376 EEV. J. T. GULICK ON 



tliese instincts ; it will, however, be found that Segregation is the 

 cause, or at least the necessary condition, on which the divergence 

 depends. 



In the present paper I have mentioned cases, representative of 

 multitudes of others, in which there is divergence between two 

 varieties or species occupying different districts, but surrounded 

 by the same environments. In such cases, the differences pre- 

 sented by the separate forms and the divergence by which the 

 differences have been produced, cannot be regarded as advan- 

 tageous ; for if the forms should exchange districts, the environ- 

 ment being the same, no disadvantage would be experienced ; 

 and this is equally true whether the differences relate to industrial 

 adaptations, or to adaptations between the sexual instincts and 

 the other secondary sexual characters of the group, or to characters 

 that are absolutely non-utilitarian. 



Mr. "Wallace says that, in my previous paper, he looks in vain 

 for any proof that cumulative segregation produces cumulative 

 divergence ; but at the same time, he claims that the segregation 

 of which 1 speak, and which I have illustrated by a supposed case 

 in the breeding of pigeons, is a form of selection which he calls 

 " selection by separation." Adopting his phrase for the moment, 

 I understand that he fully admits that in domestication " selection 

 by separation " will produce divergence. Does he then doubt 

 that the same process produced by natural causes will result in 

 divergence? Or does he deny that "selection by separation " 

 ever takes place in nature ? He will probably grant that where- 

 ever natural causes act upon the representatives of a species in 

 such a way that in each generation those presenting one style of 

 variation are led to breed together and are prevented from breeding 

 with other kinds, there divergence will certainly follow. This is 

 what 1 call begiegatioii. That without it there is no cumulative 

 divergence, and that with it there is always divergence, is amply 

 proved by the universal experience ol man in the domestication 

 of plants and animals. All that is lacking is the consistent 

 application of our knowledge to the theory of evolution. 



Segregation is a process of much deeper significance than 

 indiscriminate isolation, with which he seems to confound it ; and 

 one which in nature arises from a wide range of causes, some 

 of which 1 have pointed out. But isolation without assortment 

 of the forms according to any principle by w hich those of a kind 

 are brought together, is often transformed into Segregation by 



