190 INTRODUCTION. 



distant period, and all the terrestrial moUusks were then, 

 like ours at the present time, living in the forest. The 

 progress of agriculture there was verj slow compared 

 with its advances in this country, and thus time was 

 given to the animals to accustom themselves to the 

 change ; and thej have thus, by slow degrees, adopted 

 their present habits. In the United States, the advance 

 of agriculture in newly settled parts is very rapid ; 

 large tracts of forest are almost simultaneously subjected 

 to the axe and to fire, and a very few years produces an 

 entire change in the vegetation of a whole section. 

 Consequently these animals are at once exterminated, 

 or the few that survive are brought suddenly under the 

 influences of new circumstances, which, from the abrupt- 

 ness of the change, are fatal to them, but which, if 

 imposed upon them more gradually, might have been 

 sustained. A few spots and some limited tracts of land, 

 remaining unchanged, in the midst of cultivation, protect 

 some individuals of every species ; and it is from this 

 comparatively small number, thus preserved, that their 

 subsequent increase is derived. But, at this period, the 

 field is equally open for the multiphcation of those 

 foreign species which accompany man as for the native 

 species, and it is not surprismg that the former, whose 

 habits are already adapted to the existing state of things, 

 should increase more rapidly than the latter. The native 

 species however, become gradually familiarized with the 

 circumstances around them, and some few of them 

 advance, and after a time establish themselves in the 



