SEXUAL SEGREGATION. 161 
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. 
