332 EEV. J. T. gtjltck: on 



.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 differences 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 develop- 

 ment of such distinctions, it is easy to believe that they will be 

 developed in different 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 exagge- 

 rating fancies of the other sex, and are thus first preserved 

 through isolation and then exaggerated by Sexual Selection. In 

 other words, Independent Sexual Selection prodxices Segregation 

 and Divergenee. 



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 co-operation in securing sustentation and 

 defence. 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 different forms of 

 social organization are often exhibited by tlie same or closely 

 allied species ; and we find that, in such cases as elsewhere, the 

 prosperity of the individual is largely dependent on his con- 

 formity to the social organization to which he belongs. Social 

 Selection must, therefore, 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 



