SEXUAL SEGREGATION. l6l 



variety, any variation of instinct that tends to segregation will be 

 preserved by the segregation. It needs no experiments to prove that 

 if the members of a species are impelled to consort only with the mem- 

 bers of other species, they will either fail to leave offspring or their 

 offspring will fail to inherit the characteristics of the species. The 

 same is true concerning the continuance of a variety that is not some- 

 how segregated. The power of variation on the one hand, and the 

 power of divergent accumulation of variations on the other hand, are 

 prime necessities for creatures that are wresting a living from a vast 

 and complex environment ; and the former is secured by the advan- 

 tage over rivals possessed by the variations that favor crossing, and 

 the latter by the better escape from the swamping effect, and some- 

 times from the competition of certain rivals, secured by the more 

 segregative variations. We must, therefore, believe that whenever 

 in the history of an organism there arise segregative variations which 

 are able to secure sufficient sustentation and propagation to continue 

 the species, the segregative quality of the forms thus endowed will be 

 preserved and accumulated through the self-accumulated effect of the 

 segregative endowments. 



It is probable that in many of the higher vertebrates sexual in- 

 stincts tend to bring together those of somewhat divergent character, 

 but the difference preferred is within very narrow limits ; and beyond 

 those limits it may be said that the general law for sexual attraction 

 is that it varies inversely as the difference in the characters of the races 

 represented, if not inversely as some power of such difference. The 

 action of such a law is necessarily segregative whenever the diver- 

 gence has, through other causes, passed beyond the limit of higher 

 attraction. Before sexual segregation can arise, there must arise 

 distinctive characteristics by means of which the members of any 

 section may discriminate between those of their own and other 

 sections. If there are no constant characteristics there can be no 

 constant aversion between members of different groups, no constant 

 preference of those of one's own group. From this it follows that 

 before sexual segregation can arise, some form of segregation that 

 is not dependent on distinct characteristics must have produced 

 the divergence on which the sexual segregation depends. Such 

 forms are local, social, and some kinds of industrial segregation. 

 When varieties have arisen through these causes it often happens that 

 sexual segregation comes in and perpetuates the segregation which the 

 initial causes can no longer sustain. As long as the groups are held 

 apart by divergent sexual instincts, it is evident that divergent forms 

 of sexual selection are almost sure to arise, leading to a further ac- 

 cumulation of the divergence initiated by the previous causes. 



