120 BULLETIN OF THE 



example, A b, var. c. This method is, indeed, already in more or less 

 common use. 



The division of the middle and western portions of North America 

 into faunal areas is still attended with many difficulties, partly from 

 the absence of data, and partly from the peculiarly varied character 

 of the surface. The presence or absence of forests, directly resulting 

 from the peculiarities of climate, seems to be among the most effective 

 influences in the modification of the range of species. If a nearly un- 

 broken forest extended from the Atlantic to the Pacific, with the nearly 

 uniform conditions of humidity that would naturally follow, undoubtedly 

 many species would range across the continent, or at least to the base 

 of the Rocky Mountains, that now extend westward from the Atlantic 

 coast only to the edge of the Great Plains ; the western slope of the 

 continent would differ less in its animal life from the eastern than it 

 now does, and the middle region would lack the widely different zoologi- 

 cal aspect it now presents from that of either the Atlantic or the Pacific 

 coast regions. With the present elevation of the interior, and the 

 resulting climatic conditions, nearly all the woodland species of the East 

 not only range westward to the treeless districts of the interior, but 

 extend up the rivers that descend from the central plateau as far as 

 these streams are skirted to any considerable degree with trees ; a few 

 not only reach the base of the Rocky Mountains, but pass around the 

 higher elevations of the chain in Colorado, by means of the northern 

 valleys, and occur on the Pacific coast, while others reach the same 

 coast by gaps in the mountains at the southward. On the other hand, 

 most of the field and prairie species, or those which are but slightly 

 dependent upon woodlands for shelter or sustenance, do not disappear at 

 the edge of the plains, as do the strictly woodland species, but range 

 not only over the plains of the middle region, but also over the plains to 

 the westward of the main chain of the Rocky Mountains, and thence 

 generally to the Pacific coast. In the wooded parts of the Rocky 

 Mountains a few species occur that are peculiar to that region, but the 

 greater part arc either but slightly modified forms of eastern species, or 

 forms that, while they differ widely from both the eastern and western, 

 still freely " hybridize " or intergrade with them. These strictly western 

 forms, unless of alpine or subalpine distribution, also generally occur 

 along the streams of the western edge of the Plains, as far as the streams 

 are bordered with trees. 



