Mr. Wallace meets the difficulty by arguing that sterility between allied species may have been brought about by the direct influence of natural selection. But, as previously remarked, this view is expressly opposed to that of Darwin, who held that Wallace's contention is erroneous.
It will be seen, then, that both Darwin, and Wallace, fully recognize the necessity of finding some explanation of the infertility of allied species, over and above the mere reaction of morphological differentiation on the physiology of the reproductive system, and they both agree in suggesting additional causes, though they entirely disagree as to what these causes are. Now, the theory of physiological selection likewise suggests an additional cause—or, rather, a new explanation—and one which is surely the most probable. For what is to be explained? The very general association of a certain physiological peculiarity with that amount of morphological change which distinguishes species from species, of whatever kind the change may be, and in whatever family of the animal or vegetable kingdom it may occur. Well, the theory of physiological selection explains this very general association by the simple supposition that, at least in a large number of cases, it was the physiological peculiarity which first of all led to the morphological divergence, by interposing the bar of sterility between two sections of a previously uniform species; and by thus isolating the two sections one from another, started each upon a subsequently independent course of divergent evolution.
Or, to put it in another way, if the occurrence of this physiological peculiarity has been often the only possible means of isolating two sections of a species occupying a common area, and thus giving rise to a divergence of specific type (as obviously must have been the case wherever there was an absence of any other form of isolation), it is nothing less than a necessary consequence that many allied species should now present the physiological peculiarity in question. Thus the association between the physiological peculiarity and the morphological divergence is explained by the simple hypothesis, that the former has acted as a necessary condition to the occurrence of the latter. In the absence of other forms of isolation, the morphological divergence could not have taken place at all, had not the physiological peculiarity arisen; and hence it is that we now meet with so many cases where such divergence is associated with this peculiarity.
So far we have been considering the physiological change as historically the prior one. Here, at first sight, it may seem that the segregative power of physiological selection must end; for it may well seem impossible that the physiological change can ever be necessary for the divergence of morphological varieties into true species in cases where it has not been the prior change, but has only set in after morphological changes have proceeded far enough to have already constituted definite varieties. A little thought, however, will show that physiological selection is quite as potent a condition to the differentiation of species when it occurs after varietal divergence has begun, as it is when it occurs before the divergence—and hence that it really makes no difference to the theory of physiological selection whether, in particular cases, the cross-infertility arises before or after any structural or other modifications with which it is associated.
For the theory does not assert that all varieties have been due to physiological selection. There are doubtless many other causes of the origin of varieties besides cross-infertility with parent forms; but, as a general rule, it does not appear that they are by themselves capable of carrying divergence beyond a merely varietal stage. In order to carry divergence to the stage of producing species, it appears to be a general condition that, sooner or later, cross-infertility should arise—seeing that, when varieties do succeed in becoming species, we almost invariably find that, as a matter of fact, cross-infertility has arisen. Hence, if cross-infertility has thus usually been a necessary condition to a varietal divergence becoming specific, it can make no material difference when the incipient infertility arose.
It may be asked, however, whether I suppose that, when the physiological change is subsequent, it is directly caused by change of structure, size, colour, &c., or that it arises, so to speak, accidentally, from other causes which may have affected the sexual system in the required way. To this question I may briefly reply, that, looking to the absence of any influence exercised on the reproductive systems of our domesticated plants and animals by the great and varied changes which so many of these forms present, it would seem that among natural varieties such closely analogous changes are presumably not the usual causes of the physiological change, even where the latter are subsequent to the former. Nevertheless, I do not deny that in some of these cases changes of structure, size, colour, &c., may be the causes of the physiological change by reacting on the sexual system in the required way. But in such cases free intercrossing will have prevented the perpetuation of any morphological changes, save those which have the power of so reacting on the reproductive system as to produce the physiological change, and thus to protect themselves against the full and adverse power of free intercrossing. We know that slight or initial changes of structure, colour, &c., frequently occur as varieties, and yet that on common areas very few of these varieties become distinct species: free intercrossing prevents any such further divergence of character. But if in the course of many such abortive attempts, as it were, to produce a new species, nature happens to hit upon a structural or a colour variation which is capable of reacting on the sexual system in the particular way required, then this variation will be enabled to protect itself against free intercrossing in proportion to its own development. Or, in other words, the more it develops as a morphological change, the more will it increase the physiological change; while the more the physiological change is thus increased, the more will it in turn promote the morphological. By such action and reaction the development of each furthers the development of the other, till from an almost imperceptible variety, apparently quite fertile with its parent form, there arises a distinct species absolutely sterile with its parent form. In such cases, therefore, it is still the physiological conditions which have selected the particular morphological changes capable of so reacting on the reproductive system as to produce cross-infertility, and thus to protect themselves against the destructive power of free intercrossing. So to speak, free intercrossing is always on the watch to level down any changes which natural selection, or any other cause of varietal divergence, may attempt to produce; and therefore, in order to produce—or to increase—such divergence in the absence of any other form of isolation, natural selection must hit upon such changes of structure, form, or colour, as are so correlated with the reproductive system as to create the physiological isolation that is required.
To show how the principle of selective fertility may be combined with what apparently is the most improbable form of isolation for this purpose—the geographical—I quote the following suggestion made by Professor Lloyd Morgan in his Animal Life and Intelligence:—
Suppose two divergent local varieties were to arise in adjacent areas, and were subsequently (by stress of competition or by geographical changes) driven together into a single area.... If their unions be fertile, the isolation will be annulled by intercrossing—the two varieties will form one mean or average variety. But if the unions be infertile, the isolation will be preserved, and the two varieties will continue separate. Suppose now, and the supposition is by no means an improbable one, that this has taken place again and again in the evolution of species; then it is clear that those varietal forms which had continued to be fertile together would be swamped by intercrossing; while those varietal forms which had become infertile would remain isolated. Hence, in the long run, isolated forms occupying a common area would be infertile, (p. 107.)