146 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 bui slightly restrained, 

 by many of the causes which, to the inferior classes, 

 constitute insurmountable obstacles ; and consequently, 

 the geographical space wliich 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 be divided and subdivided into others of more 

 limited extent, which shall represent the more Umited 

 spheres of the less diffused species. It follows, also, that 

 within each of these minor spheres or zoiilogical 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 appljdng 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 



