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 codperating 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, tn order to refute 
the theory wt 1s 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 
