HABITS AND FACULTIES. ^9^ 
open country, where they seek such shelter as they can 
find. This transition is very slow, but there are suffi- 
cient indications, in the exceptions which are found to 
the general habits of the species in this particular, to 
show that it is going on ; and therefore, it is reasonable 
to believe, that when a period shall have elapsed as long 
as that since the south and west of Europe were cov- 
ered with forests, our species will have become able to 
sustain themselves in the open country, and will have 
spread themselves in great numbers over those populous 
parts where they are now wanting. The power of 
adaptation to new circumstances, which is a prominent 
quality of nearly all the shell-bearing species of this 
order, and which, combined with a remarkable tenacity 
of life, enables them to resist successfully the many 
dangers to which they are exposed, is illustrated in the 
extremes of their mode of life on the two continents. 
W e know of no other instances of animals living in a 
natural condition, not domesticated nor accompanying 
man, where the same diversity of habitat in analogous 
species exists. The presumption of changes wdiich shall 
approximate the habits of both, in proportion as the 
physical circumstances of both approach each other, is 
therefore not a violent one. But it is by no means cer- 
tain that all the species will survive the violent change 
to which they are at first exposed. Those of them which 
are in a state of decline and nearly run out, and those 
which are strictly local in their habitats will be least 
able to sustain themselves, and their entire extinction 
will be very likely to follow. 
