104 INTRODUCTION. 



hence we may look upon any inference Avliich may be 

 properly drawn from tliem here, as one of general 

 application. 



Mountain ranges, and particularly systems of moun- 

 tains, are of course among the most effective natural 

 barriers to the extension of animals on the surface of 

 the globe ; but the completeness of the effect is in pro- 

 portion to their height and continuity, and is modified 

 by the respective locomotive powers of the species 

 "which they restrain. A low range of mountains which 

 would completely intercept the progress of fishes, and 

 other animals inhabiting the waters separated by them, 

 would hardly place an obstacle in the way of quad- 

 rupeds or birds, which have the means of surmounting 

 them. A higher range, which would yet not impede the 

 birds, would begin to limit the quadrupeds. It would 

 define the range of the smaller species, and those of 

 feeble locomotive powers, which are confined by their 

 habits to the immediate neighborhood in which they 

 are born ; while the larger and more powerful species, 

 accustomed to roam over extensive tracts of country La 

 search of food or for change of climate, would scarcely 

 be checked in their migrations. It does not appear, 

 from any circumstance noticed in the condition of the 

 terrestrial moUusks, that any height of mountains which 

 could be surmounted by quadrupeds, would oppose an 

 insuperable boundary to them, or that there is anytliing 

 in their organization to prevent their compassing the 

 highest elevations, provided the other circumstances are 



