It is founded on the fact, already noticed, that certain individuals of some species possess what may be termed selective sterility--that is, while fertile with some individuals of the species they are 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.Romanes thinks that "it would not be nearly so remarkable, or physiologically improbable, that such incompatibility should run through a whole race or strain."[63] Admitting that this may be 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.Romanes thinks that they would persist, and urges that "whenever this one kind of variation occurs _it cannot escape the preserving 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 preserved_ whenever it does occur, its influence in the manufacture of specific types _must be cumulative_." The very positive statements which I have italicised would lead most readers to believe that the alleged fact had been demonstrated by a careful working out of the process in some definite supposed cases.

This, however, has nowhere been done in Mr.Romanes' 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.