The Monist, Vol. 1, 1890-1891

by Various

It is now time to show, by means of quotations, how unequivocal and complete is Mr. Wallace's adoption of our theory:

"The simplest case to consider will be that in which two forms or varieties of a species, occupying an extensive area, are in process of adaptation to somewhat different modes of life within the same area. If these two forms freely intercross with each other, and produce mongrel offspring which are quite fertile inter se, then the further differentiation of the forms into two distinct species will be retarded, or perhaps entirely prevented; for the offspring of the crossed unions will be, perhaps, more vigorous on account of the cross, although less perfectly adapted to the conditions of existence than either of the pure breeds; and this would certainly establish a powerful antagonistic influence to the further differentiation of the two forms.

"Now, let us suppose that a partial sterility of the hybrids between the two forms arises, in correlation with the different modes of life and the slight external or internal peculiarities that exist between them, both of which we have seen to be real causes of infertility. The result will be that, even if the hybrids between the two forms are still freely produced, these hybrids will not themselves increase so rapidly as the two pure forms; and as these latter are, by the terms of the problem, better suited to their conditions of life than are the hybrids between them, they will not only increase more rapidly, but will also tend to supplant the hybrids altogether whenever the struggle for existence becomes exceptionally severe. Thus, the more complete the sterility of the hybrids the more rapidly will they die out and leave the two parent forms pure. Hence it will follow that, if there is greater infertility between the two forms in one part of the area than the other, these forms will be kept more pure wherever this greater infertility prevails, will therefore have an advantage at each recurring period of severe struggle for existence, and will thus ultimately supplant the less infertile or completely fertile forms that may exist in other portions of the area. It thus appears that, in such a case as here supposed, natural selection would preserve those portions of the two breeds which were most infertile with each other, or whose hybrid offspring were most infertile; and would, therefore, if variations in fertility continued to arise, tend to increase that infertility. It must particularly be noted that this effect would result, not by the preservation of the infertile variations on account of their infertility, but by the inferiority of the hybrid offspring, both as being fewer in numbers, less able to continue their race, and less adapted to the conditions of existence than either of the pure forms. It is this inferiority of the hybrid offspring that is the essential point; and as the number of these hybrids will be permanently less where the infertility is greatest, therefore those portions of the two forms in which infertility is greatest will have the advantage, and will ultimately survive in the struggle for existence."

We have here a full acceptance of the theory of physiological selection. For it is represented, as Mr. Gulick and I have represented, that, if "two forms or varieties" occupying a common area are to undergo further differentiation at the hands of natural selection, it becomes a highly favoring condition to the process that some degree of segregate fecundity should arise (if it has not already arisen) between these two forms or varieties; seeing that "if these two forms freely intercross with each other, and produce mongrel offspring which are quite fertile inter se, then the further differentiation of the forms into two distinct species will be retarded, or perhaps entirely prevented." Here the importance of segregate fecundity, or physiological selection, as a factor in the differentiation of specific types on common areas is fully recognised; and the only respect in which Mr. Wallace alleges that his view of the matter differs from the view of Mr. Gulick and myself, is in drawing special attention to the part which is played by the infertility, or other "inferiority," of the mongrels. But clearly, this infertility, or other inferiority, of the mongrels, in all cases where it occurs, is part and parcel of the segregate fecundity of the parent forms. Whether the segregate fecundity has reference to first crosses alone, or likewise to second crosses, it is segregate fecundity all the same; and the only difference is that for the same degree of segregate fecundity in first crosses, the process of physiological selection will become the more effective in proportion to the degree in which the infertility extends also to second crosses. But I think it is very doubtful whether such infertility (or inferiority) on the part of mongrels can react upon the sexual system of their parent forms, so as directly to increase whatever degree of segregate fecundity may have already arisen between these forms. Does the high sterility of mules and mutes, for instance, tend to diminish the degree of fertility that obtains between horses and asses? The only way in which even an absolute degree of sterility (or other inferiority) on the part of mongrels or hybrids may clearly be seen to operate in this direction, is as a negative condition; not as an active cause. In the proportion that mongrels are impotent with one another, they will not so much compete with their parent forms for food, etc.; and in the proportion that they are impotent with their parent forms, they will not counteract any tendency which the latter may continue to develop in the direction of a still further segregation. If the mongrels are fully vigorous and fully fertile, both inter se and with their parent forms, the effect will be to retard, if not altogether to prevent, any further progress of physiological separation between the parent forms; because the free intercrossing of the mongrels with one another, and also with their parent forms, will be continually supplying progeny in which the physiological peculiarity is either attenuated or altogether abolished. But this is quite a different thing from supposing that infertility (or inferiority) of the mongrels can react upon the generative system of the parent forms, so as to increase in them the physiological peculiarity on which their segregate breeding depends: infertility (or inferiority) of the mongrels is but a negative condition which favors the preservation of further degrees of this segregate breeding, if such further degrees should be induced by any other causes.

Now, it does not appear that Mr. Wallace has clearly perceived this important distinction, because he throughout speaks of "this inferiority of the hybrid offspring as the essential point." Obviously, however, the essential point is the physiological variation in the parent forms, i. e., the original occurrence and subsequent development of infertility between the first crosses. Granting to Mr. Wallace, for the sake of argument, that this development could not proceed at all, were it not for the inferiority of the mongrels; still the inferiority of the mongrels need not be the cause of this development. Therefore it is most incorrect to say, "it must be particularly noted that this effect (i. e., increase of infertility between the parent forms) would result, not by the preservation of the infertile variations on account of their infertility, but by the inferiority of the hybrid offspring." "This effect" must be due to causes which act upon the generative systems of the parent forms, even though such causes might be counteracted by the withdrawal of the negative condition in question.

I trust, then, it has now been rendered sufficiently clear that, no matter how infertile the hybrid progeny may become, and no matter at how great a disadvantage they may thus (or otherwise) be placed in their struggle for existence with the parent varieties, it is not apparent that their infertility (or their extinction) can ever become the cause of a further increase of infertility arising between their parent forms. Consequently, although this is the cause assigned by Mr. Wallace, when he comes to "the essential point" of showing how it is to act so as to increase cross-sterility between the parent forms, he naïvely substitutes the sentence which I have printed in italics—which assumes a "greater infertility between the two forms" as arising through any other causes that we may choose to suppose. The very thing that his entire argument professes to explain (i. e., the rise and development of cross-sterility between the parent varieties) is slipped in as granted, or given by other causes than those which are said to explain it.[6]

[6] The only conceivable way in which infertility (or other inferiority) of hybrids could react on the sexual system of their parent forms, is one which Mr. Wallace appears to have missed: at all events he has nowhere stated it. This way is as follows. Suppose A and B to be two varieties which produce comparatively infertile hybrids. In the proportion that the hybrids are infertile, or otherwise inferior, it must be a disadvantage to both varieties for individuals belonging to one to cross with individuals belonging to the other, because by so doing they are wasting their time and their energy in propagating comparatively poor offspring—thereby failing to impress their characters on the next generation as effectually as they might have done by pairing homogamously. Hence, those individuals which do pair homogamously will leave a larger number—or better quality—of offspring to the next generation, than is left by those which fail to pair homogamously. Hence, also, in the course of many generations a selective premium will be set on the homogamous pairing, A plus A, B plus B, whether such pairing be due to a sexual instinct or to a sexual incompatibility. For example, if horses and asses were to occupy the same area for a sufficient length of time, it is conceivable that the instinct which many horses now present of preferring asses to their own kind would become obsolete; because the horses or mares which have such an instinct would always fail to leave progeny that could transmit it, while such would not be the case with the horses and mares which preferred to pair homogamously, and so it might be if a physiological instead of a psychological character were concerned. But now observe, if this consideration were adduced, I should not be concerned to dispute it. For, even if such a principle of segregation does obtain, to what category does the principle belong? Clearly it does not belong to natural selection, inasmuch as a mere failure to impress individual characters on the next generation is not a matter of life and death in the struggle for existence. But, no less clearly, it does belong to physiological selection; and therefore, if it be an active principle in nature, it is an additional cause of segregate fecundity in first crosses. Moreover, such a principle, if it ever acts, presupposes some considerable degree of sexual differentiation as already given by some other cause.

Having thus endeavored to make it as clear as I can, that the causes of segregate fecundity, both in its origin and subsequent "increase," must be causes acting on the physiology of the segregating forms themselves, and not the effects of these causes in the character of their mongrel offspring; I must next comment upon the extraordinary idea which underlies the whole of Mr. Wallace's exposition, and which in one place he expressly states. This extraordinary idea is that the theory of physiological selection, as held both by Mr. Gulick and myself, takes no cognizance of the possible effects of cross-sterility in leading to infertility or inferiority on the part of mongrel progeny. I call this an extraordinary idea, because it appears to me most extraordinary that Mr. Wallace can have read our papers, and then have supposed that he was adding anything to our theory by arguing the points which he does argue in the above quotation. When once this argument is correctly stated, it amounts, as we have just seen, to nothing more than pointing out how a segregate fecundity of first crosses will have a better chance of increasing, if the mongrel progeny are infertile or inferior. But surely this goes without saying; or, if it be said, let it be added that physiological selection, when it thus extends to second crosses, is really or ultimately due to physiological selection as regards the first crosses. If the segregate fecundity of the first crosses is of such a kind, that, besides tending to a physiological isolation of the parent forms, it leads to inferiority of the mongrel progeny; this is merely a further expression of the segregate fecundity in question. Its effect is that of so far extinguishing the influence of progeny in the subsequent history of parental segregation: therefore, its effect is just the same as if, owing to a somewhat higher degree of segregate fertility in the first instance (i. e., in the first crosses), a proportionately smaller number of mongrel offspring had been produced at all. In either case the result (physiological differentiation) is equally due to causes acting on the sexual system of the parent forms; and whether this effect is brought about by a suppression of progeny as to their numbers alone, or likewise as to their efficiency, is quite immaterial to the theory of physiological selection. Which shows once more how wide of the mark is Mr. Wallace's statement, that "the inferiority of the hybrid offspring is the essential point" in any process of sexual segregation. The "essential point" must always be the original occurrence and subsequent "preservation of the infertile variations" arising between the parent forms, whether these variations are only in the direction of producing a smaller number of mongrels, or also in that of suppressing their efficiency when produced.

Upon the whole, then, it is surely the oddest of misconceptions on Mr. Wallace's part that has led him to present the above-quoted "argument" as a substitute for the theory of physiological selection. As far as it goes, and as far as it is sound, it is the theory of physiological selection pure and simple—neither adding to, nor detracting from it one iota. Nevertheless, the "argument" has not yet gone far enough to embody some of the other elements of the theory. Therefore I will now continue the quotation:

"The differentiation of the two forms into distinct species, with the increase of infertility between them, would be greatly assisted by two other important factors in the problem. It has already been shown that, with each modification of form and habits, and especially with modifications of color, there arises a disinclination of the two forms to pair together; and this would produce an amount of isolation which would greatly assist the specialisation of the forms in adaptation to their different conditions of life. Again, evidence has been adduced that change of conditions or of mode of life is a potent cause of disturbance of the reproductive system, and, consequently, of infertility. We may therefore assume that, as the two forms adopted more and more different modes of life, and perhaps acquired also decided peculiarities of form and coloration, the infertility between them would increase or become more general; and as we have seen that every such increase of infertility would give that portion of the species in which it arose an advantage over the remaining portions in which the two varieties were more fertile together, all this induced infertility would maintain itself, and still further increase the general infertility between the two forms of the species."