GEOGBAPHICaL DiaTBIBUTION. 659 



tropics, as a whole, being unlike those of the temperate 

 zones ; while arctic and antarctic animals have features in 

 common. Mountains serve as most important barriers, re- 

 straining animals within their limits ; thus the basins be- 

 tween or surrounded by continuous ranges of mountains 

 harbor fauns differing from those on the opposite sides of 

 the mountains. For example, the majority of the animals 

 of the Great Basin between the Eocky Mountains and the 

 Sierra Nevada differ from those of the Pacific slope or the 

 prairie lands lying east of the Rocky Mountains, as the 

 meteorological and geological features are different. The 

 Cordilleras of South America form a barrier to the diffusion 

 westward of Brazilian animals. Still this fact is not to be 

 taken too literally, as the mountains are divided by valleys 

 and rivers, which afford means of communication and an 

 interchange of specific forms ; thus certain species of ani- 

 mals of the Rocky Mountain plateau occur on each side of 

 the range, as do those in the Alleghany district of the At- 

 lantic coast. In the West Indian and especially the Hawa- 

 iian Islands, where the species of land snails are very numer- 

 ous, certain forms are restricted to the deep narrow valleys, 

 being confined to very restricted areas. So also the cold 

 Alpine summits of the White Mountains of New Hamp- 

 shire, of the Rocky Mountains, of the Alps and Scandina- 

 vian mountains harbor a few species either peculiar to those 

 extremely limited tracts or found northward in the Arctic 

 regions. 



Deserts may act much as inland seas to separate the ani- 

 mals of the adjoining more fertile tracts, and they afford 

 dwelling-places for animals which are incapable of living 

 elsewhere. Desert faunse have a general fades the world 

 over, though the original elements out of which the fauna; 

 have been made up may radically differ. 



The distribution of plants also has much to do with that 

 of those animals which are dependent on them for food ; 

 as a rule, the distribution of both plants and animals de- 

 pends on the same physical causes. 



Large rivers sometimes act as barriers, but more often, 

 perhaps, aid in the diffusion of the smaller forms, such as 



