THROUGH CUMULATIVE SEGREGATION- 
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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 he 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 prouounced, 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 of 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 wffiich 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 wrestiug a living from a 
