100 
INTRODUCTION. 
individuals ; and to consider their respective impor- 
tance, as shown by their effects upon the indigenous 
species of land mollusks. It is necessary to pre- 
mise, however, that our remarks on these causes are 
derived solely from observation in this country, and 
relate only to their influence on the species existing 
here. 
Geographical features of the country. The effect 
of the combined influences which determine the range 
of these animals is, to confine each species to a cer- 
tain natural station or habitation ; hi other words, to 
restrict it within geographical limits more or less defi- 
nite, beyond which it never extends ; and hence their 
range appears to be very closely dependent upon the 
geographical character of the country. It is proper 
therefore to inquire in the first place, how far they are 
in reality restrained by merely geographical causes, 
and whether the geographical features of the -country 
exercise of themselves a positive influence on their 
distribution; and in order to a correct understanding 
of the subject it seems to be necessary to give a brief 
sketch of the principal geographical outlines of the 
region to which our notice is limited. 
The territory of the United States is bounded on 
the north by the Great Lakes and British America, on 
the east by the Atlantic Ocean, on the south by the 
Gulf of Mexico and the Republic of Mexico, and on 
the west by that State and the Pacific Ocean. It 
extends from 67° "W. to 125° W. from Greenwich, 
