v. 
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 g%neral inferences to be drawn from them. In 
the United States especially, it would be premature to 
attempt to define even the limits of species, so long as 
extensive portions of the country remain unexplored 
by naturalists, 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 being proved to be in 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 
