190 INTRODUCTION. 
distant period, and all the terrestrial mollusis -were then, 
like ours at the present time, living in the forest. The 
progress of agriculture there was very slow compared 
with its advances in this country, and tints time was 
given to the animals to accustom themselves to the 
change; and they have thus, by shra degrees, adopted 
then- 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 multiplication of those 
■u species which accompany man as for the native 
species, and it is not surprising that the former, whose 
habits are already adapted to the existing state of t 1 
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 
