OF THE GEOGRAPHICAL DISTRIBUTION OF GENERA AND 

 SPECIES, AND THE EXISTING CAUSES INFLUENCING 

 IT. 



The causes affecting, and the laws regulating the 

 distribution of species upon the earth, together with 

 the modifications of character produced by their influ- 

 ence, have been much discussed of late ; but the facts 

 hitherto recorded are too few in number, and the field 

 of observation is too restricted, to permit any except 

 very general inferences to be dra^vn from them. In 

 the United States especially, it would be premature to 

 attempt to define even the hmits of species, so long as 

 extensive portions of the country remain unexplored 

 by natui'alists, and so little attention is given to this 

 subject; .but, a few remarks based upon the present 

 state of our information may be hazarded, although at 

 the risk of bemg proved to be m part erroneous by fu- 

 ture investigation. We proceed, therefore, to mention 

 various causes which have been supposed to exercise 

 an influence upon the diffusion of genera and spe- 

 cies, and upon the multiplication and perfection of 



