S GRE GA TION 6 1 



their common ancestral condition was somewhat influenced 

 by the fact that the foxes east of the Atlantic Ocean were 

 unable to breed with their relatives on this continent. 

 The Rocky Mountains have been a most effective cause 

 of segregation in this country, and to their presence is 

 due probably a considerable part of the difference between 

 eastern and western forms with common ancestrv. The 



j 



fauna and flora of some of the islands off the west coast 

 of South America give us fine examples of the effects of 

 isolation. We find the species distinct from those on the 

 continent, but closely related to the latter. It is hardly 

 possible that the island forms are not different from what 

 they would have been if they had not been so separated 

 from the continental members of the species that inter- 

 breeding with the latter was impossible. Even the species 

 of the several islands within the Galapagos group are 

 different, as is well illustrated by the locusts (Fig. 9). The 

 divergence of these allied species has not been due to 

 segregation alone. The environmental conditions in the 



o o 



different areas being different, natural selection must have 

 been constantly at work to produce differences between 

 the individuals residing in the two regions. But, though 

 natural selection may have been the cause of divergence, 

 we can readily see that its results must have been mate- 

 rially affected by segregation. Segregation operates in 

 conjunction with the other factors of evolution. 



Another cause of segregation is climate. Conditions of 



O O 



drouth or of excessive humidity, of heat or cold, often 

 raise effective barriers to the migrations of both animals 



O 



and plants, and so segregate widely distributed species into 

 groups which are separate from one another so far as 



