It 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 the case, then natural selection would act with double strength; and those which were better adapted to survive both structurally and physiologically would certainly do so.] [Footnote 63: Cases of this kind are referred to at p.155.It must, however, be noted, that such sterility in first crosses appears to be equally rare between different 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 themselves infertile or quite sterile; and it is this infertility or sterility of the hybrids that is the characteristic--and was once thought to be the criterion--of species, not the sterility of their first crosses.

Hence we should not expect to find any constant infertility in the first crosses between the distinct strains or varieties that formed the starting-point of new species, but only a slight amount of infertility in their mongrel offspring.

It follows, that Mr.Romanes' theory of _Physiological Selection_--which assumes sterility or infertility between first crosses as the fundamental fact in the origin of species--does not accord with the general phenomena of hybridism in nature.] [Footnote 64: The exact number is 1219.51, but the fractions are omitted for clearness.].