EVIDENCES OF PHYSIOLOGICAL SELECTION.
I will now give an outline sketch of the evidences in favour of the
theory which has been set forth in the preceding chapter, stating first
what is the nature of the verification which it requires.
The theory is deduced from a highly general association between
distinctive specific characters of _any_ kind and a relatively constant
specific character of a _particular_ kind--namely, sexual exclusiveness.
For it is from this highly general association that the theory infers
that this relatively constant specific character has been at least one
of the needful conditions to the development of the other specific
characters with which it is found associated. Hence the necessary
verification must begin by showing the strength of the theory on these
merely deductive, or antecedent, grounds. It may then proceed to show
how far the facts of organic nature corroborate the theory in other and
independent ways.
First, let it be carefully observed that here we have to do only with
the _fact_ of selective fertility, and with its _consequences_ as
supposed by the theory: we have nothing to do either with its _causes_
or its _degrees_. Not with its causes, because in this respect the
theory of physiological selection is in just the same position as that
of natural selection: it is enough for both if the needful variations
are provided, without its being incumbent on either to explain the
causes which produce them. Not with its degrees, because, in the first
place, it can only be those degrees of variation which in particular
cases are supposed adequate to induce specific divergence, that fall
within the scope of the theory; and because, in the second place,
degrees which are adequate only to induce--or to assist in inducing,
_varietal_ divergence, must always tend to increase, or pass into higher
degrees.
_Antecedent Standing of the Theory._
The antecedent standing or logical basis of the theory has already been
in large measure displayed in the preceding chapter; for it was
impossible to state the theory without thereby showing in how
considerable a degree it is self-evident. A brief recapitulation is
therefore all that is here necessary.
It has been shown that divergent or polytypic evolution on common areas
is inexplicable by natural selection alone. Hence the question arises:
What form of isolation has, under such circumstances, rendered possible
divergent evolution? In answer to this question the theory of
physiological selection suggests that variations in the reproductive
function occur in such a way as to isolate more or less perfectly from
each other different sections of a species. While cross-fertility
remains unimpaired among the members of each section, there is more or
less cross-infertility when members of either section mate with those of
the other. Thus a physiological barrier is interposed between the two
sections; and any divergences of structure, colouring, or instinct
arising in the members of either section will not in any way be affected
by such divergences as arise among the members of the other.
In support of this suggestion, it has been shown in the preceding
chapter that the very general association of cross-infertility with
specific differentiation points most strongly to the inference that the
former has usually been an indispensable condition to the occurrence of
the latter. It cannot be denied that in many cases the specific
distinction is now maintained by means of that sexual isolation which
cross-infertility confers: it is therefore probable that such isolation
has been instrumental in securing its initial attainment.
This probability is strengthened by the observed fact that the general
association in question is conspicuously absent in the case of
domesticated varieties, notwithstanding that their multitudinous and
diverse varietal characters usually equal, and frequently surpass,
specific characters in their degrees of divergence.
Since, then, it would seem to be impossible for divergent evolution on
common areas to take place in the absence of some mode of isolation;
since cross-infertility appears to be the only possible mode under the
given circumstances; and since among domesticated varieties, where
isolation is otherwise secured by artificial means, cross-infertility is
usually absent, the logical foundations of the theory of physiological
selection would seem to be securely laid.
We may therefore pass to more special lines of evidence.
_Evidence from Geographical Distribution._
Darwin has adduced very good evidence to show that large areas,
notwithstanding the disadvantages which (on his theory) must arise from
free intercrossing, are what he terms better manufactories of species
than smaller areas, such as oceanic islands. On the other hand, as a
matter of fact, oceanic islands are comparatively rich in peculiar
species. These two statements, however, are not incompatible. Smaller
areas are, as a rule, rich in peculiar species relatively to the number
of their inhabitants; but it does not follow that they are rich in
species as contrasted with larger areas containing very many more
inhabitants. Therefore, the rules are that large areas turn out an
absolutely greater number of specific types than small areas; although,
relatively to the number of individuals or amount of population, the
small areas turn out a larger number of species than the large areas.
Now, these two complementary rules admit of being explained as Darwin
explains them. Small and isolated areas are rich in species relatively
to the amount of population, because, as we have before seen, this
population has been permitted to develop an independent history of its
own, shielded from intercrossing with parent forms, and from competition
with exotic forms; while, at the same time, the homogamy thus secured,
combined with change of environment, will give natural selection an
improved chance of finding new points of departure for its operation. On
the other hand, large and continuous areas are favourable to the
production of numerous species, first, because they contain a large
population, thus favouring the occurrence of numerous variations; and,
secondly, because the large area furnishes a diversity of conditions in
its different parts, as to food, climate, attitude, &c., and thus so
many different opportunities for the occurrence of sundry forms of
homogamy. Now, it is obvious that of all these sundry forms of homogamy,
physiological selection must have what may be termed a first-rate
opportunity of assisting in the manufacture of species on large areas.
For not only is it upon large and continuous areas that the antagonistic
effects of intercrossing are most pronounced (and, therefore, that the
influence of physiological selection must be most useful in the work of
species-making); but here also the diversity in the external conditions
of life, which the large area supplies to different parts of the
extensive population, cannot fail to furnish physiological selection
with a greater abundance of that particular variation in the
reproductive system on which its action depends. Again, and of still
more importance, on large areas there are a greater _number_ of species
already differentiated from one another as such; thus a greater number
of already sexually differentiated forms are presented for further
differentiation at the hands of physiological selection. For all these
reasons, therefore, we might have expected, upon the new theory, that
large and continuous areas would be good manufactories of species.
Again, Darwin has shown that not only large areas, but likewise
"dominant" genera within those areas, are rich in species. By dominant
genera he meant those which are represented by numerous individuals, as
compared with other genera inhabiting the same area. This general rule
he explains by the consideration that the qualities which first led to
the form being dominant must have been useful; that these would be
transmitted to the otherwise varying offspring; and, therefore, that
when these offspring had varied sufficiently to become new species, they
would still enjoy their ancestral advantages in the struggle for
existence. And this, doubtless, is in part a true explanation; but I
also think that the reason why dominant genera are rich in species, is
chiefly because they everywhere present a great number of individuals
exposed to relatively great differences in their conditions of life: or,
in other words, that they furnish the best raw material for the
manufacture of species by physiological selection, as explained in the
last paragraph. For, if the fact of dominant genera being rich in
species is to be explained _only_ by natural selection, it appears to me
that the useful qualities which have already led to the dominance of the
ancestral type ought rather to have proved inimical to its splitting up
into a number of subordinate types. If already so far "in harmony with
its environment" as to have become for this reason dominant, one would
suppose that there is all the more reason for its not undergoing change
by the process of natural selection. Or, at least, I do not see why the
fact of its being in an unusual degree of harmony with its environment
should in itself constitute any unusual reason for its modification by
survival of the fittest. On the other hand, as just observed, I do very
plainly see why such a reason is furnished for the modifying influence
of physiological selection.
Let us next turn to another of Darwin's general rules with reference to
distribution. He took a great deal of trouble to collect evidence of the
two following facts, namely, (1) that "species of the larger genera in
each country vary more frequently than the species of the smaller
genera"; and (2) that "many of the species included within the larger
genera resemble varieties in being very closely, but unequally, related
to each other, and in having restricted ranges[22]." By larger genera he
means genera containing many species; and he accounts for these general
facts by the principle, "that where many species of a genus have been
formed, on an average many are still forming." But _how_ forming? If we
say by natural selection alone, we should expect to find the
multitudinous species differing from one another in respect of features
presenting well-marked adaptive meanings; yet this is precisely what we
do not find. For Darwin's argument here is that "in large genera the
amount of difference between the species is often exceedingly small, so
that in this respect the species of the larger genera resemble varieties
more than do the species of the smaller genera." Therefore the argument,
while undoubtedly a very forcible one in favour of the fact of
_evolution_, appears to me scarcely consistent with the view of this
evolution being due solely to natural selection. On the other hand, the
argument tells strongly (though unconsciously) in favour of
physiological selection. For the larger a genus, or the greater the
number of its species, the greater must be the opportunity for the
occurrence of that particular kind of variation on which the principle
of physiological selection depends. The species of a genus may be
regarded as so many varieties which have already been separated from one
another physiologically; therefore each of them may now constitute a new
starting-point for a further and similar separation--particularly as, in
virtue of their previous segregation, many are now exposed to different
conditions of life. Thus, it seems to me, we can well understand why it
is that genera already rich in species tend to grow richer; while such
is not the case in so great a degree with genera that are poor in
species. Moreover, we can well understand that, multiplication of
species being as a rule, and in the first instance, determined by
changes in the reproductive system, wherever a large number of new
species are being turned out, the secondary differences between them
should be "often exceedingly small"--a general correlation which, so far
as I can see, we are not able to understand on the theory of natural
selection.
[22] _Origin of Species_, pp. 44, 45.
The two subsidiary facts, that very closely allied species have
restricted ranges, and that dominant species are rich in varieties, both
seem to tell more in favour of physiological than of natural selection.
For "very closely allied species" is but another name for species which
scarcely differ from one another at all except in their reproductive
systems; and, therefore, the more restricted their ranges, the more
certainly would they have become fused by intercrossing with one
another, had it not been for the barrier of sterility imposed by the
primary distinction. Or rather, I should say, had it not been for the
original occurrence of this barrier, these now closely-allied species
could never have become species. Again, that dominant species should be
rich in varieties is what might have been expected; for the greater the
number of individuals in a species, the greater is the chance of
variations taking place in all parts of the organic type, and
particularly in the reproductive system, seeing that this system is the
most sensitive to small changes in the conditions of life, and that the
greater the number of individuals composing a specific type, the more
certainty there is of some of them encountering such changes. Hence, the
richness of dominant species in varieties is, I believe, mainly due to
the greater opportunity which such species afford of some degree of
cross-infertility arising between their constituent members.
Here is another general fact, also first noticed by Darwin, and one
which he experiences some difficulty an explaining on the theory of
natural selection. He says:--
In travelling from north to south over a continent, we generally
meet at successive intervals with closely-allied or representative
species, evidently filling the same place in the economy of the
land. These representative species often meet and interlock, and as
one becomes rarer and rarer, the other becomes more and more
frequent, till the one replaces the other. But if we compare these
species where they intermingle, they are generally as absolutely
distinct from each other in every detail of structure as are
specimens taken from the metropolis of each.... In the
intermediate region, having intermediate conditions of life, why do
we not now find closely-linking intermediate varieties? This
difficulty for a long time quite confounded me. But I think it can
in large part be explained[23].
[23] _Origin of Species_, ed. 6, pp. 134, 135.
[Illustration:]
His explanation is that, "as the neutral territory between two
representative species is generally narrow in comparison with the
territory proper to each, ... and as varieties do not essentially differ
from species, the same rule will probably apply to both; and, therefore,
if we take a varying species inhabiting a very large area, we shall have
to adapt two varieties to two large areas, and a third variety to a
narrow intermediate zone." It is hence argued that this third or
intermediate variety, on account of its existing in lesser numbers, will
probably be soon overrun and exterminated by the larger populations on
either side of it. But how is it possible "to adapt two varieties to two
large areas, and a third [transitional] variety to a narrow intermediate
zone," in the face of free intercrossing on a continuous area? Let _A_,
_B_, and _C_ represent the three areas in question. According to the
argument, variety _A_ passes first into variety _B_, and then into
variety _C_, while variety _B_ eventually becomes exterminated by the
inroads both from _A_ and _C_. But how can all this have taken place
with nothing to prevent intercrossing throughout the entire area _A_,
_B_, _C_? I confess that to me it seems this argument can only hold on
the supposition that the analogy between varieties and species extends
to the reproductive system; or, in a sense more absolute than the
argument has in view, that "varieties do not essentially differ from the
species" which they afterwards form, but from the first show some degree
of infertility towards one another. And, if so, we have of course to do
with the principles of physiological selection.
That in all such cases of species-distribution these principles have
played an important part in the species-formation, appears to be
rendered further probable from the suddenness of transition on the area
occupied by contiguous species, as well as from the completeness of
it--i. e. the absence of connecting forms. For these facts combine to
testify that the transition was originally due to that particular change
in the reproductive systems of the forms concerned, which still enables
those forms to "interlock" without intercrossing. On the other hand,
neither of these facts appears to me compatible with the theory of
species-formation by natural selection alone.
But this leads us to another general fact, also mentioned by Darwin, and
well recognized by all naturalists, namely, that closely allied species,
or species differing from one another in trivial details, usually occupy
contiguous areas; or, conversely stated, that contiguity of geographical
position is favourable to the appearance of species closely allied to
one another. Now, the large body of facts to which I here allude, but
need not at present specify, appear to me to constitute one of the
strongest of all my arguments in favour of physiological selection.
Take, for instance, a large continental area, and follow across it a
chain of species, each link of which differs from those on either side
of it by the minute and trivial distinctions of a secondary kind, but
all the links of which differ from one another in respect of the primary
distinction, so that no one member of the series is perfectly fertile
with any other member. Can it be supposed that in every case this
constant primary distinction has been superinduced by the secondary
distinctions, distributed as they are over different parts of all these
kindred organisms, and yet nowhere presenting any but a trifling amount
of morphological change?
For my own part, I cannot believe--any more than Darwin could
believe--that all these numerous, diverse, and trivial changes have
always had the accidental effect of inducing the same peculiar change in
the reproductive system, and so producing it without any reference to
the process of specific divergence. Nor can I believe, as Darwin
incidentally and provisionally suggested, that prolonged exposure to
uniform conditions of life have so generally induced an equally
meaningless result. I can only believe that all the closely allied
species inhabiting our supposed continent, and differing from one
another in so many and such divers points of small detail, are merely so
many records of the fact that selective fertility has arisen among their
ancestry, and has thus given as many opportunities for the occurrence of
morphological differentiations as it has furnished cases of efficient
isolation. Of course, I do not deny that many, or probably most, of
these trivial morphological differentiations have been produced by
natural selection on account of their utility: I merely deny that they
could have been so produced on this common area, but for the sexual
isolation with which every distinct set of them is now found to be
associated.
_Evidence from Topographical Distribution of Species._
By topographical distribution I mean the distribution of organisms with
reference to comparatively small areas, as distinguished from larger
regions with reference to which the term geographical distribution is
appropriate.
It will be at once apparent that a study of the topographical
distribution of organic types is of even more importance for us than a
study of their geographical distribution. For while the former study is
conducted, as it were, with a low power of our observing microscope, the
latter is conducted with a high power. The larger facts of geographical
distribution yield, indeed, all the general characters which we might
expect them to yield, on the theory that divergence of specific types on
common areas has been in chief part determined by physiological
conditions. But for the purpose of testing this theory in a still more
exacting manner, it is of the first importance to consider the more
detailed facts of topographical distribution, since we here come to
closer quarters with the problem of specific differentiation. Therefore,
as we have already considered this problem under the most general points
of view, we will now consider it under more special points of view.
* * * * *
It is self-evident, as we have seen in the preceding section, that the
greater the number of individuals of the same species on a given area,
the less must be the power of natural selection to split that species
into two or more allied types; because, the more crowded the population,
the greater must be the uniformitarian effect of free intercrossing.
This obvious fact has been insisted upon by several previous writers on
Darwinism; and the only reason why it has not been recognized by all
naturalists is that so few of them have observed the all-important
distinction between monotypic and polytypic evolution. The denser the
population, and therefore the greater the intercrossing and the severer
the struggle for existence within the species, the better will it be for
_transmutation_ of the species by natural selection; but the worse it
will be for _differentiation_ of the species by this form of homogamy.
On the other hand, if physiological selection be entertained as a form
of homogamy, the denser the population, the better opportunity it will
have of differentiating the species, first, because a greater number of
individuals will be present in which the physiological change may arise,
and, secondly, because, if it does arise, the severity of the struggle
for existence will _then_ give natural selection a better chance of
acting rapidly and effectually on each of the isolated sections.
Hence, where the question is whether selective fertility has played any
large or general part in the differentiation of specific types, the best
criterion we can apply is to ascertain whether it is a general rule that
closely allied species occur in intimate association, so that their
individual members constitute, as it were, a single population, or, on
the other hand, whether they occur rather on different sides of
physical barriers. If they occur intimately associated, the form of
homogamy to which their differentiation was due must have presumably
been the physiological form; whereas, if they are proved to be
correlated with physical barriers, the form of homogamy which was
concerned in their differentiation must presumably have been the
geographical form.
Now, at first this consideration was a trouble to me, because Moritz
Wagner had strenuously argued--and supported his argument by a
considerable wealth of illustration--that allied species are always
found correlated with physical barriers or discontinuous areas.
Weismann's answer, indeed, had shown that Wagner's statement was much
too general: nevertheless, I was disappointed to find that so much could
be said in favour of the geographical (or topographical) form of
isolation where closely allied species are concerned. Subsequently,
however, I read the writings of Nägeli on this subject, and in them I
find a very different state of matters represented.
Seeing as clearly as Wagner that it is impossible under any
circumstances for natural selection to cause specific _differentiation_
unless assisted by some other forms of homogamy, but committing the same
oversight as Wagner and Weismann in supposing that the only other form
of homogamy in nature is geographical isolation, Nägeli, with great
force of reasoning, and by many examples, founded his argument against
the theory of natural selection on the ground that in the vegetable
kingdom closely allied species are most frequently found in intimate
association with one another, not, that is to say, in any way isolated
by means of physical barriers. This argument is everywhere logically
intact; and, as he sustains it by a large knowledge of topographical
botany, his indictment against natural selection as a cause of specific
_differentiation_ appeared to be insurmountable. And, in point of fact,
it _was_ insurmountable; so that the whole problem of the origin of
species by _differentiation on common areas_ has hitherto been left in
utter obscurity. Nor is there now any escape from this obscurity, unless
we entertain the "supplementary factor" of selective fertility. And,
apparently, the only reason why this has not been universally
recognized, is because Darwinians have hitherto failed to perceive the
greatness of the distinction between the _differentiation_ and the
_transmutation_ of species; and hence have habitually met such
overwhelming difficulties as Nägeli presented by an illogical
confounding of these two totally distinct things.
But if the idea of selective fertility had ever occurred to Nägeli as a
form of segregation which gives rise to specific differentiation, I can
have no doubt that so astute and logical a thinker would have perceived
that his whole indictment against natural selection was answered. For it
is incredible that he should not have perceived how this physiological
form of homogamy (supposing it to arise _before_ or _during_, and not
_after_ the specific differentiation) would perform exactly the same
function on a continuous area, as he allowed that "isolation" does on a
discontinuous one.
However, be this as it may, there cannot be any question touching the
immense value of his facts and arguments as evidence in favour of
physiological selection--albeit this evidence was given unconsciously,
or, as it were, prophetically. Therefore I will here quote a few
examples of both, from his paper _Du Développement des Espèces
Sociales_[24].
[24] _Archives des Sciences physiques et naturelles_ (Genève), vol.
liii. (1875), pp. 211-236.
After stating the theory of natural selection, he says that if the
theory is (of itself) a true explanation of the origin (or divergence)
of specific forms, it ought to follow that
two closely allied forms, derived the one from the other, would
necessarily occupy two different geographical areas [or
topographical stations], since otherwise they would soon become
blended. Until they had already become sufficiently consolidated as
distinct species to render mutual intercrossing highly improbable,
they could not be intermingled without disadvantage [to
differentiation]. Had Darwin endeavoured to support his hypothesis
by facts, he would, at least in the vegetable kingdom, have found
little to favour his cause. I can cite many hundreds of cases, in
which species in every stage of development have been found closely
mingling with one another, and not in any way isolated. Therefore,
I do not think that one can rightly speak of natural selection in
the Darwinian sense in the vegetable kingdom; and, in my
estimation, there is a great difference between the formation of
species by nature and the production of stock by a breeder.... (p.
212).
Of the two kinds of distribution (i. e. growing apart and growing
together), Synoicy (or growing together) is by far the most usual
in nature. I reckon that out of a hundred allied vegetable forms,
at least ninety-five would be found to be synoical (p. 219).
This is a most important point. That so enormous a proportion of
vegetable species should have originated in intimate association with
their parent or sister types, is clearly unintelligible on the theory of
natural selection alone; there obviously _must_ be some other form of
homogamy which, whether or not in all places _associated_ with natural
selection, is the primary condition to the differentiation. Such I hold
with Nägeli, is a logical necessity; and this whether or not I am right
in believing the other form of homogamy in question to be selective
fertility. But I go further and say, Surely there can be no rational
question that this other form of homogamy must have been, at any rate as
a highly general rule, the one which I have assigned. For how is it that
in these ninety-five per cent. of cases, where vegetable species are
growing intimately associated with their nearest allies, there is no
hybridizing, or blending and relapsing to the original undifferentiated
types? We know well the answer. These are fully differentiated species,
and, as such, are protected from mutual intercrossing by the barrier of
mutual sterility. But now, if this bar is thus necessary for preserving
the specific distinctions when they have been fully developed, much more
must it have been so to admit of their development; or, otherwise
stated, since we know that this barrier is associated with "synoical"
species, and since we clearly perceive that were it withdrawn these
species would soon cease to exist, can we reasonably doubt that their
existence (or origin) is due to the previous erection of this barrier?
If synoical species were comparatively rare, the validity of such
reasoning might be open to question; or, even if we should not doubt it
in such cases, at any rate we might well doubt the importance or extent
of selective fertility as a factor in the origination of species. But
the value of Nägeli's writings on the present subject consists in
showing that synoical species constitute so overwhelming a majority of
the vegetable kingdom, that here, at all events, it appears impossible
to rate too highly the importance of the principle I have called
physiological selection.