HABITS AND FACULTIES. 191 



open country, Avhere 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 

 quahty 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 Hfe on the two contuients. 

 We 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 which 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 Avill survive the violent change 

 to which they are at first exposed. Those of them which 

 are in a state of declme and nearly run out, and those 

 which are strictly local in then- habitats will be least 

 able to sustain themselves, and their entire extinction 

 will be very likely to follow. 



