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Show 180 DARWINISM OUAP. Physiological Selection. Another form of infertility has been suggested by Professor G. J. Romanes as having aided in bringing about the characteristic infertility or sterility of hybrids. It is founded on the fact, already noticed, that certain individuals of some species possess !"hat may be termed selective sterility-that is, while fertile with some individuals of the species they arc sterile with others, and this altogether independently of any differences of form, colour, or structure. The phenomenon, in the only form in which it has been observed, is that of "infertility or absolute sterility between two individuals, each of which is perfectly fertile with all other individuals;" but Mr. Romancs thinks that "it would not be nearly so remarkable, or physiologically improbable, that such incompatibility should run through a whole race or strain."1 Admitting that this may bo 7. In t11e other part of the area, however, where hybridism occurs with perfect freedom, hybrids of various degrees may increa ·e till they equal or even exceed in number the pure species-that is, the incipient species will be liable to be swamped by intercrossing. 8_ The first result, then, of a partial sterility of crosses appearing in one part of the area occupied by the two form,·, will be- that the great majority of the individuals will there con~ist of the two pure forms only, while in the remaining part the. c will be in a minority,-which is the same as &'tying that the new physiological vct?·iety of the two forms will be better suited to the· conditions of existence than the remaining portion which has not varied physiologically. 9. But when thr. struggle for existence becomes severe, that variety wl1ich is best adapted to the conditions of existence always supplants that which is imperfectly adapted; therefore, by natw·al selection the varieties which arc sterile when crossed will become established as the only ones. 10. Now let variations in the amount of ste?"il'ily and in tl1e disinclin etlio11 to crossed tmions continue to occur- also in certain parts of the nren : exactly the same result must recur, and the progeny of this new physiological variety will in time occupy the whole area. 11. There is yet another consideration that would facilitate the process. ft seems probable that the sterility variations would, to some extent, concur with, and perhaps depend upon, the specific variations; so that, just in proportion as the two forms diverged and became better adapted to the conditions of existence, they would become more sterile when intercrossed. If this were theca e, then natural selection would act with double strength; aJHl those which were better adapted to survive both structurally and physiologically would certainly do so. 1 Cases of this kind are referred to at p. 155. It must, however, be noted, that such sterility in first cros .. es appears to be equally rare between ditfercnt species of the same genus as between individuals of the same species. Mules and other hybrids are freely produced between very distinct species, but are VII ON THE INFERTILITY OF CROSSES 181 so, though we have at present no evidence whatever in support of it, it remains to be considered whether such physiological varieties could maintain themselves, or whether, as in the cases of sporadic infertility already discussed, they would necessarily die out unless correlated with useful characters. Mr. Romaucs thinks that they would persist, and urges that "whenever this 011e kind of variation occurs it cannot escape the presen;ing agency of physiological selection. Hence, even if it be granted that the variation which affects the reproductive system in this particular way is a variation of comparatively rare occurrence, still, as it must always be preserve£? whenever it docs occur, its influence in the manufacture of specific types must be cumulative." The very positive statements which I have italicised would lead mo. t readers to believe that the alleged fact had been demonstrated by a. careful working out of the process in some definite suppo eel cases. This, however, has nowhere been clone in Mr. Homanes' paper ; and as it is the vital theoretical point on which any possible value of the new theory rests, and as it appears so opposed to the self-destructive effects of simple infertility, which we have already demonstrated when it occurs between the intermingled portion of two varieties, it must be carefully examined. In doing so, I will suppose that the required variation is not of "rare occurrence," but of considerable amount, and that it appears afresh each year to ahout tho same extent, thus giving the theory every possible advantage. Lot us then suppose that a given species consists of 100,000 individuals of each sex, with only the usual amount of fl~ctuating external variability. Let a. physiological variation ansc, so that 10 per cent of the whole numbcr-10,000 in lividuals of each sex-while rcma.ining fertile inte1· se become quito sterile with the remaining 90,000. This peculiarity is not correlated with any external differences of themselves infertile or quite sterile; and it is this infertility or sterility of tho hybrids that is the characteristic-and was once thought to be the criteri on -of species, not the sterility of th eir first crosse . Hell(•e we shoul<l not expect to lind any constant infertility in the fir~t crosse between the distinct strains or varieties that formed the Rtarting- point of new species, but only a slight amount of infertility in their mongt·el offspring. It follow .. , tlmt l\fr. Homanes' theory of Physiological Selectl:on-which assumes terility or infertility between tlrs~ crosses as the fumlmn ental fact in the origin of species -~loes not acconl wtth the generall•henomena of hybridism in nature, |