1914] Grinnell: Mammals and Birds of the Colorado Valley 97 



THE COLORADO RIVER AS A HIGHWAY OF DISPERSAL 

 AND CENTER OF DIFFERENTIATION OF SPECIES 



According to Gilbert and Scofield (1898, pp. 487, 488) the peculi- 

 arities of the fish fauna of the Colorado River bespeak a very long 

 period of absolute isolation. A remarkably high percentage of its 

 fishes are specifically distinct from those of the other river basins of 

 western North America. The same is to be said of the riparian birds 

 and mammals. 



So far as known to the present writer, none of the species listed 

 with a star in the following table ranges beyond the confines of the 

 Colorado River basin, including of course its various tributaries and 

 distributaries such as the Gila and New rivers, except sporadically, 

 or as accounted for by distal invasion through passes or along sea- 

 coastal tracts. In other words, the Colorado River has been in existence 

 so long that the conditions imposed by its presence have figured in 

 the differentiation of representative species of several families, both 

 mammalian and avian. 



The great age of the Colorado River is indicated geologically by 

 the vast extent and slow rate of the erosion involved in the formation 

 of the Grand Canon. This time-element is justly inferred to have 

 been an essential condition in the formation of these species. 



Species which Belong to the Riparian Belt, and which are Therefore 



Hemmed in by the Paralleling Desert Tracts. Starred 



Species are Peculiar to the Colorado Subpauna 



BREEDING BIRDS MAMMALS 



Agelaius phoenieeus sonoriensis* Castor canadensis frondator* 



Melospiza melodia saltonis* Peromyscus manieulatus sonoriensis 



Pipilo aberti* Sigmodon hispidus eremieus* 



fiuiraea caerulea lazula Reithrodontomys megalotis deserti 



Piranga rubra cooperi Neotoma albigula venusta* 



Vireo belli arizonae* Ondatra zibethiea pallida* 



Vermivora luciae* Mephitis estor 



Dendroica aestiva sonorana* Procyon pallidus* 

 Toxostoma crissale* 



It may not be amiss to consider these riparian species somewhat with 

 regard to origin. By confining our attention to the north-and-south 

 valley of the lower Colorado, this becomes, with a knowledge of the 

 general status in North America of each group represented, a com- 



