Gvliok — Divergent Involution andtht Darwinian Theory. 27 



Darwin's Theory that Exposure to Different Environments is 

 Ess* ntial to diversity of Natural Selection, 



Diversity of natural selection in different portions of the 



same species depends upon diversity in the relations of the 

 different portions to the environment. Now, observation 

 shows that cumulative diversity in the relations of the species 

 to the environment may be introduced, (1) by dissimilar 

 changes in the environment presented by the different areas 

 occupied by the different portions; (2) by different portions of 

 the species entering different environments; or (3) by dissimi- 

 lar changes in the habits of the different portions of the species 

 in using the same environment. Certainly in this third class 

 of cases, if not in the other classes, without prevention of free 

 crossing between the different portions, there can be no cumu- 

 lative diversity in relations to the environment, and therefore 

 no cumulative diversity in the natural selection ; and without 

 the same condition, there can be no accumulation of divergent 

 effects of natural selection, in any case. Darwin, however, 

 forgetting the possibility of divergent changes in the habits 

 of isolated portions of a species exposed to the same environ- 

 ment, maintains that exposure to different environments is 

 ntial to diversity of natural selection and to divergence. 

 Without change in the climate, soil, or organic forms lying 

 outside of the species there is, according to him, nothing to 

 produce modification. 



11 If a number of species after having long competed with 

 each other in their old home were to migrate in a body into a 

 new and afterwards isolated country, they would be little liable 

 to modification ; for neither migration nor isolation in them- 

 selves effect any thing. These principles come into play only 

 by bringing organisms into new relations with each other, and 

 in a lesser degree with the surrounding physical conditions." — 

 [Origin of Species. On the 4th and 5th pages of the first 

 chapter on Geographical Distribution.]* " Each separate island 

 of the Galapagos Archipelago is tenanted, and the fact is a 

 marvelous one, by many distinct species ; but these species are 

 related to each other in a very much closer manner than to the 

 inhabitants of the American continent, or of any other quarter 

 of the world. This is what might have been expected, for 

 islands situated so near each other would almost necessarily 

 receive immigrants from the same original source, and from 

 each other. But how is it that many of the immigrants have 

 been differently modified, though only in a small degree, in 

 islands situated within sight of each other, having the same 

 geological nature, the same height, climate, etc. ( This long 



* See ed. 6, p. 319. 



