160 APPENDIX I DIVERGENT EVOLUTION. 



10. Social Segregation Produced by Discriminative Action of Social Instincts.* 



The law of social instinct is preference for that which is familiar in 

 one's companions; and as in most cases the greatest familiarity is 

 gained with those that are near of kin, it tends to produce breeding 

 within the clan, which is a form of segregate breeding. If the clan 

 never grows beyond the powers of individual recognition, or if the 

 numbers never become so great as to impede each other in gaining 

 sustenance, there will be but little occasion for segregation; but 

 multiplication will lead to subdivision. 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 group 

 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, often holding apart groups but very slightly differ- 

 entiated; while in the case of many animals the sexual instincts 

 of the males tend to break up these minor groups. Though the 

 barriers raised by social instincts are often broken over, their in- 

 fluence is not 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 0} 



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 pre- 

 vents the destruction of the species through the complete isolation of 

 individuals, and which cooperates with migrational instincts in secur- 

 ing dispersal without extinction, may be perfected by the accumulat- 

 ing 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 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 selection 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 crossing within the same 



* Numerals are used to designate causes of segregation not depending on human 

 purpose. Of these nine were mentioned in the section on environal segregation. 



