162 Rev. J. T. Gulick on Divergent Evolution 



leads many naturalists to maintain that isolation does not 

 tend to produce divergence unless accompanied by exposure to 

 different environments. But their reasoning is inconclusive, 

 inasmuch as they have never shown that divergence depends 

 on its being advantageous. In my study of Sandwich- 

 Island mollusks I have found very strong reasons for believing 

 that divergence may arise in the representatives of one species 

 during exposure to the same environment, producing not only 

 non-adaptive but also adaptive differences. But whether 

 adaptive or non-adaptive, whether due to natural selection or 

 to some other principle, differences that arise under the same 

 environment cannot be advantageous differences, and the 

 divergence through which the differences are reached is not 

 advantageous divergence. It seems to me evident that neither 

 is divergence always advantageous, nor is the advantage of 

 access to unappropriated resources necessarily dependent on 

 divergence ; that neither does the accumulation of diver- 

 gence depend on its being advantageous, nor is advantageous 

 divergence always accumulated. 



Darwin's Theory that Exposure to different Environments 

 is Essential 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 dis- 

 similar 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 pre- 

 vention of free crossing between the different portions there 

 can be no cumulative diversity in relation to the environ- 

 ment, 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 diver- 

 gent changes in the habits of isolated portions of a species 

 exposed to the same environment, maintains that exposure 

 to different environments is essential 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. 



