GEOGRAPHICAL DISTRIBUTION. JQJ 



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, Avhich to 

 this time, with one or t>yo 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 prvfimda, common in 

 the Central Region, but hitherto only found eastward 

 of the Alleghany Mountains in a smgle 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 Mssissippi 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 



