ISOLATION (_continued_).
I will now recapitulate the main doctrines which have been set forth in
the foregoing chapter, and then proceed to consider the objections which
have been advanced against them.
It must be remembered that by isolation I mean exactly what Mr. Gulick
does by "Segregation," and approximately what Professor Weismann does by
"Amixia "--i. e. the prevention of intercrossing.
Isolation occurs in very many forms besides the geographical, as will be
more fully shown at the end of this chapter; and in all its forms it
admits of degrees.
It also occurs in two very different species or kinds--namely,
discriminate and indiscriminate. These I have called respectively
Homogamy and Apogamy. This all-important distinction has been clearly
recognized by Mr. Gulick, as a result of his own thought and
observation, independently of anything that I have published upon the
subject.
In view of this distinction Isolation takes rank with Heredity and
Variability as one of the most fundamental principles of organic
evolution. For, if these other two principles be granted, the whole
theory of descent resolves itself into an inquiry touching the causes,
forms, and degrees of Homogamy.
Save in cases where very large populations are concerned, apogamy must
sooner or later give rise _per se_ to homogamy, owing to the Law of
Delboeuf, which is the principle that I have called Independent
Variability, and Gulick has called Independent Generation. But of course
this does not hinder that under apogamy various other causes of homogamy
are likely to arise--in particular natural selection.
That natural selection differs from most of the other forms of isolation
in not being capable of causing _divergent_ or _polytypic_ evolution
must at once become evident, if we remember that the only way in which
isolation of any form can cause such evolution is by partitioning a
given group of intergenerants into two or more groups, each of which is
able to survive as thus separated from the other, and so to carry on the
evolution in divergent lines. But the distinguishing peculiarity of
natural selection, considered as a form of isolation, is that it effects
the isolation _by killing off all the individuals which it fails to
isolate_: consequently, this form of isolation differs from other forms
in prohibiting the possibility of any ramification of a single group of
intergenerants into two or more groups, for the purpose of carrying on
the evolution in divergent lines. Therefore, under this form of
isolation alone, evolution must proceed, palm-like, in a single line of
growth. So to speak, the successive generations continuously ascend to
higher things on the steps supplied by their own "dead selves"; but in
doing so they must climb a single ladder, no rung of which can be
allowed to bifurcate in the presence of the uniformity secured _for that
generation_ by the free intercrossing of the most fit. Even though
beneficial variations may arise in two or more directions
simultaneously, and all be simultaneously selected by survival of the
fittest, the effect of free intercrossing (in the absence of any other
form of isolation) will be to fuse all these beneficial variations into
one common type, and so to end in _monotypic_ evolution as before. In
order to secure _polytypic_ evolution, intercrossing between the
different beneficial variants which may arise must be prevented; and
there is nothing to prevent such intercrossing in the process of natural
selection _per se_. In order that the original group of intergenerants
should be divided and sub-divided into two or more groups of
intergenerants, some additional form of isolation must necessarily
supervene--when, of course, polytypic evolution will result. And, as Mr.
Gulick has shown, the conclusion thus established by deductive reasoning
is verified inductively by the facts of geographical distribution.
How, then, are we to account for the fact that Darwin attributed to
natural selection the power to cause divergence of character? The answer
is sufficiently simple. _He does so by tacitly invoking the aid of some
other form of homogamy in every case._ If we carefully read pp. 86-97 of
the _Origin of Species_, where this subject is under consideration, we
shall find that in every one of the arguments and illustrations which
are adduced to prove the power of natural selection to effect
"divergence of character," he either pre-supposes or actually names some
other form of homogamy as the originating cause of the diversity that
is afterwards presented to natural selection for further
intensification. To give only one example. At the starting-point of the
whole discussion the priority of such other forms of homogamy is assumed
in the following words:--
But how, it may be asked, can any analogous principle [to that of
diversity caused by artificial selection] apply in nature? I
believe it can and does apply most efficiently (though it was a
long time before I saw how), from the simple circumstance that the
more diversified the descendants from any one species become in
structure, constitution, and habits, by so much will they be better
enabled to seize on many and widely diversified places in the
polity of nature, and so be enabled to increase in numbers.
Now, without question, so soon as segregate breeding in two or more
lines of homogamy has been in any sufficient degree determined by some
"change of structure, constitution, or habits," natural selection will
forthwith proceed to increase the divergence in as many different lines
as there are thus yielded discriminately isolated sections of the
species. And this fact it must have been that Darwin really had before
his mind when he argued that diversification of character is caused by
natural selection, through the benefit gained by the diversified forms
being thus "enabled to increase in number." Nevertheless he does not
expressly state the essential point, that although diversification of
character, _when once begun_, is thus _promoted_ by natural selection,
which forthwith proceeds to cultivate each of the resulting branches,
yet diversification of character can never be _originated_ by natural
selection. The change of "structure," of "constitution," of "habits," of
"station," of geographical area, of reciprocal fertility, and so
on--this change, _whatever_ it may have been, must clearly have been
antecedent to any operation of natural selection through the benefit
which arose from the change. Therefore the change must in all cases have
been due, in the first instance, to some other form of isolation than
the superadded form which afterwards arose from superior fitness in the
possession of superior benefit--although, so long as the prior form of
isolation endured, or continued to furnish the necessary condition to
the co-operation of survival of the fittest, survival of the fittest
would have continued to increase the divergence of character in as many
ramifying lines as there were thus given to its action separate cases of
isolation by other means.
In short, as divergence of character must in all cases be due to a
prevention of intercrossing, and as in the process of natural selection
there is, _ex hypothesi_, nothing to prevent the intercrossing until the
divergence has already arisen, to suppose that natural selection alone
can have caused the divergence, is to suppose that natural selection can
have caused the conditions of its own activity, which is absurd.
Seeing, then, that even in cases where any "benefit" arises from
divergence of character, such benefit can arise only after the
divergence has already commenced, and seeing that on this as on other
accounts previously mentioned it is plainly impossible to attribute the
origin of such divergence to natural selection, we find that natural
selection must be in all cases assisted by some other form of isolation,
if it is to be concerned in polytypic as distinguished from monotypic
evolution. But this does not hinder that, when it is so assisted,
natural selection may become--and, I believe, does become--the most
efficient of all the forms of isolation in promoting divergence of
character. For, in the first place, of all the forms of isolation
natural selection is probably the most energetic in promoting monotypic
evolution; so that under the influence of such isolation monotypic
evolution probably advances more rapidly than it does under any other
form of isolation. In the second place, when polytypic evolution has
been begun by any of these other forms of isolation, and natural
selection then sets to work on each of the resulting branches, although
natural selection is thus engaged in as many different acts of monotypic
evolution as there are thus separate cases supplied to it by these other
forms of isolation, the joint result of all these different acts is to
hurry on the polytypic evolution which was originally started by the
other forms of isolation. So to speak, natural selection is the forcing
heat, acting simultaneously on each of the separate branches which has
been induced to sprout by other means; and in thus rapidly advancing the
growth of all the branches, it is still entitled to be regarded as the
most important _single_ cause of diversification in organic nature,
although we must henceforth cease to regard it as in any instance the
_originating_ cause--or even so much as the _sustaining_ cause.
So much by way of summary and recapitulation. I will now briefly
consider the only objections which, so far as I can see, admit of being
brought against the foregoing doctrine of Isolation as held by Mr.
Gulick and myself. These possible objections are but two in
number--although but one of them has been hitherto adduced. This,
therefore, I will take first.
Mr. Wallace, with his customary desire to show that natural selection is
everywhere of itself capable of causing organic evolution, seeks to
minimize the swamping effects of free intercrossing, and the consequent
importance of other forms of isolation. His argument is as follows.
Alluding to the researches of Mr. J. A. Allen, and others, on the amount
of variation presented by individuals of a species in a state of nature,
Mr. Wallace shows that, as regards any given part of the animal under
consideration, there is always to be found a considerable range of
individual variation round the average mean which goes to constitute the
specific character of the type. Thus, for example, Mr. Allen says of
American birds, "that a variation of from fifteen to twenty per cent. in
general size, and an equal degree of variation in the relative size of
different parts, may be ordinarily expected among specimens from the
same species and sex, taken at the same locality, while in some cases
the variation is even greater than this." Now, Mr. Wallace is under the
impression that these facts obviate the difficulty which arises from the
presence of free intercrossing--the difficulty, that is, against the
theory of natural selection when natural selection is supposed to have
been the exclusive means of modification. For, as he says, "if less size
of body would be beneficial, then, as half the variations in size are
above and half below the mean or existing standard of the species, there
would be ample beneficial variations"; and similarly with regard to
longer or shorter legs, wings, tails, &c., darker or lighter colour, and
so on through all the parts of any given organism.
Well, although I have no wish at all to disparage the biological value
of these actual measurements of the range of individual variation, I
must point out that they are without any value at all in the connexion
which Mr. Wallace adduces them. We did not require these measurements to
tell us the broad and patent fact that "no being on this earthly ball is
like another all in all"--or, in less Tennysonian words, that as regards
every specific structure there is a certain amount of individual
variability round an average mean. Indeed, in my own paper on
_Physiological Selection_--against which Mr. Wallace is here specially
arguing--I expressly said, as previously remarked, "that a specific type
may be regarded as _the average mean of all individual variations_." The
fact of such individual variability round a specific mean has always
been well known to anatomists; it constitutes one of the basal pillars
of the whole Darwinian theory; and is besides a matter of universal
recognition as regards human stature, features, and so forth. The value
of Mr. Allen's work consists in accurately measuring the _amount_ or
_range_ of individual variation; but the question of its amount or range
is without relevancy in the present connexion. For the desirability of
isolation as an aid to natural selection even where monotypic evolution
is concerned, does not arise with any reference to the amount or range
of variation: it arises with reference to the _number_ of variations
which are--or are not--_similar_ and _simultaneous_. If there be a
sufficient number which are both similar and simultaneous, the
desirability of any co-operating form of isolation is correspondingly
removed, because natural selection may then have sufficient material
wherewith to overcome the adverse influence of free intercrossing, and
so of itself to produce monotypic evolution. Now, variations may be
numerous, similar, and simultaneous, either on account of some common
cause acting on many individuals at the same time, or on account of the
structures in question being more or less variable round a specific
mean. In the latter case--which is the only case that Mr. Allen's
measurements have to do with--the law of averages will of course
determine that half the whole number of variations in any given
structure, in any given generation, will be above the mean line. But,
equally of course, no one has ever denied that where, for either of
these reasons, natural selection is provided with sufficient material,
it is correspondingly capable of improving the specific type without the
assistance of any other form of homogamy; so to speak, they protect
themselves by their very numbers, and their superiority over others
leads to their survival and accumulation. But what is the result? _The
result can only be monotypic evolution._ No matter how great the number,
or how great the range, of variations round an average specific mean,
out of such material natural selection can never produce _polytypic_
evolution: it may _change_ the type to any extent during successive
generations, and in a single line of change; but it cannot _branch_ the
type, unless some other form of homogamy intervenes. Therefore, when Mr.
Wallace adduces the well-known fact that all structures vary more or
less round a specific mean as proof that natural selection need not be
incommoded by free intercrossing, but can of itself produce all the
known phenomena of specific evolution, he fails to perceive that his
argument refers only to one aspect of such evolution (viz. the
transformation of species in time), and does not apply to the aspect
with which alone my paper on _Physiological Selection_ was concerned
(viz. the multiplication of species in space).
The same thing may be shown in this way. It is perfectly obvious that
where the improvement of type in a linear series is concerned (monotypic
evolution), free intercrossing, far from being a hindrance to the
process, _is the very means by which the process is accomplished_.
Improvement here ascends by successive steps, in successive generations,
simply _because_ of the general intercrossing of the generally most fit
with the result that the species, _as a whole_, gradually becomes
transformed into another species, _as a whole_. Therefore, it would be
mere fatuity in any one to adduce free intercrossing as a "difficulty"
against natural selection alone being competent to produce evolution of
this kind. But where the kind of evolution is that whereby the species
is _differentiated_--where it is required, for instance, to produce
different structures in different portions of the species, such as the
commencement of a fighting spur on the wing of a duck, or _novel_
characters of any sort in different groups of the species--free
intercrossing is no longer a condition to, but an absolute preventive
of, the process; and, therefore, unless checked as between each portion
of the species by some form of homogamy other than natural selection,
it must effectually inhibit any _segregation_ of specific types, or
divergence of character.
Hence it is that, while no Darwinian has ever questioned the power of
unaided selection to cause _improvement of character in successive
generations_, in common now with not a few other Darwinians I have
emphatically denied so much as the abstract possibility of selection
alone causing a _divergence of character in two or more simultaneous
lines of change_.
And, although these opposite views cannot be reconciled, I am under the
impression that they do admit of being explained. For I take them to
indicate a continued failure to perceive the all-important distinction
between evolution as monotypic and polytypic. Unless one has fully
grasped this distinction, and constantly holds it in mind, he is not in
a position to understand the "difficulty" in question; nor can he avoid
playing fast and loose with natural selection as possibly the sole cause
of evolution, and as necessarily requiring the co-operation of some
other cause. But if he once clearly perceives that "evolution" is a
logical genus, of which the monotypic and the polytypic forms are
species, he will immediately escape from his confusion, and find that
while the monotypic form may be caused by natural selection alone the
polytypic form can never be so caused.
* * * * *
The second difficulty which I have to mention as at first sight
attaching to the views of Mr. Gulick and myself on the subject of
Isolation is, that in an isolated section of a species Mr. Francis
Galton's law of regression in the average character of offspring to the
typical character of the group through reversion or atavism (_Natural
Inheritance_, p. 97) must have the effect of neutralizing the
segregative influence of mere apogamy. That such, however, cannot be the
case has been well shown by Mr. Gulick in his paper on _Intensive
Segregation_. Without at all disputing the validity of Mr. Galton's law,
he proves that "it can hold in full force only where there is free
crossing, otherwise no divergent race could ever be formed by any amount
of selection and independent breeding[14]." This is so self-evident that
I need not quote his demonstration of the point.
[14] _Zool. Journal Lin. Soc._, vol. xxiii. p. 313.
* * * * *
In conclusion, then, and having regard to the principle of isolation as
a whole, or in all the many and varied forms in which this principle
obtains, I trust that I have redeemed the promise with which I set
out--viz. to show that in relation to the theory of descent this
principle is of an importance second to no other, not even excepting
heredity, variability, and the struggle for existence. This has now been
fully shown, inasmuch as we have clearly seen that the importance of the
struggle for existence, and consequent survival of the fittest, arises
just because survival of the fittest is a form, and a very stringent
form, of isolation; while, as regards both heredity and variability, we
are now in a position to see that the more fully we recognize their
supreme importance as principles concerned in organic evolution, the
more must we also recognize that any rational theory of such evolution
becomes, in the last resort, a theory of the different modes in which
efficient isolation can be secured. For, in whatever degree the process
of organic evolution has been dependent upon heredity with variability,
in that degree must it also have been dependent upon the means of
securing homogamy, whereby alone the force of heredity can be made to
expend itself in the innumerable directions of progressive change,
instead of continually neutralizing the force of variability by
promiscuous intercrossing.