GENERAL ASPECTS OF THE EVOLUTION-HYPOTHESIS.
§ 116. Just as the supposition that races of organisms have been specially
created, is discredited by its origin; so, conversely, the supposition that
races of organisms have been evolved, is credited by its origin. Instead of
being a conception suggested and accepted when mankind were profoundly
ignorant, it is a conception born in times of comparative enlightenment.
Moreover, the belief that plants and animals have arisen in pursuance of
uniform laws, instead of through breaches of uniform laws, is a belief
which has come into existence in the most-instructed class, living in these
better-instructed times. Not among those who have disregarded the order of
Nature, has this idea made its appearance; but among those who have
familiarized themselves with the order of Nature. Thus the derivation of
this modern hypothesis is as favourable as that of the ancient hypothesis
is unfavourable.
§ 117. A kindred antithesis exists between the two families of beliefs, to
which the beliefs we are comparing severally belong. While the one family
has been dying out the other family has been multiplying. As fast as men
have ceased to regard different classes of phenomena as caused by special
personal agents, acting irregularly; so fast have they come to regard these
different classes of phenomena as caused by a general agency acting
uniformly--the two changes being correlatives. And as, on the one hand, the
hypothesis that each species resulted from a supernatural act, having lost
nearly all its kindred hypotheses, may be expected soon to die; so, on the
other hand, the hypothesis that each species resulted from the action of
natural causes, being one of an increasing family of hypotheses, may be
expected to survive.
Still greater will the probability of its survival and establishment
appear, when we observe that it is one of a particular genus of hypotheses
which has been rapidly extending. The interpretation of phenomena as
results of Evolution, has been independently showing itself in various
fields of inquiry, quite remote from one another. The supposition that the
Solar System has been evolved out of diffused matter, is a supposition
wholly astronomical in its origin and application. Geologists, without
being led thereto by astronomical considerations, have been step by step
advancing towards the conviction that the Earth has reached its present
varied structure by modification upon modification. The inquiries of
biologists have proved the falsity of the once general belief, that the
germ of each organism is a minute repetition of the mature organism,
differing from it only in bulk; and they have shown, contrariwise, that
every organism advances from simplicity to complexity through insensible
changes. Among philosophical politicians, there has been spreading the
perception that the progress of society is an evolution: the truth that
"constitutions are not made but grow," is seen to be a part of the more
general truth that societies are not made but grow. It is now universally
admitted by philologists that languages, instead of being artificially or
supernaturally formed, have been developed. And the histories of religion,
of science, of the fine arts, of the industrial arts, show that these have
passed through stages as unobtrusive as those through which the mind of a
child passes on its way to maturity. If, then, the recognition of evolution
as the law of many diverse orders of phenomena, has been spreading; may we
not say that there thence arises the probability that evolution will
presently be recognized as the law of the phenomena we are considering?
Each further advance of knowledge confirms the belief in the unity of
Nature; and the discovery that evolution has gone on, or is going on, in so
many departments of Nature, becomes a reason for believing that there is no
department of Nature in which it does not go on.
§ 118. The hypotheses of Special Creation and Evolution, are no less
contrasted in respect of their legitimacy as hypotheses. While, as we have
seen, the one belongs to that order of symbolic conceptions which are
proved to be illusive by the impossibility of realizing them in thought;
the other is one of those symbolic conceptions which are more or less fully
realizable in thought. The production of all organic forms by the
accumulation of modifications and of divergences by the continual addition
of differences to differences, is mentally representable in outline, if not
in detail. Various orders of our experiences enable us to conceive the
process. Let us look at one of the simplest.
There is no apparent similarity between a straight line and a circle. The
one is a curve; the other is defined as without curvature. The one encloses
a space; the other will not enclose a space though produced for ever. The
one is finite; the other may be infinite. Yet, opposite as the two are in
their characters, they may be connected together by a series of lines no
one of which differs from the adjacent ones in any appreciable degree.
Thus, if a cone be cut by a plane at right angles to its axis we get a
circle. If, instead of being perfectly at right angles, the plane subtends
with the axis an angle of 89° 59', we have an ellipse which no human eye,
even when aided by an accurate pair of compasses, can distinguish from a
circle. Decreasing the angle minute by minute, this closed curve becomes
perceptibly eccentric, then manifestly so, and by and by acquires so
immensely elongated a form so as to bear no recognizable resemblance to a
circle. By continuing this process the ellipse changes insensibly into a
parabola. On still further diminishing the angle, the parabola becomes an
hyperbola. And finally, if the cone be made gradually more obtuse, the
hyperbola passes into a straight line as the angle of the cone approaches
180°. Here then we have five different species of line--circle, ellipse,
parabola, hyperbola, and straight line--each having its peculiar properties
and its separate equation, and the first and last of which are quite
opposite in nature, connected together as members of one series, all
producible by a single process of insensible modification.
But the experiences which most clearly illustrate the process of general
evolution, are our experiences of special evolution, repeated in every
plant and animal. Each organism exhibits, within a short time, a series of
changes which, when supposed to occupy a period indefinitely great, and to
go on in various ways instead of one way, give us a tolerably clear
conception of organic evolution at large. In an individual development, we
see brought into a comparatively infinitesimal time, a series of
metamorphoses equally great with each of those which the hypothesis of
evolution assumes to have taken place during immeasurable geologic epochs.
A tree differs from a seed in every respect--in bulk, in structure, in
colour, in form, in chemical composition. Yet is the one changed in the
course of a few years into the other: changed so gradually, that at no
moment can it be said--Now the seed ceases to be and the tree exists. What
can be more widely contrasted than a newly-born child and the small,
semi-transparent, gelatinous spherule constituting the human ovum? The
infant is so complex in structure that a cyclopædia is needed to describe
its constituent parts. The germinal vesicle is so simple that it may be
defined in a line. Nevertheless, nine months suffice to develop the one out
of the other; and that, too, by a series of modifications so small, that
were the embryo examined at successive minutes, even a microscope would not
disclose any sensible changes. Aided by such facts, the conception of
general evolution may be rendered as definite a conception as any of our
complex conceptions can be rendered. If, instead of the successive minutes
of a child's foetal life, we take the lives of successive generations of
creatures--if we regard the successive generations as differing from one
another no more than the foetus differs in successive minutes; our
imaginations must indeed be feeble if we fail to realize in thought, the
evolution of the most complex organism out of the simplest. If a single
cell, under appropriate conditions, becomes a man in the space of a few
years; there can surely be no difficulty in understanding how, under
appropriate conditions, a cell may, in the course of untold millions of
years, give origin to the human race.
Doubtless many minds are so unfurnished with those experiences of Nature
out of which this conception is built, that they find difficulty in forming
it. Looking at things rather in their statical than in their dynamical
aspects, they never realize the fact that, by small increments of
modification, any amount of modification may in time be generated. The
surprise they feel on finding one whom they last saw as a boy, grown into a
man, becomes incredulity when the degree of change is greater. To such, the
hypothesis that by any series of changes a protozoon can give origin to a
mammal, seems grotesque--as grotesque as Galileo's assertion of the Earth's
movement seemed to his persecutors; or as grotesque as the assertion of the
Earth's sphericity seems now to the New Zealanders. But those who accept a
literally-unthinkable proposition as quite satisfactory, may not
unnaturally be expected to make a converse mistake.
§ 119. The hypothesis of evolution is contrasted with the hypothesis of
special creations, in a further respect. It is not simply legitimate
instead of illegitimate, because representable in thought instead of
unrepresentable; but it has the support of some evidence, instead of being
absolutely unsupported by evidence. Though the facts at present assignable
in _direct_ proof that by progressive modifications, races of organisms
which are apparently distinct from antecedent races have descended from
them, are not sufficient; yet there are numerous facts of the order
required. Beyond all question unlikenesses of structure gradually arise
among the members of successive generations. We find that there is going on
a modifying process of the kind alleged as the source of specific
differences: a process which, though slow, does, in time, produce
conspicuous changes--a process which, to all appearance, would produce in
millions of years, any amount of change.
In the chapters on "Heredity" and "Variation," contained in the preceding
Part, many such facts were given, and more might be added. Although little
attention has been paid to the matter until recent times, the evidence
already collected shows that there take place in successive generations,
alterations of structure quite as marked as those which, in successive
short intervals, arise in a developing embryo--nay, often much more marked;
since, besides differences due to changes in the relative sizes or parts,
there sometimes arise differences due to additions and suppressions of
parts. The structural modification proved to have taken place since
organisms have been observed, is not less than the hypothesis
demands--bears as great a ratio to this brief period, as the total amount
of structural change seen in the evolution of a complex organism out of a
simple germ, bears to that vast period during which living forms have
existed on the Earth.
We have, indeed, much the same kind and quantity of direct evidence that
all organic beings have arisen through the actions of natural causes, which
we have that all the structural complexities of the Earth's crust have
arisen through the actions of natural causes. Between the known
modifications undergone by organisms, and the totality of modifications
displayed in their structures, there is no greater disproportion than
between the observed geological changes, and the totality of geological
changes supposed to have been similarly caused. Here and there are
sedimentary deposits now slowly taking place. At this place a shore has
been greatly encroached on by the sea during recorded times; and at another
place an estuary has become shallower within some generations. In one
region an upheaval is going on at the rate of a few feet in a century;
while in another region occasional earthquakes cause slight variations of
level. Appreciable amounts of denudation by water are visible in some
localities; and in other localities glaciers are detected in the act of
grinding down the rocky surfaces over which they glide. But these changes
are infinitesimal compared with the aggregate of changes to which the
Earth's crust testifies, even in its still extant systems of strata. If,
then, the small changes now being wrought on the Earth's crust by natural
agencies, yield warrant for concluding that by such agencies acting through
vast epochs, all the structural complexities of the Earth's crust have been
produced; do not the small known modifications produced in races of
organisms by natural agencies, yield warrant for concluding that by natural
agencies have been produced all those structural complexities which we see
in them?
The hypothesis of Evolution then, has direct support from facts which,
though small in amount, are of the kind required; and the ratio which these
facts bear to the generalization based on them, seems as great as is the
ratio between facts and generalization which, in another case, produces
conviction.
§ 120. Let us put ourselves for a moment in the position of those who, from
their experiences of human modes of action, draw differences respecting the
mode of action of that Ultimate Power manifested to us through phenomena.
We shall find the supposition that each kind of organism was separately
designed and put together, to be much less consistent with their professed
conception of this Ultimate Power, than is the supposition that all kinds
of organisms have resulted from one unbroken process. Irregularity of
method is a mark of weakness. Uniformity of method is a mark of strength.
Continual interposition to alter a pre-arranged set of actions, implies
defective arrangement in those actions. The maintenance of those actions,
and the working out by them of the highest results, implies completeness of
arrangement. If human workmen, whose machines as at first constructed
require perpetual adjustment, show their increasing skill by making their
machines self-adjusting; then, those who figure to themselves the
production of the world and its inhabitants by a "Great Artificer," must
admit that the achievement of this end by a persistent process, adapted to
all contingencies, implies greater skill than its achievement by the
process of meeting the contingencies as they severally arise.
So, too, it is with the contrast under its moral aspect. We saw that to the
hypothesis of special creations, a difficulty is presented by the absence
of high forms of life during immeasurable epochs of the Earth's existence.
But to the hypothesis of evolution, absence of them is no such obstacle.
Suppose evolution, and this question is necessarily excluded. Suppose
special creations, and this question can have no satisfactory answer. Still
more marked is the contrast between the two hypotheses, in presence of that
vast amount of suffering entailed on all orders of sentient beings by their
imperfect adaptations to their conditions of life, and the further vast
amount of suffering entailed on them by enemies and by parasites. We saw
that if organisms were severally designed for their respective places in
Nature, the inevitable conclusion is that these innumerable kinds of
inferior organisms which prey on superior organisms, were intended to
inflict all the pain and mortality which results. But the hypothesis of
evolution involves us in no such dilemma. Slowly, but surely, evolution
brings about an increasing amount of happiness. In all forms of
organization there is a progressive adaptation, and a survival of the most
adapted. If, in the uniform working out of the process, there are evolved
organisms of low types which prey on those of higher types, the evils
inflicted form but a deduction from the average benefits. The universal
multiplication of the most adapted must cause the spread of those superior
organisms which, in one way or other, escape the invasions of the inferior;
and so tends to produce a type less liable to the invasions of the
inferior. Thus the evils accompanying evolution are ever being
self-eliminated. Though there may arise the question--Why could they not
have been avoided? there does not arise the question--Why were they
deliberately inflicted? Whatever may be thought of them, it is clear that
they do not imply gratuitous malevolence.
§ 121. In all respects, then, the hypothesis of evolution contrasts
favourably with the hypothesis of special creation. It has arisen in
comparatively-instructed times and in the most cultivated class. It is one
of those beliefs in the uniform concurrence of phenomena, which are
gradually supplanting beliefs in their irregular and arbitrary concurrence;
and it belongs to a genus of these beliefs which has of late been rapidly
spreading. It is a definitely-conceivable hypothesis; being simply an
extension to the organic world at large, of a conception framed from our
experiences of individual organisms; just as the hypothesis of universal
gravitation was an extension of the conception which our experiences of
terrestrial gravitation had produced. This definitely-conceivable
hypothesis, besides the support of numerous analogies, has the support of
direct evidence. We have proof that there is going on a process of the kind
alleged; and though the results of this process, as actually witnessed, are
minute in comparison with the totality of results ascribed to it, yet they
bear to such totality a ratio as great as that by which an analogous
hypothesis is justified. Lastly, that sentiment which the doctrine of
special creations is thought necessary to satisfy, is much better satisfied
by the doctrine of evolution; since this doctrine raises no contradictory
implications respecting the Unknown Cause, such as are raised by the
antagonist doctrine.
And now, having observed how, under its most general aspects, the
hypothesis of organic evolution commends itself to us by its derivation, by
its coherence, by its analogies, by its direct evidence, by its
implications; let us go on to consider the several orders of facts which
yield indirect support to it. We will begin by noting the harmonies between
it and sundry of the inductions set forth in Part II.