Evidences of Physiological Selection. 65 



absent, the logical foundations of the theory of 

 physiological selection would seem to be securely laid. 

 We may therefore pass to more special lines of 

 evidence. 



Evidence from Geographical Distribution. 



Darwin has adduced very good evidence to show 

 that large areas, notwithstanding the disadvantages 

 which (on his theory) must arise from free inter- 

 crossing, are what he terms better manufactories of 

 species than smaller areas, such as oceanic islands. 

 On the other hand, as a matter of fact, oceanic 

 islands are comparatively rich in peculiar species. 

 These two statements, however, are not incompatible. 

 Smaller areas are, as a rule, rich in peculiar species 

 relatively to the number of their inhabitants ; but 

 it does not follow that they are rich in species as 

 contrasted with larger areas containing very many 

 more inhabitants. Therefore, the rules are, that 

 large areas turn out an absolutely greater number 

 of specific types than small areas ; although, relatively 

 to the number of individuals or amount of population, 

 the small areas turn out a larger number of species 

 than the large areas. 



Now, these two complementary rules admit of 

 being explained as Darwin explains them. Small 

 and isolated areas are rich in species relatively to 

 the amount of population, because, as we have before 

 seen, this population has been permitted to develop 

 an independent history of its own, shielded from 

 intercrossing with parent forms, and from competition 

 with exotic forms; while, at the same time, the 

 homogamy thus secured, combined with change of 



III. F 



