GEOGRAPHICAL DISTRIBUTION 



317 



readily pass. On the other hand, the fauna of California or 

 Colorado differs materially from that of the adjoining regions, 

 because the mountainous country is full of barriers which 



obstruct the diffusion of life. Distinctness is in direct propor- 

 tion to isolation. What is true in this regard of the fauna of 

 any region is likewise true of its individual species. The degree 



