IN ORGANIC EVOLUTION 85 



favor reinvasion, and each group will have sortie 

 representatives in all available locations. The 

 other fact is that the forms in the small area tend 

 to be more sharply defined. They are better spe- 

 cies, from the point of view of taxonomy, and the 

 causes of their existence can be better traced. In 

 our current studies of evolution, we are of neces- 

 sity more analytical than Darwin. We would 

 view as separate factors elements which to him 

 were simply phases of Natural Selection. In 

 artificial selection, segregation or isolation was 

 taken by Darwin for granted. Natural Selec- 

 tion was to Darwin the same cause or factor re- 

 lated to natural processes. In his chapter on 

 Geographical Distribution, Darwin shows an 

 essentially modern grasp of the subject, though 

 without analysis of the reasons why variations in 

 distribution naturally persist. 

 Darwin says : — 



" The preservation of favorable variations and the 

 rejection of injurious variations, I call Natural Selec- 

 tion. Variations neither useful nor injurious would be 

 unaffected by natural selection, and would be left a 

 fluctuating element." 



It is clear that the completed process of Nat- 

 ural Selection as here indicated implies segrega- 

 tion also, especially if we are to explain how those 

 forms bearing " fluctuating elements " are to be 

 coordinated as species. It is, moreover, certain 

 that in most groups, probably in all, the charac- 

 ters that distinguish species are these elements,. 



