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, indepen- 

 dent sexual selection produces segregation and divergence. 



(n) Social 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 cooperation 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 or species. When a species or a section of a species 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 thejnherited instincts of the race are brought into increasing 



