160 APPENDIX I—DIVERGENT EVOLUTION. 
10. Soctal Segregation Produced by Discriminative Action of Social Instincts.* 
The Jaw 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 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 pre- 
vents the destruction of the species through the complete isolation of 
individuals, and which codperates 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. 
