104 INTRODUCTION. 
hence we may look upon any inference which may he 
properly drawn from them 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 glohe ; 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 in 
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 mollusks, that any height of mountains which 
could be surmounted by quadrupeds, would oppose an 
insuperable boundary to them, or that there is anything 
in their organization to prevent their compassing the 
highest elevations, provided the other circumstances are 
