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INTRODUCTION. 
the utmost importance in the consideration of this sub- 
ject. The facts, in relation to the zoological influence 
of the two systems of mountains within the United 
States, accord fully with these views. The Rocky 
Mountains, rising through the level of vegetable bar- 
renness to that of perpetual snow, are, at their highest 
elevations, unsuitable to the existence and support of 
animal life ; and constitute a barrier impenetrable to 
nearly every class of animals. The country westward 
of those mountains is therefore separated zoologically, 
as well as geographically, from that eastward of them ; 
the species common in the more eastern divisions are 
there replaced by other and different forms ; and it is 
thus a distinct zoological region. The Appalachian 
ranges on the other hand, of moderate elevation, cov- 
ered for the most part to then- summits with forests, 
and presenting no limit to the support of animal life, 
are easily penetrated at many points through their de- 
files, and present but few obstacles to the extension of 
species. They constitute no zoological barrier to the 
land-mollusks, although they do to some other animals ; 
and if, owing to their altitude and the consequent di- 
minution of temperature, individuals are less numerous 
upon their summits than in the valleys, this effect is 
climatic alone. Of the native species inhabiting the 
Atlantic Region, with the exception of those of the ter- 
tiary section in the south and south-eastern parts of it, 
nearly all have been noticed in the Central Region ; 
and, so far as observation extends, both species and 
