GEOGRAPHICAL DISTRIBUTION. 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 faune differing from those on the opposite sides of 
the mountains. For example, the majority of the animals 
of the Great Basin between the Rocky 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 faune have a general facies the world 
over, though the original elements out of which the faune 
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 
