202 APPENDIX II—INTENSIVE SEGREGATION. 
the same respects between those of the same sex in closely allied varie- 
ties and species, and no clear understanding of the subject will ever 
be reached till those who study and discuss the subject discriminate 
between these two classes of phenomena. The formation of differ- 
ences of the former kind is simple transformation without divergence, 
while the entrance of differences of the latter kind is divergent evolu- 
tion tending to the production of separate species. 
If a species deficient in secondary sexual distinctions, after being 
divided into segregated sections, attains a high development of such 
distinctions, it is easy to believe that they will be developed in differ- 
ent ways in the different sections, and that thus they will become 
specific distinctions; but it is not so easy to see why a species in which 
sexual distinctions have already been fully developed should undergo 
divergent changes in the different sections into which it may be 
divided. It is in such cases that we discover the important influence 
of what I have called unstable equilibrium. It seems probable that in 
some cases small differences originating through indefinite variation 
in only a few isolated individuals are seized upon by the exaggerating 
fancies of the other sex, and are thus first preserved through isolation 
and then exaggerated by sexual selection. In other words, zndepen- 
dent sexual selection produces segregation and divergence. 
(11) Soczal selection is the exclusive breeding of those better fitted 
to the social constitution and instincts of the race through the failure 
to breed of those less fitted. Social organization has reference 
chiefly to codperation in securing sustentation and defense. If for 
each species there were but one possible form of social organization 
through which sustentation could be secured, there would be no need 
of considering social selection, for the form of social organization 
would be rigorously determined by natural selection, and the success 
of the individual through conformity to that organization would be 
sufficiently explained by the principle of natural selection. But dif- 
ferent forms of social organization are often exhibited by the same or 
closely allied species; and we find that, in such cases as elsewhere, the 
prosperity of the individual is largely dependent on his conformity to 
the social organism to which he belongs. Social selection must, there- 
fore, in some cases, have been an important factor in maintaining a 
correspondence between the capacities and the social organization of a 
race orspecies. Whenaspecies or asection of aspecies is undergoing 
a change of social habits, there will be individuals that fail through 
reverting to the old instincts and methods which put them out of 
accord with the rest of the community. But through the failure of 
these the inherited instincts of the race are brought into increasing 
