306 DIVERGENT EVOLUTION THROUGH SEGREGATION. 
numbers never become so great as to impede each other in gaining sus- 
tenance, there will be but little occasion for segregation; but maul- 
tiplication will lead to segmentation. Wherever the members of a 
species, ranging freely over a given area, divide up into separate herds, 
flocks, or swarms, of which the members produced in any one clan 
breed with each other more than with others, there we have social 
segregation. 
It should always be. kept in mind that social segregation arises at a 
very early stage, holding apart groups not at all or but very slightly 
differentiated; while in the case of many animals, the eager sexual 
instincts of the males constantly tend to break up these minor groups. 
Though the barriers raised by social instincts are often broken over, 
their influence isnot wholly overcome; and in many instances the social 
segregation becomes more and more pronounced, till in time decided 
sexual segregation comes in to secure and strengthen the divergence. 
11. Sexual segregation is produced by the discriminative action of 
sexual instincts. 
There can be no doubt that sexual instincts often differ in such a 
way as to produce segregation. But how shall we account for these 
differences? In the case of social segregation there is no difficulty, for 
it seems to be, like migration, due to a constant instinet, always tend- 
ing to segregation. We also see that an endowment which prevents 
the destruction of the species through the complete isolation of indi- 
viduals, and which co-operates with migrational instincts in securing 
dispersal without extinction, may be perfected by the accumulating 
effects of its own action. And is there any greater difficulty in ae- 
counting for the law that regulates sexual instincts? If it ean be 
shown that vigor and variation, the conditions on which adaptation 
depends, are in their turn dependent on some degree of crossing, there 
will be no difficulty in attributing the development of an instinct that 
secures the crossing to the superior success of the individuals that 
possess it in even a small degree. On the other hand, whenever there 
arises a variety that can maintain itself by crosses within the same 
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 
members 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 
otherwise 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 ad- 
vantage over rivals possessed by the variations that favor crossing, 
and the latter by the better eseape from the swamping effect, and 
sometimes from the competition of certain rivals, secured by the more 
