,
being at one end but mere pebbles, are at the other end immense
boulders. In this case, then, the breakers and the undertow have
effected a selection--have at each place left behind those stones which
were too large to be readily moved, while taking away others small
enough to be moved easily. But now, if we contemplate exclusively this
selective action of the sea, we overlook certain important effects which
the sea simultaneously works. While the stones have been differently
acted upon in so far that some have been left here and some carried
there; they have been similarly acted upon in two allied, but
distinguishable, ways. By perpetually rolling them about and knocking
them one against another, the waves have so broken off their most
prominent parts as to produce in all of them more or less rounded forms;
and then, further, the mutual friction of the stones simultaneously
caused, has smoothed their surfaces. That is to say in general terms,
the actions of environing agencies, so far as they have operated
indiscriminately, have produced in the stones a certain unity of
character; at the same time that they have, by their differential
effects, separated them: the larger ones having withstood certain
violent actions which the smaller ones could not withstand.
Similarly with other assemblages of objects which are alike in their
primary traits but unlike in their secondary traits. When simultaneously
exposed to the same set of actions, some of these actions, rising to a
certain intensity, may be expected to work on particular members of the
assemblage changes which they cannot work in those which are markedly
unlike; while others of the actions will work in all of them similar
changes, because of the uniform relations between these actions and
certain attributes common to all members of the assemblage. Hence it is
inferable that on living organisms, which form an assemblage of this
kind, and are unceasingly exposed in common to the agencies composing
their inorganic environments, there must be wrought two such sets of
effects. There will result a universal likeness among them consequent on
the likeness of their respective relations to the matters and forces
around; and there will result, in some cases, the differences due to the
differential effects of these matters and forces, and in other cases,
the changes which, being life-sustaining or life-destroying, eventuate
in certain natural selections.
I have, above, made a passing reference to the fact that Mr. Darwin did
not fail to take account of some among these effects directly produced
on organisms by surrounding inorganic agencies. Here are extracts from
the sixth edition of the _Origin of Species_ showing this.
"It is very difficult to decide how far changed conditions, such as
of climate, food, &c., have acted in a definite manner. There is
reason to believe that in the course of time the effects have been
greater than can be proved by clear evidence.... Mr. Gould believes
that birds of the same species are more brightly coloured under a
clear atmosphere, than when living near the coast or on islands;
and Wollaston is convinced that residence near the sea affects the
colours of insects. Moquin-Tandon gives a list of plants which,
when growing near the sea-shore, have their leaves in some degree
fleshy, though not elsewhere fleshy" (pp. 106-7). "Some observers
are convinced that a damp climate affects the growth of the hair,
and that with the hair the horns are correlated" (p. 159).
In his subsequent work, _Animals and Plants under Domestication_, Mr.
Darwin still more clearly recognizes these causes of change in
organization. A chapter is devoted to the subject. After premising that
"the direct action of the conditions of life, whether leading to
definite or indefinite results, is a totally distinct consideration from
the effects of natural selection;" he goes on to say that changed
conditions of life "have acted so definitely and powerfully on the
organisation of our domesticated productions, that they have sufficed
to form new sub-varieties or races, without the aid of selection by man
or of natural selection." Of his examples here are two.
"I have given in detail in the ninth chapter the most remarkable
case known to me, namely, that in Germany several varieties of
maize brought from the hotter parts of America were transformed in
the course of only two or three generations." (Vol. ii, p. 277.)
[And in this ninth chapter concerning these and other such
instances he says "some of the foregoing differences would
certainly be considered of specific value with plants in a state of
nature." (Vol. i, p. 321.)] "Mr. Meehan, in a remarkable paper,
compares twenty-nine kinds of American trees, belonging to various
orders, with their nearest European allies, all grown in close
proximity in the same garden and under as nearly as possible the
same conditions." And then enumerating six traits in which the
American forms all of them differ in like ways from their allied
European forms, Mr. Darwin thinks there is no choice but to
conclude that these "have been definitely caused by the
long-continued action of the different climate of the two
continents on the trees." (Vol. ii, pp. 281-2.)
But the fact we have to note is that while Mr. Darwin thus took account
of special effects due to special amounts and combinations of agencies
in the environment, he did not take account of the far more important
effects due to the general and constant operation of these agencies.[43]
If a difference between the quantities of a force which acts on two
organisms, otherwise alike and otherwise similarly conditioned, produces
some difference between them; then, by implication, this force produces
in both of them effects which they show in common. The inequality
between two things cannot have a value unless the things themselves have
values. Similarly if, in two cases, some unlikeness of proportion among
the surrounding inorganic agencies to which two plants or two animals
are exposed, is followed by some unlikeness in the changes wrought on
them; then it follows that these several agencies taken separately, work
changes in both of them. Hence we must infer that organisms have certain
structural characters in common, which are consequent on the action of
the medium in which they exist: using the word medium in a comprehensive
sense, as including all physical forces falling upon them as well as
matters bathing them. And we may conclude that from the primary
characters thus produced there must result secondary characters.
Before going on to observe those general traits of organisms due to the
general action of the inorganic environment upon them, I feel tempted to
enlarge on the effects produced by each of the several matters and
forces constituting the environment. I should like to do this not only
to give a clear preliminary conception of the ways in which all
organisms are affected by these universally-present agents, but also to
show that, in the first place, these agents modify inorganic bodies as
well as organic bodies, and that, in the second place, the organic are
far more modifiable by them than the inorganic. But to avoid undue
suspension of the argument, I content myself with saying that when the
respective effects of gravitation, heat, light, &c., are studied, as
well as the respective effects, physical and chemical, of the matters
forming the media, water and air, it will be found that while more or
less operative on all bodies, each modifies organic bodies to an extent
immensely greater than the extent to which it modifies inorganic bodies.
* * * * *
Here, not discriminating among the special effects which these various
forces and matters in the environment produce on both classes of bodies,
let us consider their combined effects, and ask--What is the most
general trait of such effects?
Obviously the most general trait is the greater amount of change wrought
on the outer surface than on the inner mass. In so far as the matters of
which the medium is composed come into play, the unavoidable implication
is that they act more on the parts directly exposed to them than on the
parts sheltered from them. And in so far as the forces pervading the
medium come into play, it is manifest that, excluding gravity, which
affects outer and inner parts indiscriminately, the outer parts have to
bear larger shares of their actions. If it is a question of heat, then
the exterior must lose it or gain it faster than the interior; and in a
medium which is now warmer and now colder, the two must habitually
differ in temperature to some extent--at least where the size is
considerable. If it is a question of light, then in all but absolutely
transparent masses, the outer parts must undergo more of any change
producible by it than the inner parts--supposing other things equal; by
which I mean, supposing the case is not complicated by any such
convexities of the outer surface as produce internal concentrations of
rays. Hence then, speaking generally, the necessity is that the primary
and almost universal effect of the converse between the body and its
medium, is to differentiate its outside from its inside. I say almost
universal, because where the body is both mechanically and chemically
stable, like, for instance, a quartz crystal, the medium may fail to
work either inner or outer change.
Of illustrations among inorganic bodies, a convenient one is supplied by
an old cannon-ball that has been long lying exposed. A coating of rust,
formed of flakes within flakes, incloses it; and this thickens year by
year, until, perhaps, it reaches a stage at which its exterior loses as
much by rain and wind as its interior gains by further oxidation of the
iron. Most mineral masses--pebbles, boulders, rocks--if they show any
effect of the environment at all, show it only by that disintegration of
surface which follows the freezing of absorbed water: an effect which,
though mechanical rather than chemical, equally illustrates the general
truth. Occasionally a "rocking-stone" is thus produced. There are formed
successive layers relatively friable in texture, each of which, thickest
at the most exposed parts, and being presently lost by weathering,
leaves the contained mass in a shape more rounded than before; until,
resting on its convex under-surface, it is easily moved. But of all
instances perhaps the most remarkable is one to be seen on the west bank
of the Nile at Philae, where a ridge of granite 100 feet high, has had
its outer parts reduced in course of time to a collection of
boulder-shaped masses, varying from say a yard in diameter to six or
eight feet, each one of which shows in progress an exfoliation of
successively-formed shells of decomposed granite: most of the masses
having portions of such shells partially detached.
If, now, inorganic masses, relatively so stable in composition, thus
have their outer parts differentiated from their inner parts, what must
we say of organic masses, characterized by such extreme chemical
instability?--instability so great that their essential material is
named protein, to indicate the readiness with which it passes from one
isomeric form to another. Clearly the necessary inference is that this
effect of the medium must be wrought inevitably and promptly, wherever
the relation of outer and inner has become settled: a qualification for
which the need will be seen hereafter.
* * * * *
Beginning with the earliest and most minute kinds of living things, we
necessarily encounter difficulties in getting direct evidence; since, of
the countless species now existing, all have been subject during
millions upon millions of years to the evolutionary process, and have
had their primary traits complicated and obscured by those endless
secondary traits which the natural selection of favourable variations
has produced. Among protophytes it needs but to think of the
multitudinous varieties of diatoms and desmids, with their
elaborately-constructed coverings; or of the definite methods of growth
and multiplication among such simple _Algae_ as the _Conjugatae_; to see
that most of their distinctive characters are due to inherited
constitutions, which have been slowly moulded by survival of the fittest
to this or that mode of life. To disentangle such parts of their
developmental changes as are due to the action of the medium, is
therefore hardly possible. We can hope only to get a general conception
of it by contemplating the totality of the facts.
The first cardinal fact is that all protophytes are cellular--all show
us this contrast between outside and inside. Supposing the multitudinous
specialities of the envelope in different orders and genera of
protophytes to be set against one another, and mutually cancelled, there
remains as a trait common to them--an envelope unlike that which it
envelopes. The second cardinal fact is that this simple trait is the
earliest trait displayed in germs, or spores, or other parts from which
new individuals are to arise; and that, consequently, this trait must be
regarded as having been primordial. For it is an established truth of
organic evolution that embryos show us, in general ways, the forms of
remote ancestors; and that the first changes undergone, indicate, more
or less clearly, the first changes which took place in the series of
forms through which the existing form has been reached. Describing, in
successive groups of plants, the early transformations of these
primitive units, Sachs[44] says of the lowest _Algae_ that "the
conjugated protoplasmic body clothes itself with a cell-wall" (p. 10);
that in "the spores of Mosses and Vascular Cryptogams" and in "the
pollen of Phanerogams" ... "the protoplasmic body of the mother-cell
breaks up into four lumps, which quickly round themselves off and
contract, and become enveloped by a cell-membrane only after complete
separation" (p. 13); that in the _Equisetaceae_ "the young spores, when
first separated, are still naked, but they soon become surrounded by a
cell-membrane" (p. 14); and that in higher plants, as in the pollen of
many Dicotyledons, "the contracting daughter-cells secrete cellulose
even during their separation" (p. 14). Here, then, in whatever way we
interpret it, the fact is that there quickly arises an outer layer
different from the contained matter. But the most significant evidence
is furnished by "the masses of protoplasm that escape into water from
the injured sacs of _Vaucheria_, which often instantly become rounded
into globular bodies," and of which the "hyaline protoplasm envelopes
the whole as a skin" (p. 41) which "is denser than the inner and more
watery substance" (p. 42). As in this case the protoplasm is but a
fragment, and as it is removed from the influence of the parent-cell,
this differentiating process can scarcely be regarded as anything more
than the effect of physico-chemical actions: a conclusion which is
supported by the statement of Sachs that "not only every vacuole in a
solid protoplasmic body, but also every thread of protoplasm which
penetrates the sap-cavity, and finally the inner side of the
protoplasm-sac which encloses the sap-cavity, is also bounded by a skin"
(p. 42). If then "every portion of a protoplasmic body immediately
surrounds itself, when it becomes isolated, with such a skin," which is
shown in all cases to arise at the surface of contact with sap or water,
this primary differentiation of outer from inner must be ascribed to the
direct action of the medium. Whether the coating thus initiated is
secreted by the protoplasm, or whether, as seems more likely, it
results from transformation of it, matters not to the argument. Either
way the action of the medium causes its formation; and either way the
many varied and complex differentiations which developed cell-walls
display, must be considered as originating from those variations of this
physically-generated covering which natural selection has taken
advantage of.
The contained protoplasm of a vegetal cell, which has self-mobility and
when liberated sometimes performs amoeba-like motions for a time, may
be regarded as an imprisoned amoeba; and when we pass from it to a
free amoeba, which is one of the simplest types of first animals, or
_Protozoa_, we naturally meet with kindred phenomena. The general trait
which here concerns us, is that while its plastic or semi-fluid sarcode
goes on protruding, in irregular ways, now this and now that part of its
periphery, and again withdrawing into its interior first one and then
another of these temporary processes, perhaps with some small portion of
food attached, there is but an indistinct differentiation of outer from
inner (a fact shown by the frequent coalescence of the pseudopodia in
Rhizopods); but that when it eventually becomes quiescent, the surface
becomes differentiated from the contents: the passing into an encysted
state, doubtless in large measure due to inherited proclivity, being
furthered, and having probably been once initiated, by the action of the
medium. The connexion between constancy of relative position among the
parts of the sarcode, and the rise of a contrast between superficial and
central parts, is perhaps best shown in the minutest and simplest
_Infusoria_, the _Monadinae_. The genus _Monas_ is described by Kent as
"plastic and unstable in form, possessing no distinct cuticular
investment; ... the food-substances incepted at all parts of the
periphery";[45] and the genus _Scytomonas_ he says "differs from _Monas_
only in its persistent shape and accompanying greater rigidity of the
peripheral or ectoplasmic layer."[46] Describing generally such low
forms, some of which are said to have neither nucleus nor vacuole, he
remarks that in types somewhat higher "the outer or peripheral border of
the protoplasmic mass, while not assuming the character of a distinct
cell-wall or so-called cuticle, presents, as compared with the inner
substance of that mass, a slightly more solid type of composition."[47]
And it is added that these forms having so slightly differentiated an
exterior, "while usually exhibiting a more or less characteristic normal
outline, can revert at will to a pseud-amoeboid and repent state."[48]
Here, then, we have several indications of the truth that the permanent
externality of a certain part of the substance, is followed by
transformation of it into a coating unlike the substance it contains.
Indefinite and structureless in the simplest of these forms, as instance
again the _Gregarina_,[49] the limiting membrane becomes, in higher
_Infusoria_, definite and often complex: showing that the selection of
favourable variations has had largely to do with its formation. In such
types as the _Foraminifera_, which, almost structureless internally
though they are, secrete calcareous shells, it is clear that the nature
of this outer layer is determined by inherited constitution. But
recognition of this consists with the belief that the action of the
medium initiated the outer layer, specialized though it now is; and that
even still, contact with the medium excites secretion of it.
A remarkable analogy remains to be named. When we study the action of
the medium in an inorganic mass, we are led to see that between the
outer changed layer and the inner unchanged mass, comes a surface where
active change is going on. Here we have to note that, alike in the
plant-cell and in the animal-cell, there is a similar relation of parts.
Immediately inside the envelope comes the primordial utricle in the
one case, and in the other case the layer of active sarcode. In either
case the living protoplasm, placed in the position of a lining to the
cuticle of the cell, is shielded from the direct action of the medium,
and yet is not beyond the reach of its influences.
* * * * *
Limited, as thus far drawn, to a certain common trait of those minute
organisms which are mostly below the reach of unaided vision, the
foregoing conclusion appears trivial enough. But it ceases to appear
trivial on passing into a wider field, and observing the implications,
direct and indirect, as they concern plants and animals of sensible
sizes.
Popular expositions of science have so far familiarized many readers
with a certain fundamental trait of living things around, that they have
ceased to perceive how marvellous a trait it is, and, until interpreted
by the Theory of Evolution, how utterly mysterious. In past times, the
conception of an ordinary plant or animal which prevailed, not
throughout the world at large only but among the most instructed, was
that it is a single continuous entity. One of these livings things was
unhesitatingly regarded as being in all respects a unit. Parts it might
have, various in their sizes, forms, and compositions; but these were
components of a whole which had been from the beginning in its original
nature a whole. Even to naturalists fifty years ago, the assertion that
a cabbage or a cow, though in one sense a whole, is in another sense a
vast society of minute individuals, severally living in greater or less
degrees, and some of them maintaining their independent lives
unrestrained, would have seemed an absurdity. But this truth which, like
so many of the truths established by science, is contrary to that common
sense in which most people have so much confidence, has been gradually
growing clear since the days when Leeuwenhoeck and his contemporaries
began to examine through lenses the minute structures of common plants
and animals. Each improvement in the microscope, while it has widened
our knowledge of those minute forms of life described above, has
revealed further evidence of the fact that all the larger forms of life
consist of units severally allied in their fundamental traits to these
minute forms of life. Though, as formulated by Schwann and Schleiden,
the cell-doctrine has undergone qualifications of statement; yet the
qualifications have not been such as to militate against the general
proposition that organisms visible to the naked eye, are severally
compounded of invisible organisms--using that word in its most
comprehensive sense. And then, when the development of any animal is
traced, it is found that having been primarily a nucleated cell, and
having afterwards become by spontaneous fission a cluster of nucleated
cells, it goes on through successive stages to form out of such cells,
ever multiplying and modifying in various ways, the several tissues and
organs composing the adult.
On the hypothesis of evolution this universal trait has to be accepted
not as a fact that is strange but unmeaning. It has to be accepted as
evidence that all the visible forms of life have arisen by union of the
invisible forms; which, instead of flying apart when they divided,
remained together. Various intermediate stages are known. Among plants,
those of the _Volvox_ type show us the component protophytes so feebly
combined that they severally carry on their lives with no appreciable
subordination to the life of the group. And among animals, a parallel
relation between the lives of the units and the life of the group is
shown us in _Uroglena_ and _Syncrypta_. From these first stages upwards,
may be traced through successively higher types, an increasing
subordination of the units to the aggregate; though still a
subordination leaving to them conspicuous amounts of individual
activity. Joining which facts with the phenomena presented by the
cell-multiplication and aggregation of every unfolding germ, naturalists
are now accepting the conclusion that by this process of composition
from _Protozoa_, were formed all classes of the _Metazoa_[50]--(as
animals formed by this compounding are now called); and that in a
similar way from _Protophyta_, were formed all classes of what I suppose
will be called _Metaphyta_, though the word does not yet seem to have
become current.
And now what is the general meaning of these truths, taken in connexion
with the conclusion reached in the last section. It is that this
universal trait of the _Metazoa_ and _Metaphyta_, must be ascribed to
the primitive action and re-action between the organism and its medium.
The operation of those forces which produced the primary differentiation
of outer from inner in early minute masses of protoplasm, pre-determined
this universal cell-structure of all embryos, plant and animal, and the
consequent cell-composition of adult forms arising from them. How
unavoidable is this implication, will be seen on carrying further an
illustration already used--that of the shingle-covered shore, the
pebbles on which, while being in some cases selected, have been in all
cases rounded and smoothed. Suppose a bed of such shingle to be, as we
often see it, solidified, along with interfused material, into a
conglomerate. What in such case must be considered as the chief trait of
such conglomerate; or rather--what must we regard as the chief cause of
its distinctive characters? Evidently the action of the sea. Without the
breakers, no pebbles; without the pebbles, no conglomerate. Similarly
then, in the absence of that action of the medium by which was effected
the differentiation of outer from inner in those microscopic portions of
protoplasm constituting the earliest and simplest animals and plants,
there could not have existed this cardinal trait of composition which
all the higher animals and plants show us.
So that, active as has been the part played by natural selection, alike
in modifying and moulding the original units--largely as survival of
the fittest has been instrumental in furthering and controlling the
combination of these units into visible organisms, and eventually into
large ones; yet we must ascribe to the direct effect of the medium
on the first forms of life, that character of which this
everywhere-operative factor has taken advantage.
* * * * *
Let us turn now to another and more obvious attribute of higher
organisms, for which also there is this same general cause. Let us
observe how, on a higher platform, there recurs this differentiation of
outer from inner--how this primary trait in the living units with which
life commences, re-appears as a primary trait in those aggregates of
such units which constitute visible organisms.
In its simplest and most unmistakable form, we see this in the early
changes of an unfolding ovum of primitive type. The original fertilized
single cell, having by spontaneous fission multiplied into a cluster of
such cells, there begins to show itself a contrast between periphery and
centre; and presently there is formed a sphere consisting of a
superficial layer unlike its contents. The first change, then, is the
rise of a difference between that outer part which holds direct converse
with the surrounding medium, and that inclosed part which does not. This
primary differentiation in these compound embryos of higher animals,
parallels the primary differentiation undergone by the simplest living
things.
Leaving, for the present, succeeding changes of the compound embryo, the
significance of which we shall have to consider by-and-by, let us pass
now to the adult forms of visible plants and animals. In them we find
cardinal traits which, after what we have seen above, will further
impress us with the importance of the effects wrought on the organism by
its medium.
From the thallus of a sea-weed up to the leaf of a highly developed
phaenogam, we find, at all stages, a contrast between the inner and
outer parts of these flattened masses of tissue. In the higher _Algae_
"the outermost layers consist of smaller and firmer cells, while the
inner cells are often very large, and sometimes extremely long;"[51] and
in the leaves of trees the epidermal layer, besides differing in the
sizes and shapes of its component cells from the parenchyma forming the
inner substance of the leaf, is itself differentiated by having a
continuous cuticle, and by having the outer walls of its cells unlike
the inner walls.[52] Especially significant is the structure of such
intermediate types as the Liverworts. Beyond the differentiation of the
covering cells from the contained cells, and the contrast between upper
surface and under surface, the frond of _Marchantia polymorpha_ clearly
shows us the direct effect of incident forces; and shows us, too, how it
is involved with the effect of inherited proclivities. The frond grows
from a flat disc-shaped gemma, the two sides of which are alike. Either
side may fall uppermost; and then of the developing shoot, the side
exposed to the light "is under all circumstances the upper side which
forms stomata, the dark side becomes the under side which produces
root-hairs and leafy processes."[53] So that while we have undeniable
proof that the contrasted influences of the medium on the two sides,
initiate the differentiation, we have also proof that the completion of
it is determined by the transmitted structure of the type; since it is
impossible to ascribe the development of stomata to the direct action of
air and light. On turning from foliar expansions, to stems and roots,
facts of like meaning meet us. Speaking generally of epidermal tissue
and inner tissue, Sachs remarks that "the contrast of the two is the
plainer the more the part of the plant concerned is exposed to air and
light."[54] Elsewhere, in correspondence with this, it is said that in
roots the cells of the epidermis, though distinguished by bearing hairs,
"are otherwise similar to those of the fundamental tissue" which they
clothe,[55] while the cuticular covering is relatively thin; whereas in
stems the epidermis (often further differentiated) is composed of layers
of cells which are smaller and thicker-walled: a stronger contrast of
structure corresponding to a stronger contrast of conditions. By way of
meeting the suggestion that these respective differences are wholly due
to the natural selection of favourable variations, it will suffice if I
draw attention to the unlikeness between imbedded roots and exposed
roots. While in darkness, and surrounded by moist earth, the outermost
protective coats, even of large roots, are comparatively thin; but when
the accidents of growth entail permanent exposure to light and air,
roots acquire coverings allied in character to the coverings of
branches. That the action of the medium causes these and converse
changes, cannot be doubted when we find, on the one hand, that "roots
can become directly transformed into leaf-bearing shoots," and, on the
other hand, that in some plants certain "apparent roots are only
underground shoots," and that nevertheless "they are similar to true
roots in function and in the formation of tissue, but have no root-cap,
and, when they come to the light above ground, continue to grow in the
manner of ordinary leaf-shoots."[56] If, then, in highly developed
plants inheriting pronounced structures, this differentiating influence
of the medium is so marked, it must have been all-important at the
outset while types were undetermined.
As with plants so with animals, we find good reason for inferring that
while the specialities of the tegumentary parts must be ascribed to the
natural selection of favourable variations, their most general traits
are due to the direct action of surrounding agencies. Here we come upon
the border of those changes which are ascribable to use and disuse. But
from this class of changes we may fitly exclude those in which the parts
concerned are wholly or mainly passive. A corn and a blister will
conveniently serve to illustrate the way in which certain outer actions
initiate in the superficial tissues, effects of very marked kinds, which
are related neither to the needs of the organism nor to its normal
structure. They are neither adaptive changes nor changes towards
completion of the type. After noting them we may pass to allied, but
still more instructive, changes. Continuous pressure on any portion of
the surface causes absorption, while intermittent pressure causes
growth: the one impeding circulation and the passage of plasma from the
capillaries into the tissues, and the other aiding both. There are yet
further mechanically-produced effects. That the general character of the
ribbed skin on the under surfaces of the feet and insides of the hands
is directly due to friction and intermittent pressure, we have the
proofs:--first, that the tracts most exposed to rough usage are the most
ribbed; second, that the insides of hands subject to unusual amounts of
rough usage, as those of sailors, are strongly ribbed all over; and
third, that in hands which are very little used, the parts commonly
ribbed become quite smooth. These several kinds of evidence, however,
full of meaning as they are, I give simply to prepare the way for
evidence of a much more conclusive kind.
Where a wide ulcer has eaten away the deep-seated layer out of which the
epidermis grows, or where this layer has been destroyed by an extensive
burn, the process of healing is very significant. From the subjacent
tissues, which in the normal order have no concern with outward growth,
there is produced a new skin, or rather a pro-skin; for this substituted
outward-growing layer contains no hair-follicles or other specialities
of the original one. Nevertheless, it is like the original one in so far
that it is a continually renewed protective covering. Doubtless it may
be contended that this make-shift skin results from the inherited
proclivity of the type--the tendency to complete afresh the structure
of the species when injured. We cannot, however, ignore the immediate
influence of the medium, on recalling the facts above named, or on
remembering the further fact that an inflamed surface of skin, when not
sheltered from the air, will throw out a film of coagulable lymph. But
that the direct action of the medium is a chief factor we are clearly
shown by another case. Accident or disease occasionally causes permanent
eversion, or protrusion, of mucous membrane. After a period of
irritability, great at first but decreasing as the change advances, this
membrane assumes the general character of ordinary skin. Nor is this
all: its microscopic structure changes. Where it is a mucous membrane of
the kind covered by cylinder-epithelium, the cylinders gradually
shorten, becoming finally flat, and there results a squamous epithelium:
there is a near approach in minute composition to epidermis. Here a
tendency towards completion of the type cannot be alleged; for there is,
contrariwise, divergence from the type. The effect of the medium is so
great that, in a short time, it overcomes the inherited proclivity and
produces a structure of opposite kind to the normal one.
With but little break we come here upon a significant analogy, parallel
to an analogy already described. As was pointed out, an inorganic body
that is modifiable by its medium, acquires, after a time, an outer coat
which has already undergone such change as surrounding agencies can
effect; has a contained mass which is as yet unchanged, because
unreached; and has a surface between the two where change is going on--a
region of activity. And we saw that alike in the vegetal cell and the
animal cell there exist analogous distributions: of course with the
difference that the innermost part is not inert. Now we have to note
that in those aggregates of cells constituting the _Metaphyta_ and
_Metazoa_, analogous distributions also exist. In plants they are of
course not to be looked for in leaves and other deciduous portions, but
only in portions of long duration--stems and branches. Naturally, too,
we need not expect them in plants having modes of growth which early
produce an outer practically dead part, that effectually shields the
inner actively living part of the stem from the influence of the
medium--long-lived acrogens such as tree-ferns and long-lived endogens
such as palms. But in the highest plants, exogens, which have the
actively living part of their stems within reach of environing agencies,
we find this part,--the cambium layer,--is one from which there is a
growth inwards forming wood, and a growth outwards forming bark: there
is an increasingly thick covering (where it does not scale off) of
tissue changed by the medium, and inside this a film of highest
vitality. In so far as concerns the present argument, it is the same
with the _Metazoa_, or at least all of them which have developed
organizations. The outer skin grows up from a limiting plane, or layer,
a little distance below the surface--a place of predominant vital
activity. Here perpetually arise new cells, which, as they develop, are
thrust outwards and form the epidermis: flattening and drying up as they
approach the surface, whence, having for a time served to shield the
parts below, they finally scale off and leave younger ones to take their
places. This still undifferentiated tissue forming the base of the
epidermis, and existing also as a source of renewal in internal organs,
is the essentially living substance; and facts above given imply that it
was the action of the medium on this essentially living substance,
which, during early stages in the organization of the _Metazoa_,
initiated that protective envelope which presently became an inherited
structure--a structure which, though now mainly inherited, still
continues to be modifiable by its initiator.
Fully to perceive the way in which these evidences compel us to
recognize the influence of the medium as a primordial factor, we need
but conceive them as interpreted without it. Suppose, for instance, we
say that the structure of the epidermis is wholly determined by the
natural selection of favourable variations; what must be the position
taken in presence of the fact above named, that when mucous membrane is
exposed to the air its cell-structure changes into the cell-structure of
skin? The position taken must be this:--Though mucous membrane in a
highly-evolved individual organism, thus shows the powerful effect of
the medium on its surface; yet we must not suppose that the medium had
the effect of producing such a cell-structure on the surfaces of
primitive forms, undifferentiated though they were; or, if we suppose
that such an effect was produced on them, we must not suppose that it
was inheritable. Contrariwise, we must suppose that such effect of the
medium either was not wrought at all, or that it was evanescent: though
repeated through millions upon millions of generations it left no
traces. And we must conclude that this skin-structure arose only in
consequence of spontaneous variations not physically initiated (though
like those physically initiated) which natural selection laid hold of
and increased. Does any one think this a tenable position?
* * * * *
And now we approach the last and chief series of morphological phenomena
which must be ascribed to the direct action of environing matters and
forces. These are presented to us when we study the early stages in the
development of the embryos of the _Metazoa_ in general.
We will set out with the fact already noted in passing, that after
repeated spontaneous fissions have changed the original fertilized
germ-cell into that cluster of cells which forms a gemmule or a
primitive ovum, the first contrast which arises is between the
peripheral parts and the central parts. Where, as with lower creatures
which do not lay up large stores of nutriment with the germs of their
offspring, the inner mass is inconsiderable, the outer layer of cells,
which are presently made quite small by repeated subdivisions, forms a
membrane extending over the whole surface--the blastoderm. The next
stage of development, which ends in this covering layer becoming double,
is reached in two ways--by invagination and by delamination; but which
is the original way and which the abridged way, is not quite certain. Of
invagination, multitudinously exemplified in the lowest types, Mr.
Balfour says:--"On purely _a priori_ grounds there is in my opinion more
to be said for invagination than for any other view";[57] and, for
present purposes, it will suffice if we limit ourselves to this: making
its nature clear to the general reader by a simple illustration.
Take a small india-rubber ball--not of the inflated kind, nor of the
solid kind, but of the kind about an inch or so in diameter with a small
hole through which, under pressure, the air escapes. Suppose that
instead of consisting of india-rubber its wall consists of small cells
made polyhedral in form by mutual pressure, and united together. This
will represent the blastoderm. Now with the finger, thrust in one side
of the ball until it touches the other: so making a cup. This action
will stand for the process of invagination. Imagine that by continuance
of it, the hemispherical cup becomes very much deepened and the opening
narrowed, until the cup becomes a sac, of which the introverted wall is
everywhere in contact with the outer wall. This will represent the
two-layered "gastrula"--the simplest ancestral form of the _Metazoa_: a
form which is permanently represented in some of the lowest types; for
it needs but tentacles round the mouth of the sac, to produce a common
hydra. Here the fact which it chiefly concerns us to remark, is that of
these two layers the outer, called in embryological language the
epiblast, continues to carry on direct converse with the forces and
matters in the environment; while the inner, called the hypoblast, comes
in contact with such only of these matters as are put into the
food-cavity which it lines. We have further to note that in the embryos
of _Metazoa_ at all advanced in organization, there arises between these
two layers a third--the mesoblast. The origin of this is seen in types
where the developmental process is not obscured by the presence of a
large food-yolk. While the above-described introversion is taking place,
and before the inner surfaces of the resulting epiblast and hypoblast
have come into contact, cells, or amoeboid units equivalent to them,
are budded off from one or both of these inner surfaces, or some part of
one or other; and these form a layer which eventually lies between the
other two--a layer which, as this mode of formation implies, never has
any converse with the surrounding medium and its contents, or with the
nutritive bodies taken in from it. The striking facts to which this
description is a necessary introduction, may now be stated. From the
outer layer, or epiblast, are developed the permanent epidermis and its
out-growths, the nervous system, and the organs of sense. From the
introverted layer, or hypoblast, are developed the alimentary canal and
those parts of its appended organs, liver, pancreas, &c., which are
concerned in delivering their secretions into the alimentary canal, as
well as the linings of those ramifying tubes in the lungs which convey
air to the places where gaseous exchange is effected. And from the
mesoblast originate the bones, the muscles, the heart and blood-vessels,
and the lymphatics, together with such parts of various internal organs
as are most remotely concerned with the outer world. Minor
qualifications being admitted, there remain the broad general facts,
that out of that part of the external layer which remains permanently
external, are developed all the structures which carry on intercourse
with the medium and its contents, active and passive; out of the
introverted part of this external layer, are developed the structures
which carry on intercourse with the quasi-external substances that are
taken into the interior--solid food, water, and air; while out of the
mesoblast are developed structures which have never had, from first to
last, any intercourse with the environment. Let us contemplate these
general facts.
Who would have imagined that the nervous system is a modified portion of
the primitive epidermis? In the absence of proofs furnished by the
concurrent testimony of embryologists during the last thirty or forty
years, who would have believed that the brain arises from an infolded
tract of the outer skin, which, sinking down beneath the surface,
becomes imbedded in other tissues and eventually surrounded by a bony
case? Yet the human nervous system in common with the nervous systems of
lower animals is thus originated. In the words of Mr. Balfour, early
embryological changes imply that--
"the functions of the central nervous system, which were originally
taken by the whole skin, became gradually concentrated in a special
part of the skin which was step by step removed from the surface,
and has finally become in the higher types a well-defined organ
imbedded in the subdermal tissues.... The embryological evidence
shows that the ganglion-cells of the central part of the nervous
system are originally derived from the simple undifferentiated
epithelial cells of the surface of the body."[58]
Less startling perhaps, though still startling enough, is the fact that
the eye is evolved out of a portion of the skin; and that while the
crystalline lens and its surroundings thus originate, the "percipient
portions of the organs of special sense, especially of optic organs, are
often formed from the same part of the primitive epidermis" which forms
the central nervous system.[59] Similarly is it with the organs for
smelling and hearing. These, too, begin as sacs formed by infoldings of
the epidermis; and while their parts are developing they are joined from
within by nervous structures which were themselves epidermic in origin.
How are we to interpret these strange transformations? Observing, as we
pass, how absurd from the point of view of the special-creationist,
would appear such a filiation of structures, and such a round-about
mode of embryonic development, we have here to remark that the process
is not one to have been anticipated as a result of natural selection.
After numbers of spontaneous variations had occurred, as the hypothesis
implies, in useless ways, the variation which primarily initiated a
nervous centre might reasonably have been expected to occur in some
internal part where it would be fitly located. Its initiation in a
dangerous place and subsequent migration to a safe place, would be
incomprehensible. Not so if we bear in mind the cardinal truth above set
forth, that the structures for holding converse with the medium and its
contents, arise in that completely superficial part which is directly
affected by the medium and its contents; and if we draw the inference
that the external actions themselves initiate the structures. These once
commenced, and furthered by natural selection where favourable to life,
would form the first term of a series ending in developed sense organs
and a developed nervous system.[60]
Though it would enforce the argument, I must, for brevity's sake, pass
over the analogous evolution of that introverted layer, or hypoblast,
out of which the alimentary canal and attached organs arise. It will
suffice to emphasize the fact that having been originally external, this
layer continues in its developed form to have a quasi-externality, alike
in its digesting part and in its respiratory part; since it continues to
deal with matters alien to the organism. I must also refrain from
dwelling at length on the fact already adverted to, that the
intermediate derived layer, or mesoblast, which was at the outset
completely internal, originates those structures which ever remain
completely internal, and have no communication with the environment save
through the structures developed from the other two: an antithesis which
has great significance.
Here, instead of dwelling on these details, it will be better to draw
attention to the most general aspect of the facts. Whatever may be the
course of subsequent changes, the first change is the formation of a
superficial layer or blastoderm; and by whatever series of
transformations the adult structure is reached, it is from the
blastoderm that all the organs forming the adult originate. Why this
marvellous fact?
Meaning is given to it if we go back to the first stage in which
_Protozoa_, having by repeated fissions formed a cluster, then arranged
themselves into a hollow sphere, as do the protophytes forming a
_Volvox_. Originally alike all over its surface, the hollow sphere of
ciliated units thus formed, would, if not quite spherical, assume a
constant attitude when moving through the water; and hence one part of
the spheroid would more frequently than the rest come in contact with
nutritive matters to be taken in. A division of labour resulting from
such a variation being advantageous, and tending therefore to increase
in descendants, would end in a differentiation like that shown in the
gemmules of various low types of _Metazoa_, which, ovate in shape, are
ciliated over one part of the surface only. There would arise a form in
which the cilium-bearing units effected locomotion and aeration; while
on the others, assuming an amoeba-like character, devolved the
function of absorbing food: a primordial specialization variously
indicated by evidence.[61] Just noting that an ancestral origin of this
kind is implied by the fact that in low types of _Metazoa_ a hollow
sphere of cells is the form first assumed by the unfolding embryo, I
draw attention to the point here of chief interest; namely that the
primary differentiation of this hollow sphere is in such case determined
by a difference in the converse of its parts with the medium and its
contents; and that the subsequent invagination arises by a continuance
of this differential converse.
Even neglecting this first stage and commencing with the next, in which
a "gastrula" has been produced by the permanent introversion of one
portion of the surface of the hollow sphere, it will suffice if we
consider what must thereafter have happened. That which continued to be
the outer surface was the part which from time to time touched quiescent
masses and occasionally received the collisions consequent on its own
motions or the motions of other things. It was the part to receive the
sound-vibrations occasionally propagated through the water; the part to
be affected more strongly than any other by those variations in the
amounts of light caused by the passing of small bodies close to it; and
the part which met those diffused molecules constituting odours. That is
to say, from the beginning the surface was the part on which there fell
the various influences pervading the environment, the part by which
there was received those impressions from the environment serving for
the guidance of actions, and the part which had to bear the mechanical
re-actions consequent upon such actions. Necessarily, therefore, the
surface was the part in which were initiated the various
instrumentalities for carrying on intercourse with the environment. To
suppose otherwise is to suppose that such instrumentalities arose
internally where they could neither be operated on by surrounding
agencies nor operate on them,--where the differentiating forces did not
come into play, and the differentiated structures had nothing to do; and
it is to suppose that meanwhile the parts directly exposed to the
differentiating forces remained unchanged. Clearly, then, organization
could not but begin on the surface; and having thus begun, its
subsequent course could not but be determined by its superficial origin.
And hence these remarkable facts showing us that individual evolution is
accomplished by successive in-foldings and in-growings. Doubtless
natural selection soon came into action, as, for example, in the removal
of the rudimentary nervous centres from the surface; since an
individual in which they were a little more deeply seated would be less
likely to be incapacitated by injury of them. And so in multitudinous
other ways. But nevertheless, as we here see, natural selection could
operate only under subjection. It could do no more than take advantage
of those structural changes which the medium and its contents initiated.
See, then, how large has been the part played by this primordial factor.
Had it done no more than give to _Protozoa_ and _Protophyta_ that
cell-form which characterizes them--had it done no more than entail the
cellular composition which is so remarkable a trait of _Metazoa_ and
_Metaphyta_--had it done no more than cause the repetition in all
visible animals and plants of that primary differentiation of outer from
inner which it first wrought in animals and plants invisible to the
naked eye; it would have done much towards giving to organisms of all
kinds certain leading traits. But it has done more than this. By causing
the first differentiations of those clusters of units out of which
visible animals in general arose, it fixed the starting place for
organization, and therefore determined the course of organization; and,
doing this, gave indelible traits to embryonic transformations and to
adult structures.
* * * * *
Though mainly carried on after the inductive method, the argument at the
close of the foregoing section has passed into the deductive. Here let
us follow for a space the deductive method pure and simple. Doubtless in
biology _a priori_ reasoning is dangerous; but there can be no danger in
considering whether its results coincide with those reached by reasoning
_a posteriori_.
Biologists in general agree that in the present state of the world, no
such thing happens as the rise of a living creature out of non-living
matter. They do not deny, however, that at a remote period in the past,
when the temperature of the Earth's surface was much higher than at
present, and other physical conditions were unlike those we know,
inorganic matter, through successive complications, gave origin to
organic matter. So many substances once supposed to belong exclusively
to living bodies, have now been formed artificially, that men of science
scarcely question the conclusion that there are conditions under which,
by yet another step of composition, quaternary compounds of lower types
pass into those of highest types. That there once took place gradual
divergence of the organic from the inorganic, is, indeed, a necessary
implication of the hypothesis of Evolution, taken as a whole; and if we
accept it as a whole, we must put to ourselves the question--What were
the early stages of progress which followed, after the most complex form
of matter had arisen out of forms of matter a degree less complex?
At first, protoplasm could have had no proclivities to one or other
arrangement of parts; unless, indeed, a purely mechanical proclivity
towards a spherical form when suspended in a liquid. At the outset it
must have been passive. In respect of its passivity, primitive organic
matter must have been like inorganic matter. No such thing as
spontaneous variation could have occurred in it; for variation implies
some habitual course of change from which it is a divergence, and is
therefore excluded where there is no habitual course of change. In the
absence of that cyclical series of metamorphoses which even the simplest
living thing now shows us, as a result of its inherited constitution,
there could be no _point d'appui_ for natural selection. How, then, did
organic evolution begin?
If a primitive mass of organic matter was like a mass of inorganic
matter in respect of its passivity, and differed only in respect of its
greater changeableness; then we must infer that its first changes
conformed to the same general law as do the changes of an inorganic
mass. The instability of the homogeneous is a universal principle. In
all cases the homogeneous tends to pass into the heterogeneous, and the
less heterogeneous into the more heterogeneous. In the primordial units
of protoplasm, then, the step with which evolution commenced must have
been the passage from a state of complete likeness throughout the mass
to a state in which there existed some unlikeness. Further, the cause of
this step in one of these portions of organic matter, as in any portion
of inorganic matter, must have been the different exposure of its parts
to incident forces. What incident forces? Those of its medium or
environment. Which were the parts thus differently exposed? Necessarily
the outside and the inside. Inevitably, then, alike in the organic
aggregate and the inorganic aggregate (supposing it to have coherence
enough to maintain constant relative positions among its parts), the
first fall from homogeneity to heterogeneity must always have been the
differentiation of the external surface from the internal contents. No
matter whether the modification was physical or chemical, one of
composition or of decomposition, it comes within the same
generalization. The direct action of the medium was the primordial
factor of organic evolution.
* * * * *
And now, finally, let us look at the factors in their _ensemble_, and
consider the respective parts they play: observing, especially, the ways
in which, at successive stages, they severally give place one to another
in degree of importance.
Acting alone, the primordial factor must have initiated the primary
differentiation in all units of protoplasm alike. I say alike, but I
must forthwith qualify the word. For since surrounding influences,
physical and chemical, could not be absolutely the same in all places,
especially when the first rudiments of living things had spread over a
considerable area, there necessarily arose small contrasts between the
degrees and kinds of superficial differentiation effected. As soon as
these became decided, natural selection came into play; for inevitably
the unlikenesses produced among the units had effects on their lives:
there was survival of some among the modified forms rather than others.
Utterly in the dark though we are respecting the causes which set up
that process of fission everywhere occurring among the minutest forms of
life, we must infer that, when established, it furthered the spread of
those which were most favourably differentiated by the medium. Though
natural selection must have become increasingly active when once it had
got a start; yet the differentiating action of the medium never ceased
to be a co-operator in the development of these first animals and
plants. Again taking the lead as there arose the composite forms of
animals and plants, and again losing the lead with that advancing
differentiation of these higher types which gave more scope to natural
selection, it nevertheless continued, and must ever continue, to be a
cause, both direct and indirect, of modifications in structure.
Along with that remarkable process which, beginning in minute forms with
what is called conjugation, developed into sexual generation, there came
into play causes of frequent and marked fortuitous variations. The
mixtures of constitutional proclivities made more or less unlike by
unlikenesses of physical conditions, inevitably led to occasional
concurrences of forces producing deviations of structure. These were of
course mostly suppressed, but sometimes increased, by survival of the
fittest. When, along with the growing multiplication in forms of life,
conflict and competition became continually more active, fortuitous
variations of structure of no account in the converse with the medium,
became of much account in the struggle with enemies and competitors; and
natural selection of such variations became the predominant factor.
Especially throughout the plant-world its action appears to have been
immensely the most important; and throughout that large part of the
animal world characterized by relative inactivity, the survival of
individuals that had varied in favourable ways, must all along have been
the chief cause of the divergence of species and the occasional
production of higher ones.
But gradually with that increase of activity which we see on ascending
to successively higher grades of animals, and especially with that
increased complexity of life which we also see, there came more and more
into play as a factor, the inheritance of those modifications of
structure caused by modifications of function. Eventually, among
creatures of high organization, this factor became an important one; and
I think there is reason to conclude that, in the case of the highest of
creatures, civilized men, among whom the kinds of variation which affect
survival are too multitudinous to permit easy selection of any one, and
among whom survival of the fittest is greatly interfered with, it has
become the chief factor: such aid as survival of the fittest gives,
being usually limited to the preservation of those in whom the totality
of the faculties has been most favourably moulded by functional changes.
Of course this sketch of the relations among the factors must be taken
as in large measure a speculation. We are now too far removed from the
beginnings of life to obtain data for anything more than tentative
conclusions respecting its earliest stages; especially in the absence of
any clue to the mode in which multiplication, first agamogenetic and
then gamogenetic, was initiated. But it has seemed to me not amiss to
present this general conception, by way of showing how the deductive
interpretation harmonizes with the several inferences reached by
induction.
* * * * *
In his article on Evolution in the _Encyclopaedia Britannica_, Professor
Huxley writes as follows:--
"How far 'natural selection' suffices for the production of
species remains to be seen. Few can doubt that, if not the whole
cause, it is a very important factor in that operation.... On the
evidence of palaeontology, the evolution of many existing forms of
animal life from their predecessors is no longer an hypothesis, but
an historical fact; it is only the nature of the physiological
factors to which that evolution is due which is still open to
discussion."
With these passages I may fitly join a remark made in the admirable
address Prof. Huxley delivered before unveiling the statue of Mr. Darwin
in the Museum at South Kensington. Deprecating the supposition that an
authoritative sanction was given by the ceremony to the current ideas
concerning organic evolution, he said that "science commits suicide when
it adopts a creed."
Along with larger motives, one motive which has joined in prompting the
foregoing articles, has been the desire to point out that already among
biologists, the beliefs concerning the origin of species have assumed
too much the character of a creed; and that while becoming settled they
have been narrowed. So far from further broadening that broader view
which Mr. Darwin reached as he grew older, his followers appear to have
retrograded towards a more restricted view than he ever expressed. Thus
there seems occasion for recognizing the warning uttered by Prof.
Huxley, as not uncalled for.
Whatever may be thought of the arguments and conclusions set forth in
this article and the preceding one, they will perhaps serve to show that
it is as yet far too soon to close the inquiry concerning the causes of
organic evolution.