MAIN POINTS OF AGREEMENT AND OF DIFFERENCE BETWEEN THE OLD AND NEW
THEORIES OF EVOLUTION.
Having put before the reader with some fulness the theories of the three
writers to whom we owe the older or teleological view of evolution, I
will now compare that view more closely with the theory of Mr. Darwin
and Mr. Wallace, to whom, in spite of my profound difference of opinion
with them on the subject of natural selection, I admit with pleasure
that I am under deep obligation. For the sake of brevity, I shall take
Lamarck as the exponent of the older view, and Mr. Darwin as that of the
one now generally accepted.
We have seen, that up to a certain point there is very little difference
between Lamarck and Mr. Darwin. Lamarck maintains that animals and
plants vary: so does Mr. Darwin. Lamarck maintains that variations
having once arisen have a tendency to be transmitted to offspring and
accumulated: so does Mr. Darwin. Lamarck maintains that the accumulation
of variations, so small, each one of them, that it cannot be, or is not
noticed, nevertheless will lead in the course of that almost infinite
time during which life has existed upon earth, to very wide differences
in form, structure, and instincts: so does Mr. Darwin. Finally, Lamarck
declares that all, or nearly all, the differences which we observe
between various kinds of animals and plants are due to this exceedingly
gradual and imperceptible accumulation, during many successive
generations, of variations each one of which was in the outset small: so
does Mr. Darwin. But in the above we have a complete statement of the
fact of evolution, or descent with modification--wanting nothing, but
entire, and incapable of being added to except in detail, and by way of
explanation of the causes which have brought the fact about. As regards
the general conclusion arrived at, therefore, I am unable to detect any
difference of opinion between Lamarck and Mr. Darwin. They are both bent
on establishing the theory of evolution in its widest extent.
The late Sir Charles Lyell, in his 'Principles of Geology,' bears me out
here. In a note to his _resume_ of the part of the 'Philosophie
Zoologique' which bears upon evolution, he writes:--
"I have reprinted in this chapter word for word my abstract of Lamarck's
doctrine of transmutation, as drawn up by me in 1832 in the first
edition of the 'Principles of Geology.'[333] I have thought it right to
do this in justice to Lamarck, in order to show how nearly the opinions
taught by him at the commencement of this century resembled those now in
vogue amongst a large body of naturalists respecting the infinite
variability of species, and the progressive development in past time of
the organic world. The reader must bear in mind that when I made this
analysis of the 'Philosophie Zoologique' in 1832, I was altogether
opposed to the doctrine that the animals and plants now living were the
lineal descendants of distinct species, only known to us in a fossil
state, and ... so far from exaggerating, I did not do justice to the
arguments originally adduced by Lamarck and Geoffroy St. Hilaire,
especially those founded on the occurrence of rudimentary organs. There
is therefore no room for suspicion that my account of the Lamarckian
hypothesis, written by me thirty-five years ago, derived any colouring
from my own views tending to bring it more into harmony with the theory
since propounded by Darwin."[334] So little difference did Sir Charles
Lyell discover between the views of Lamarck and those of his successors.
With the identity, however, of the main proposition which, both Lamarck
and Mr. Darwin alike endeavour to establish, the points of agreement
between the two writers come to an end. Lamarck's great aim was to
discover the cause of those variations whose accumulation results in
specific, and finally in generic, differences. Not content with
establishing the fact of descent with modification, he, like his
predecessors, wishes to explain how it was that the fact came about. He
finds its explanation in changed surroundings--that is to say, in
changed conditions of existence--as the indirect cause, and in the
varying needs arising from these changed conditions as the direct cause.
According to Lamarck, there is a broad principle which underlies
variation generally, and this principle is the power which all living
beings possess of slightly varying their actions in accordance with
varying needs, coupled with the fact observable throughout nature that
use develops, and disuse enfeebles an organ, and that the effects,
whether of use or disuse, become hereditary after many generations.
This resolves itself into the effect of the mutual interaction of mind
on body and of body on mind. Thus he writes:--
"The physical and the mental are to start with undoubtedly one and the
same thing; this fact is most easily made apparent through study of the
organization of the various orders of known animals. From the common
source there proceeded certain effects, and these effects, in the outset
hardly separated, have in the course of time become so perfectly
distinct, that when looked at in their extremest development they appear
to have little or nothing in common.
"The effect of the body upon the mind has been already sufficiently
recognized; not so that of the mind upon the body itself. The two, one
in the outset though they were, interact upon each other more and more
the more they present the appearance of having become widely sundered,
and it can be shown that each is continually modifying the other and
causing it to vary."[335]
And again, later:--
"I shall show that the habits by which we now recognize any creature
are due to the environment (_circonstances_) under which it has for a
long while existed, _and that these habits have had such an influence
upon the structure of each individual of the species as to have at
length_" (that is to say, through many successive slight variations,
each due to habit engendered by the wishes of the animal itself),
"modified this structure and adapted it to the habits contracted."[336]
These quotations must suffice, for the reader has already had Lamarck's
argument sufficiently put before him.
Variation, and consequently modification, are, according to Lamarck, the
outward and visible signs of the impressions made upon animals and
plants in the course of their long and varied history, each organ
chronicling a time during which such and such thoughts and actions
dominated the creature, and specific changes being the effect of certain
long-continued wishes upon the body, and of certain changed surroundings
upon the wishes. Plants and animals are living forms of faith, or faiths
of form, whichever the reader pleases.
Mr. Darwin, on the other hand, repeatedly avows ignorance, and profound
ignorance, concerning the causes of those variations which, or nothing,
must be the fountain-heads of species. Thus he writes of "the complex
and _little known_ laws of variation."[337] "There is also _some
probability_ in the view propounded by Andrew Knight, that variability
_may be partly_ connected with excess of food."[338] "Many laws regulate
variation, _some few of which_ can be _dimly seen_."[339] "The results
of the _unknown_, or _but dimly understood_, laws of variation are
infinitely complex and diversified."[340] "We are _profoundly ignorant_
of the cause of each slight variation or individual difference."[341]
"We are _far too ignorant_ to speculate on the relative importance of
the several known and unknown causes of variation."[342] He admits,
indeed, the effects of use and disuse to have been important, but how
important we have no means of knowing; he also attributes considerable
effect to the action of changed conditions of life--but how considerable
again we know not; nevertheless, he sees no great principle underlying
the variations generally, and tending to make them appear for a length
of time together in any definite direction advantageous to the creature
itself, but either expressly, as at times, or by implication, as
throughout his works, ascribes them to accident or chance.
In other words, he admits his ignorance concerning them, and dwells only
on the accumulation of variations the appearance of which for any length
of time in any given direction he leaves unaccounted for.
Lamarck, again, having established his principle that sense of need is
the main direct cause of variation, and having also established that the
variations thus engendered are inherited, so that divergences accumulate
and result in species and genera, is comparatively indifferent to
further details. His work is avowedly an outline. Nevertheless, we have
seen that he was quite alive to the effects of the geometrical ratio of
increase, and of the struggle for existence which thence inevitably
follows.
Mr. Darwin, on the other hand, comparatively indifferent to, or at any
rate silent concerning the causes of those variations which appeared so
all-important to Lamarck, inasmuch as they are the raindrops which unite
to form the full stream of modification, goes into very full detail upon
natural selection, or the survival of the fittest, and maintains it to
have been "the most important but not the exclusive means of
modification."[343]
It will be readily seen that, according to Lamarck, the variations which
when accumulated amount to specific and generic differences, will have
been due to causes which have been mainly of the same kind for long
periods together. Conditions of life change for the most part slowly,
steadily, and in a set direction; as in the direction of steady, gradual
increase or decrease of cold or moisture; of the steady, gradual
increase of such and such an enemy, or decrease of such and such a kind
of food; of the gradual upheaval or submergence of such and such a
continent, and consequent drying up or encroachment of such and such a
sea, and so forth. The thoughts of the creature varying will thus have
been turned mainly in one direction for long together; and hence the
consequent modifications will also be mainly in fixed and definite
directions for many successive generations; as in the direction of a
warmer or cooler covering; of a better means of defence or of attack in
relation to such and such another species; of a longer neck and longer
legs, or of whatever other modification the gradually changing
circumstances may be rendering expedient. It is easy to understand the
accumulation of slight successive modifications which thus make their
appearance in given organs and in a set direction.
With Mr. Darwin, on the contrary, the variations being accidental, and
due to no special and uniform cause, will not appear for any length of
time in any given direction, nor in any given organ, but will be just as
liable to appear in one organ as in another, and may be in one
generation in one direction, and in another in another.
In confirmation of the above, and in illustration of the important
consequences that will follow according as we adopt the old or the more
recent theory, I would quote the following from Mr. Mivart's 'Genesis of
Species.'
Shortly before maintaining that two similar structures have often been
developed independently of one another, Mr. Mivart points out that if we
are dependent upon indefinite variations only, as provided for us by Mr.
Darwin, this would be "so improbable as to be practically
impossible."[344] The number of possible variations being indefinitely
great, "it is therefore an indefinitely great number to one against a
similar series of variations occurring and being similarly preserved in
any two independent instances." It will be felt (as Mr. Mivart presently
insists) that this objection does not apply to a system which maintains
that in case an animal feels any given want it will gradually develop
the structure which shall meet the want--that is to say, if the want be
not so great and so sudden as to extinguish the creature to which it has
become a necessity. For if there be such a power of self-adaptation as
thus supposed, two or more very widely different animals feeling the
same kind of want might easily adopt similar means to gratify it, and
hence develop eventually a substantially similar structure; just as two
men, without any kind of concert, have often hit upon like means of
compassing the same ends. Mr. Spencer's theory--so Mr. Mivart tells
us--and certainly that of Lamarck, whose disciple Mr. Spencer would
appear to be,[345] admits "a certain peculiar, but limited power of
response and adaptation in each animal and plant"--to the conditions of
their existence. "Such theories," says Mr. Mivart, "have not to contend
against the difficulty proposed, and it has been urged that even very
complex extremely similar structures have again and again been developed
quite independently one of the other, and this because the process has
taken place not by merely haphazard, indefinite variations in all
directions, but by the concurrence of some other internal natural law or
laws co-operating with external influences and with Natural Selection in
the evolution of organic forms.
"_It must never be forgotten that to admit any such constant operation
of any such unknown natural cause is to deny the purely Darwinian theory
which relies upon the survival of the fittest by means of minute
fortuitous indefinite variations._
"Among many other obligations which the author has to acknowledge to
Professor Huxley, are the pointing out of this very difficulty, and the
calling his attention to the striking resemblance between certain teeth
of the dog, and of the thylacine, as one instance, and certain ornithic
peculiarities of pterodactyles as another."[346]
In brief then, changed distribution of use and disuse in consequence of
changed conditions of the environment is with Lamarck the main cause of
modification. According to Mr. Darwin natural selection, or the survival
of favourable but accidental variations, is the most important means of
modification. In a word, with Lamarck the variations are definite; with
Mr. Darwin indefinite.
FOOTNOTES:
[333] Vol. ii. chap. i.
[334] Vol. ii. chap, xxxiv., ed. 1872.
[335] 'Philosophie Zoologique,' ed. M. Martins, Paris, Lyons, 1873, tom.
i. p. 24.
[336] 'Philosophie Zoologique,' tom. i. p. 72.
[337] 'Origin of Species,' p. 3.
[338] Ibid. p. 5.
[339] 'Origin of Species,' p. 8.
[340] Ibid. p. 9.
[341] Ibid. p. 158.
[342] Ibid. p. 159.
[343] 'Origin of Species,' p. 4.
[344] 'Genesis of Species,' p. 74, 1871.
[345] See _ante_, p. 330, line 1 after heading.
[346] 'Genesis of Species,' p. 76, ed. 1871.