GEOGRAPHICAL DISTRIBUTION. 107 
individuals are quite as numerous in the insulated val- 
leys among the mountains, and upon their slopes, as in 
the country on either side of them. This remark can- 
not of course apply to the introduced species, which to 
this time, with one or two exceptions, are confined to the 
Atlantic Region alone. There are, however, in the 
Central Region, several species which hitherto have not 
been detected in the Atlantic Region ; but this obser- 
vation, if it should continue to hold good after more 
extended investigation, may probably be explained by 
other than geographical causes. And there is at least 
one species, which, in its progress from the west east- 
ward, seems barely to have reached the confines of the 
Eastern Region. This is Helix profunda, common in 
the Central Region, but hitherto only found eastward 
of the Alleghany Mountains in a single locality, on the 
Juniata River, in Pennsylvania. Neither do our rivers 
and lakes appear to present any positive obstacle to the 
extension of species, for we do not know an instance 
where the two banks of a river exhibit any considerable 
difference in this respect, both species and individuals 
being in general equally numerous upon both sides of 
them. Even the Mississippi River, separating the coun- 
try into eastern and western sections, and nearly insulat- 
ing the eastern section lying between the Great Lakes 
and the Gulf of Mexico, has no restraining effect, and 
the Great Lakes themselves have not prevented many, 
and perhaps all, of the species common to the country on 
their southern border from extending to their northern 
