1914] Grin nell: Mammals and Birds of the Colorado Valley 105 



inheritance of characters obtains among the rodents here concerned is 

 yet to be proven. 



Suffice it to say that all the evidence at hand shows the Colorado 

 River to have effectually blocked distribution, in the two directions 

 concerned in the eight cases as listed. While this hindrance to distri- 

 bution involves the species, it does so through its mechanical action 

 upon the frontier individuals of each species. Hypothetically the 

 invaders are severally hurled back or else destroyed outright. 



The divergent characters displayed by the upland rodents of the 

 two sides of the Colorado River are, in the mind of the writer, to be 

 bast explained on historical grounds. It is not necessary to believe that 

 the specific characters concerned arose in the immediate vicinity of 

 the river, though the circumstance of segregation alone is deemed by 

 some to suffice as a cause of differentiation. The climatic features 

 (zonal and faunal, as well as a.ssociational) are identical on the two 

 sides of the river. Rather is it reasonable to presuppose separate and 

 rather remote centers of differentiation, and convergent dispersal 

 through time and space which brought the resulting types to the 

 verge of the river, beyond which they were unable to spread. 



It is possible that an arm of the sea continuous with the Gulf of 

 California once extended northward into southern Nevada. A sub- 

 mergence of only 1,000 feet would divide the present desert areas of 

 western Arizona and southern California into two peninsular land 

 masses, which might have served as well-isolated centers of differentia- 

 tion for various forms w r hich later spread with the elevation of the 

 land until their ranges abutted. Unfortunately for this suggestion, 

 as I am informed by Professor John C. Merriam, geological evidence 

 fails so far to show the existence of such conditions within Pleistocene 

 or even Pliocene times. The suggested explanation must therefore be 

 discarded in our dealing with the differentiation of present-day species 

 and subspecies, especially since even the genera represented and as 

 now restricted are not known to have evolved so early as Miocene. 



But another process, recognizable far and wide in dynamic zoo- 

 geography, may be called into account without assuming any departure 

 in the past from topographic and climatic conditions as they are today. 

 Comparison of the fauna of the Lower Sonoran plains of south central 

 Arizona with that of the Mohave desert plateau in the same zone, 

 shows two prevalent character combinations among the nearly related 

 component species. The mid-Arizona representatives are usually dark 

 colored and large sized! the reverse appearing to hold in the majority 



