Volume Iii Part 4 (1/2)

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 circ.u.mstances; 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 pa.s.s 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, s.h.i.+elded from intercrossing with parent forms, and from compet.i.tion with exotic forms; while, at the same time, the h.o.m.ogamy 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, att.i.tude, &c., and thus so many different opportunities for the occurrence of sundry forms of h.o.m.ogamy. Now, it is obvious that of all these sundry forms of h.o.m.ogamy, physiological selection must have what may be termed a first-rate opportunity of a.s.sisting 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 p.r.o.nounced (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 s.e.xually 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 const.i.tute 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 mult.i.tudinous 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 const.i.tute 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 const.i.tuent 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.

[Ill.u.s.tration:]

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_ pa.s.ses 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 a.n.a.logy 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 const.i.tute 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 s.e.xual isolation with which every distinct set of them is now found to be a.s.sociated.

_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 _trans.m.u.tation_ of the species by natural selection; but the worse it will be for _differentiation_ of the species by this form of h.o.m.ogamy.

On the other hand, if physiological selection be entertained as a form of h.o.m.ogamy, 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 a.s.sociation, so that their individual members const.i.tute, 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 a.s.sociated, the form of h.o.m.ogamy 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 h.o.m.ogamy which was concerned in their differentiation must presumably have been the geographical form.