"The Rocky Mountains, rising through the level of vegetable 

 barrenness to that of perpetual snow, are, at their highest eleva- 

 tions, 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 Apalachian ranges, on the other hand, of moderate eleva- 

 tion, covered for the most part to their summits with forests, and 

 presenting no limit to the support of animal life, are easily pene- 

 trated at many points through their defiles, 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 consequent diminution of tem- 

 perature, individuals are less numerous on their summits than in the 

 valleys, this effect is climatic alone." 



AMOS BINNET, 



Terrestrial Air-Breathing Mollusks of the 

 United States, vol. i. p. 106. 



28T 



