8. The first result, then, of a partial sterility of crosses appearing in one part of the area occupied by the two forms, will be--that the great majority of the individuals will there consist of the two pure forms only, while in the remaining part these will be in a minority,--which is the same as saying that the new _physiological variety_ 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 the struggle for existence becomes severe, that variety which is best adapted to the conditions of existence always supplants that which is imperfectly adapted; therefore, _by natural selection_ the _varieties_ which are _sterile_ when crossed will become established as the only ones. 10. Now let variations in the _amount of sterility_ and in the _disinclination to crossed unions_ continue to occur--also in certain parts of the area: 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. 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.] CHAPTER VIII THE ORIGIN AND USES OF COLOUR IN ANIMALS