FURTHER EVIDENCES OF PHYSIOLOGICAL SELECTION.
_Evidence from Topographical Distribution of Varieties._
In the last section we have considered the topographical distribution of
closely allied _species_. I now propose to go still further into matters
of detail, by considering the case of natural _varieties_. And here we
come upon a branch of our inquiry where we may well expect to meet with
the most crucial tests of our theory. For if it should appear that these
nascent species more or less resemble fully developed species in
presenting the feature of cross-infertility, the theory would be
verified in the most direct and conclusive manner possible. These
nascent species may be called embryo species, which are actually in
course of differentiation from their parent-type; and therefore, if they
do not exhibit the feature in relation to that type which the present
theory infers to be necessary for the purposes of differentiation, the
theory must be abandoned. On the other hand, if they do exhibit this
feature, it is just the feature which the theory predicted as one that
would be found highly characteristic of such embryo types.
Contrariwise, the theory of natural selection can have no reason to form
any such anticipation; or rather its anticipation would necessarily
require to be the exact opposite. For, according to this theory, the
cross-infertility of allied species is due, either to correlation with
morphological changes which are being produced by the selection, or
else, as Darwin supposed, to "prolonged exposure to uniform conditions
of life"; and thus, in either case, the sterility variation ought to be,
as a general rule at all events, subsequent to the specific
differentiation, and, according to Darwin's view, _long_ subsequent.
Thus we ought not to find that the physiological change is ever, on any
large or general scale, the initial change; nor ought we to find that it
is, on any such scale, even so much as a contemporary change: there
ought, in fact, to be no constant or habitual association between
divergence of embryo-types and the concurrence of cross-infertility.
Now, it will be my endeavour to prove that there is an extraordinarily
general association between _varietal_ divergence and cross-infertility,
_wherever common areas are concerned_; and in as far as this can be
proved, I take it that the evidence will make wholly in favour of
physiological selection as the prime condition to specific divergence,
while at the same time they will make no less wholly, _and quite
independently_, against natural selection as the unaided cause of such
divergence.
I shall begin with some further quotations from Nägeli.
Species may be synoical at all stages of relationship. We come
across varieties, scarcely distinguishable from one another,
growing in the same locality (as, for example, the _Cirsium
heterophyllum_, with smooth or jagged leaves, the _Hieracium
sylvaticum_, with or without caulinary leaves); again, we meet
other varieties more accentuated (as the _H. hoppeanum_, with under
ligules of white or red, the _Campanula_, with white or lilac
flowers, &c.), other varieties even more marked, which might almost
be elevated to the rank of species (_Hieracium alpinum_, with hairs
and glands, and the new form _H. holadenium_, which has only
glands, _Campanula rotundifolia_ with smooth and hairy leaves), or
forms still more distinct, up to well-defined species. I could
enumerate endless examples at all stages.
It will be seen that in my definition of synoicy I do not mean to
assert that _all_ allied forms are invariably found together, but
that they are much more often seen in groups than singly. Take, for
instance, nine forms closely related (_A_ to _I_). _A_, _E_, _H_
will be found side by side at one point, _B_, _D_ at another, _C_,
_F_ at a third, &c. These facts are plainly opposed to the theory
of isolation and amixia, and make, on the contrary, in favour of
the social development of species (_loc. cit._, p. 221).
Not to multiply quotations to the same general effect, I will supply but
one other, referring to a particular case.
At one spot (_Rothwand_) much exposed to the sun, and difficult of
access, I remarked two closely allied forms, so nearly related to
_H. villosum_ that this would seem to be an intermediary form
between the two. One of these (_H. villosissimum_) is distinguished
by its tongue and thick pubescence, its tolerably large capitula,
and by the lengthened and separated scales of the involucrum; the
other, on the contrary (_H. elongatum_), is less pubescent, has
smaller capitula, and more compact scales on the involucrum than
_H. villosum_. Both are finally distinguishable from the type by
their longer stalks, which are more decidedly aphyllous, and by
their later flowering. At the spot where I found them the two forms
were closely intermingled, and each was represented by a
considerable number of plants. I did not find them anywhere else on
the mountain, nor could I find at the spot where these were growing
a single specimen of the true _H. villosum_, nor a single hybrid
from these two.
I concluded that these two new forms had, by joining their forces,
expelled the _H. villosum_ from its primitive abode, but had not
succeeded in displacing one another. As to their origin, they had
evidently developed in two different directions from a common point
of departure, namely _H. villosum_. They had succeeded, not only in
separating themselves from the original form, but also in
preventing any intermediary form from interposing. I thought myself
therefore justified in considering this as a case of varieties
which have come into existence subsequently to the Glacial epoch.
The morphological characteristics of the three forms are
sufficiently distinct for them to be designated as species by a
good many writers. They are better defined than some of MM. Frolich
and Fries' weaker species, and as well defined as some of MM. Koch
and Grisebach's (p. 222).
Now it is clear, without comment, that all this is exactly as it ought
to be, if allied species have been differentiated on common areas by
selective fertility. For if, as Nägeli elsewhere says, "one meets forms
in nature associated with one another, and severally distinguished by
every possible degree of differentiation," not only as Nägeli adds, does
this general fact lead to the inference that species are (usually)
developed when plants grow intimately associated together; but as
certainly it leads to the further inference that such development must
be due to a prior development of cross-infertility between the diverging
varietal forms, cross-infertility which is therefore afterwards so
characteristic of the allied species, when these are found, in their
fully differentiated condition, still occupying the same area in large
and intimately mingled populations.
To my mind there could not be any inference more strongly grounded than
this, because, with the one exception of the physiological form, no
other form of homogamy can be conceived which shall account for the
origin and permanence of these synoical varieties, in all degrees of
differentiation up to well-defined synoical species. Least of all, as we
have seen, can natural selection alone have had anything to do with such
a state of matters; while, as we have likewise seen, in all its details
it is exactly the state of matters which the theory of physiological
selection requires.
Nevertheless, although this inference is so strongly grounded, we ought
to remember that it is only an inference. In order fully to verify the
theory of physiological selection, we ought to prove by experiment the
fact of cross-infertility between these synoical varieties, as we learn
that it afterwards obtains between synoical species. It is to be
regretted that the theory of physiological selection did not occur to
the mind of Nägeli, because he would then, no doubt, have ascertained
this by actual experiment. As it is, the great value of his observations
goes no further than establishing a strong presumption, that it _must_
be selective fertility which causes the progressive differentiation of
synoical varieties; and also that, if so, this _must_ be the principal
factor in the differentiation of vegetable species, seeing that some
ninety-five per cent. are of synoical origin.
_Evidence from Experimental Research._
My paper on _Physiological Selection_ pointed out that the whole theory
would have to stand or fall with the experimental proof of the presence
or the absence of cross-infertility between varieties of the same
species growing on common areas. From the facts and considerations which
we have hitherto been dealing with, it did indeed appear to me that
there was the strongest conceivable ground for inferring that
cross-infertility between such varieties would be found by experiment to
be a phenomenon of highly general occurrence--amply sufficient ground to
prove that allied species on common areas for the most part owed their
origin to this character of mutual sterility, and not vice versa as
previously supposed. At that time I was not aware that any experiments
had been made in this direction. Soon after the paper was published,
however, my attention was directed to a laborious research which had
been directed to this very point, and carried on for more than thirty
years, by M. Jordan[25]. This had not attracted the general notice which
it undoubtedly deserved; and I have since ascertained that even Darwin
began to look into it only a few months before his death.
[25] _Remarques sur le fait de l'existence en société à l'état
sauvage des espèces végétales affines et sur d'autres faits relatifs
à la question de l'espèce_, par Alexis Jordan; lues au congrès de
l'Association Française pour l'Avancemeat des Sciences, 2^me
session, Lyon, séance de 28 Août, 1873.
Having devoted his life to closely observing in divers stations
multitudes of different species of plants--annuals and perennials,
bulbous and aquatic, trees and shrubs--M. Jordan has been able to
satisfy himself, and the French school of botanists to which this line
of observation has given rise, that in most cases (or "nearly
everywhere"), when a Linnean species is indigenous to a country and is
there of common occurrence, this species within that district is
represented by more or less numerous and perfectly constant varieties.
These varieties are constituted by such minute differences of
morphological character that their very existence eluded the
observation of botanists, until M. Jordan began to search specially for
them as the special objects of his scrutiny. Moreover, these varieties
of a Linnean species occupy common areas, and there grow in intimate
association with one another, or as M. Jordan says, "_pêle-mêle_." So
far, be it noticed, Jordan was proceeding on exactly the same lines as
Nägeli; only he carried his observations over a still wider range of
species on the one hand, and into a still minuter search for varieties
on the other. But the all-important point for us is, that he further
proceeded to test by experiment the physiological relations between
these morphological varieties; and found, in many hundreds of cases,
that they not only came true to seed (i. e. are hereditary and not
merely climatic), but likewise cross-sterile _inter se_. For these
reasons, M. Jordan, who is opposed to the theory of evolution, regards
all such varieties as separately created species; and the inspiring
motive of his prolonged investigations has been a desire to multiply
these proofs of creative energy. But it clearly makes no difference, so
far as evolutionists are concerned with them, whether all this multitude
of sexually isolated forms be denominated species or varieties.
The points which are of importance to evolutionists--and of the first
order of importance in the present connexion--may be briefly summarized
as follows:--
(1) The research embraces large numbers of species, belonging to very
numerous and very varied orders of plants; (2) in the majority of
cases--although not all--indigenous species which are of common
occurrence present constant varieties; (3) these varieties,
nevertheless, may be morphologically so slight as to be almost
imperceptible; (4) they occupy common areas and grow in intimate
association; (5) although many of them have undergone so small an amount
of morphological change, they have undergone a surprising amount of
physiological change; for (6) not only do very many of these varieties
come true to seed; but, (7) when they do, they are always more or less
cross-infertile _inter se_.
Now, it is self-evident that every one of these seven points is exactly
what the theory of physiological selection requires, while there is not
one of them which it does not require. For if the theory be sound, we
should expect to find large numbers of species belonging to numerous and
varied orders of plants presenting constant varieties on common areas;
we should expect this to be a highly general, though not a universal,
rule; and we should expect it to apply only to species which are
indigenous. Moreover, we should expect these varieties, although but
slightly differentiated morphologically, to present a great
differentiation physiologically--and this in the special direction of
selective fertility, combined, of course, with heredity.
On the other hand, as I have said, this catalogue of evidences leaves
nothing to be supplied. It gives us all the facts--and no more than all
the facts--which my paper on _Physiological Selection_ anticipated as
the eventual result of a prolonged experimental research. And if I have
to regret my ignorance of these facts when that paper was published, at
any rate it now furnishes the best proof that my anticipations were not
guided by the results of a verification which had already been supplied.
These anticipations were deduced exclusively from the theory itself, as
representing what _ought_ to be the case if the theory were true; and, I
must confess, if I had then been told that they had already been
realized--that it had actually been found to be a general rule that
endemic species present constant and hereditary varieties, intimately
commingled on common areas, morphologically almost indistinguishable,
but physiologically isolated by selective fertility--I should have felt
that the theory had been verified in advance. For there are only two
alternatives: either these things are due to physiological selection, or
else they are due--as M. Jordan himself believes--to special creation.
Which is equivalent to saying that, for evolutionists, the facts must be
held to verify the former theory in as complete a manner as it is
logically possible for the theory to be verified.
_Evidence from Prepotency._
We have now to consider the bearing of what is called "prepotency" on
the theory of physiological selection.
Speaking of the vast number of species of Compositae, Darwin says:--
There can be no doubt that if the pollen of all these species could
be simultaneously or successively placed on the stigma of any one
species, this one would elect with unerring certainty its own
pollen. This elective capacity is all the more wonderful, as it
must have been acquired since the many species of this great group
of plants branched off from a common progenitor.
Darwin is here speaking of elective affinity in its fully developed
form, as absolute cross-sterility between fully differentiated species.
But we meet with all lower degrees of cross-infertility--sometimes
between "incipient species," or permanent varieties, and at other times
between closely allied species. It is then known as "prepotency" of the
pollen belonging to the same variety or species over the pollen of the
other variety or species, when both sets of pollen are applied to the
same stigma. Although in the absence of the prepotent pollen the less
potent will fertilize the seed, yet, such is the appetency for the more
appropriate pollen, that even if this be applied to the stigma some
considerable time after the other, it will outstrip or overcome the
other in fertilizing the ovules, and therefore produce the same result
on the next generation as if it had been applied to the mother plant
without any admixture of the less potent pollen, although in some cases
such incipient degrees of cross-infertility are further shown by the
number or quality of the seeds being fewer or inferior.
Now, in different varieties and in different allied species, all degrees
of such prepotency have been noticed by many observers, from the
faintest perceptible amount up to complete impotency of the alien
pollen--when, of course, there is absolute sterility between the two
varieties or allied species. The inference is obvious. In this graduated
scale of prepotency--beginning with an experimentally almost
imperceptible amount of sexual differentiation between two varieties,
and ending in an absolute partitioning of two allied species--we have
the only remaining fact that is required to complete the case in favour
of the present theory. We are here brought back to the very earliest
stages of physiological differentiation or to the stages which lie
behind Jordan's "Physiological Species"; and therefore, when taken in
conjunction with his results, the phenomena of prepotency may be said to
give us the complete and final demonstration of one continuous
development, which, beginning in an almost imperceptible amount of
cross-infertility, ends in absolute cross-sterility. The "elective
capacity" to which Darwin alludes as having been "acquired" by all the
species of Compositae since they "branched off from a common
progenitor," is thus seen among innumerable other species actually in
process of acquisition; and so we can perfectly well understand, what is
otherwise unintelligible, that closely allied species of plants occur,
in ninety-five per cent. of cases, intimately associated on common
areas, while exhibiting towards one another the character of mutual
sterility.
But more than this. The importance of the widespread phenomena of
prepotency to the theory of physiological selection does not consist
merely in thus supplying the last link in the chain of evidence touching
the origin of species by selective fertility, or "elective capacity."
These phenomena are of further importance as showing how in plants, at
all events, physiological selection appears to be frequently capable of
differentiating specific types without the necessary assistance of any
other form of homogamy. In my original statement of the theory, I was
careful to insist upon the great value, as differentiating agents, of
even small degrees of other forms of homogamy when co-operating with
physiological selection. But I also stated my belief that in many cases
selective fertility is presumably of itself capable of splitting a
specific type; and the reason why I still believe this is, that I do not
otherwise understand these phenomena of prepotency. I cannot believe
that in all the innumerable cases where they arise, they have been
super-induced by some prior morphological changes going on in some other
part of the organism, or by "prolonged exposure to uniform conditions of
life," on the part of two well-nigh identical forms which have arisen
intimately commingled in exactly the same environment, and under the
operation of a previously universal intercrossing. Even if such a thing
could be imagined as happening occasionally, I feel it difficult to
imagine that it can happen habitually, and yet this view must be held by
those who would attribute prepotency to natural selection.
It must never be forgotten that the relatively enormous changes as to
size, structure, habit, &c., which are presented by our domesticated
plants as results of artificial selection, do not entail the
physiological character of cross-sterility in any degree, save possibly
in some small number of cases. Although in wild species any
correspondingly small percentage of cases (where natural selection
happens to hit upon parts of the organism modifications of which produce
the physiological change by way of correlation) would doubtless be the
ones to survive on common areas, still it is surely incredible that such
an accidental association between natural selection and
cross-infertility is so habitually the means of specific differentiation
as the facts of prepotency (together with the observations of Jordan
and Nägeli) would necessarily demand.
Moreover, this view of the matter is still further corroborated by
certain other facts and considerations. For example, the phenomena of
prepotency (whether as between varieties or between closely allied
species) are found to occur when the two forms occupy a common area,
i.e. are growing intermingled with one another. Therefore, but for this
physiological differentiation, there could be absolutely nothing to
prevent free intercrossing. Yet the fact that hybrids are so
comparatively rare in a state of nature--a fact which Sir Joseph Hooker
has pointed out to me as otherwise inexplicable--proves the efficacy of
even a low degree of such differentiation in preventing the
physiologically-differentiated forms from intercrossing. Even in cases
where there is no difficulty in producing artificial hybrids or mongrels
between species or varieties growing on common areas, it is perfectly
astonishing what an extremely small percentage of the hybrid or mongrel
forms are found to occur in nature. And there can be no question that
this is due to the very efficient manner in which prepotency does its
work--efficient, I mean, from the point of view of the new theory; for
upon any other theory prepotency is a meaningless phenomenon, which,
notwithstanding its frequent occurrence, plays no part whatever in the
process of organic evolution.
I attach considerable importance to the phenomena of prepotency in view
of the contrast which is presented between plants and animals in the
relation of their species to physical barriers. For animals--and
especially the higher animals--appear to depend for their specific
differentiations upon such barriers much more than in the case with
plants. This is no more than we should expect; for, in accordance with
our theory, selective fertility is not so likely to work alone in the
case of the higher animals which mate together, as in plants which are
fertilized through the agency of wind or insects. In the former case
there is no opportunity given for the first rise of cross-infertility,
in the form of prepotency; and even where selective fertility has gained
a footing in other ways, the chances against the suitable mating of
"physiological complements" must be much greater than it is in the
latter case. Hence, among the higher animals, selective fertility ought
much more frequently to be found in association with other forms of
homogamy than it is among plants. And this is exactly what we find. Thus
it seems to me that this contrast between the comparative absence and
presence of physical barriers, where allied species of plants and of
higher animals are respectively concerned, is entitled to be taken as a
further corroboration of our theory. For while it displays exactly such
a general correlation as this theory would expect, the correlation is
one which cannot possibly be explained on any other theory. It is just
where physiological selection can be seen to have the best opportunity
of acting (viz. in the vegetable kingdom) that we find the most
unequivocal evidence of its action; while, on the other hand, it is just
where it can be seen to have the least opportunity of asserting itself
(viz. among the higher animals) that we find it most associated with,
and therefore assisted by, other forms of homogamy, i. e. not only
geographical isolation, but also by sexual preference in pairing, and
the several other forms of homogamy, which Mr. Gulick has shown to arise
in different places as the result of intelligence.
_Evidence from Special Cases._
Hitherto I have been considering, from the most general point of view,
the most widespread facts and broadest principles which serve to
substantiate the theory of physiological selection. I now pass to the
consideration of one of those special cases in which the theory appears
to have been successfully applied.
Professor Le Conte has adduced the fossil snails of Steinheim as serving
to corroborate the theory of physiological selection[26].
[26] _Evolution and its Relations to Religious Thought_, &c. pp.
236-7.
The facts are these. The snail population of this lake remain for a long
time uniform and unchanged. Then a small percentage of individuals
suddenly began to vary as regards the form of their shells, and this in
two or three directions at the same time, each affected individual,
however, only presenting one of the variations. But after all these
variations had begun to affect a proportionally large number of
individuals, some individuals occur in which two or more of the
variations are blended together, evidently, as Weismann says, by
intercrossing of the varieties so blended. Later still, both the
separate varieties and their blended progeny became more and more
numerous, and eventually a single blended type, comprising in itself all
the initial varieties, supplanted the parent form. Then another long
period of stability ensued until another eruption of new variations took
place; and these variations, after having affected a greater and greater
number of individuals, eventually blended together by intercrossing and
supplanted their parent form. So the process went on, comparatively
short periods of variation alternating with comparatively long periods
of stability, the variations, moreover, always occurring suddenly in
crops, then multiplying, blending together, and in their finally blended
type eventually supplanting their parent form.
Now, the remarkable fact here is that whenever the variations arose,
they only intercrossed between themselves, they did not intercross with
their parent form; for, if they had, not only could they never have
survived (having been at first so few in number and there having been no
geographical barriers in the small lake), but we should have found
evidence of the fact in the half-bred progeny. Moreover, natural
selection can have had nothing to do with the process, because not only
are the variations in the form of the shells of no imaginable use in
themselves; but it would be preposterous to suppose that at each of
these "variation periods" several different variations should always
have occurred simultaneously, all of which were of some hidden use,
although no one of them ever occurred during any of the prolonged
periods of stability. How, then, are we to explain the fact that the
individuals composing each crop of varieties, while able to breed among
themselves, never crossed with their parent form? These varieties, each
time that they arose, were intimately commingled with their parent
form, and would certainly have been reabsorbed into it had intercrossing
in that direction been possible. With Professor Le Conte, therefore, I
conclude that there is only one conceivable answer to this question.
Each crop of varieties must have been _protected from intercrossing with
their parent form_.
They must have been the result of a variation, which rendered the
affected individuals sterile with their parent form, whilst leaving them
fertile amongst themselves. The progeny of these individuals would then
have dispersed through the lake, physiologically isolated from the
parent population, and especially prone to develop secondary variations
as a direct result of the primary variation. Thus, as we might expect,
two or three variations arose simultaneously, as expressions of so many
different lines of family descent from the original or physiological
variety; these were everywhere prevented from intercrossing with their
parent form, yet capable of blending whenever they or their
ever-increasing progeny happened to meet. Thus, without going into
further details, we are able by the theory of physiological selection to
give an explanation of all these facts, which otherwise remain
inexplicable.
* * * * *
In view of the evidence which has now been presented, I will now ask
five questions which must be suitably answered by critics of the theory
of physiological selection.
1. Can you doubt that the hitherto insoluble problem of inter-specific
sterility would be solved, supposing cross-infertility were proved to
arise before or during the process of specific differentiation, instead
of after that process had been fully completed?
2. Can you doubt, after duly considering the circumstances under which
allied species of plants have been differentiated--viz. in ninety-five
per cent. of cases intimately commingled on common areas, and therefore
under identical environments--that cross-infertility _must_ have arisen
before or during the specific differentiation?
3. Can you doubt, after duly considering the facts of prepotency on the
one hand and those of Jordan's physiological varieties on the other,
that cross-infertility _does_ arise before or during the specific
differentiation?
4. If you cannot express a doubt upon any of these points, can you
explain why you refuse to accept the theory of the origin of species by
means of physiological selection, together with the explanation which
this theory affords of the continued cross-fertility of domesticated
varieties?
5. Supposing this theory to be true, can you conceive of any other
classes of facts which, either quantitatively or qualitatively, could
more directly or more effectually prove its truth than those which have
now been adduced?
On these five heads I entertain no doubt. I am convinced that the theory
of physiological selection is the only one that can explain the facts of
inter-specific sterility on the one hand, and, on the other hand, the
contrast which these facts display to the unimpaired fertility of our
domesticated varieties.
In conclusion, it seems desirable once more to insist that there is no
antagonism or rivalry between the theories of natural and of
physiological selection. For which purpose I will quote the final
paragraph of my original paper.
So much, then, for the resemblances and the differences between the
two theories. It only remains to add that the two are
complementary. I have already shown some of the respects in which
the newer theory comes to the assistance of the older, and this in
the places where the older has stood most in need of assistance. In
particular, I have shown that segregation of the fit entirely
relieves survival of the fittest from the difficulty under which it
has hitherto laboured of explaining why it is that sterility is so
constantly found between species, while so rarely found between
varieties which differ from one another even more than many
species; why so many features of specific distinction are useless
to the species presenting them; and why it is that incipient
varieties are not obliterated by intercrossing with parent forms.
Again, we have seen that physiological selection, by preventing
such intercrossing, enables natural selection to promote diversity
of character, and thus to evolve species in ramifying branches
instead of in linear series--a work which I cannot see how natural
selection could possibly perform unless thus aided by physiological
selection. Moreover, we have seen that although natural selection
alone could not induce sterility between allied types, yet when
this sterility is given by physiological selection, the forms which
present it would be favoured in the struggle for existence; and
thus again the two principles are found playing, as it were, into
each other's hands. And here, as elsewhere, I believe that the
co-operation enables the two principles to effect very much more in
the way of species-making than either of them could effect if
working separately. On the one hand, without the assistance of
physiological selection, natural selection would, I believe, be all
but overcome by the adverse influences of free
intercrossing--influences all the more potent under the very
conditions which are required for the multiplication of species by
divergence of character. On the other hand, without natural
selection, physiological selection would be powerless to create any
differences of specific type, other than those of mutual sterility
and trivial details of structure, form, and colour--differences
wholly without meaning from a utilitarian point of view. But in
their combination these two principles appear to me able to
accomplish what neither can accomplish alone--namely, a full and
satisfactory explanation of the origin of species.