THROTJGH CUMULATIVE SEGREaATION. 



235 



over a given area, d]\dde 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 insincts are often broken over, their influence is not 

 wholly overcome ; and in many intstances 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 

 instinct, always tending to segregation. We also see that an 

 endowment which prevents the destruction o£ the species through 

 the complete isolation of individuals, 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 accounting for the law that 

 regulates sexual instincts ? If it can be shown that Vigour 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 im- 

 pelled 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 con- 

 cerning the continuance of a variety that is not otherwise segre- 

 gated. 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 



