CUMULATIVE SEGREGATION AND DIVERGENCE. 239 



produce divergence. Does he then doubt that the same process pro- 

 duced 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 wherever natural causes act upon the repre- 

 sentatives 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 cer- 

 tainly follow. This is what I call "segregation." That without it 

 there is no cumulative divergence, and that with it there is always 

 divergence, is amply proved by the universal experience of 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 indis- 

 criminate isolation, with which he seems to confound it, and one 

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

 have pointed out. But isolation without assortment of the forms 

 according to any principle by which those of a kind are brought to- 

 gether is often transformed into segregation by the operation of the 

 principles of transformation in the isolated sections of the species. 

 This change is often brought about by the difference of the environ- 

 ments to which the organism is exposed in the isolated areas. This 

 one form of segregation has been clearly pointed out by Darwin, 

 though he did not recognize segregation as a necessary condition for 

 divergence. There are, however, many other ways in which nature 

 produces a similar result. Some of these are operative when the 

 organism is distributed in isolated districts but surrounded by the 

 same environment, and some of them have to do with the develop- 

 ment of non-adaptative divergences, which can not come under the 

 cumulative influence of natural selection. 



It thus appears that independent generation cooperating with 

 natural selection is one form of the wider principle of segregation 

 which, in its many forms, is the ever-present condition preceding 

 cumulative divergence. Whatever divides the representatives of a 

 species in such a way that those of a kind are made to intergenerate 

 while prevented from intergenerating with other kinds is a cause of 

 segregation. This is my definition of segregation, and my theory is 

 that whatever causes segregation causes divergence, and without 

 segregation there is no cumulative divergence. Now, in order to refute 

 the theory it is necessary to show either that segregation does not take 

 place in nature or that it is not accompanied by divergence, or that diver- 

 gence takes place without segregation. As Mr. Wallace has not at- 

 tempted to prove any one of these counter propositions, I think his 



