14t> INTRODUCTION. 
the quadruped that seeks its food, according to the 
change of seasons, throughout the greater part of a con- 
tinent, and the bird which, guided by its instinct, spends 
its summers in the polar regions, and its winters between 
the tropics, are subjected to very different laws of dis- 
tribution from the insect whose range is often strictly 
local, or the mollusk, whose limits are defined by the 
causes we have described. The higher classes of ani- 
mals, indeed, are unaffected, or but slightly restrained, 
by many of the causes which, to the inferior classes, 
constitute insurmountable obstacles ; and consequently, 
the geographical space which they respectively occupy, 
or the circles within which they habitually move, are of 
very different extent. It would seem to result, from this 
reasoning, that, in seeking to ascertain and define the 
various zoological regions, we must make a distinct 
apportionment for each distinct class of animals ; and 
that the spacious regions ranged by the higher animals, 
must he divided and subdivided into others of more 
limited extent, which shall represent the more limited 
spheres of the less diffused species. It follows, also, that 
within each of these minor spheres or zoological sections, 
the original focus of all the species contained within it, 
must have been located. We believe that these ideas 
will be found to be consistent with facts everywhere 
observed. In applying them to North America, we find, 
that its temperate parts are considered to constitute a 
peculiar zoological region, characterized, among other 
animals, by the bison among quadrupeds, and the wild 
