100 INTRODUCTION. 



individuals ; and to consider their respective impor- 

 tance, as shown by their effects upon the mdigenous 

 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. 



Greograpldcal 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 ; in other words, to 

 restrict it witliin geographical Hmits more or less defi- 

 nite, beyond which it never extends ; and hence their 

 range ap])ears 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 wliich our notice is Hmited. 



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 ]\Iexico and the Eepublic of IMexico, and on 

 the west by that State and the Pacific Ocean. It 

 extends from 67° W. to 125° W. from Greenwich, 



