254 DURHAM 



The Botta pocket gopher shows high adaptation of color to local 

 conditions of soil color and humidity, and possibly even to light intensity 

 (see description of color under the account of species). 



The relationship between the rodents of the North Rim and the 

 South Rim of the Grand Canyon presents perplexing problems. Is the 

 barrier between these two areas an impassable chasm or altitudinal dif- 

 ference (about 1000 feet), or a combination of the two? Durrant (1952, 

 p. 515) correlated the degree of separation of juxtaposed populations 

 further up the Colorado River with the relative size of the canyon. 



Obviously, all the ground squirrels as well as all the tree squirrels, 

 all the meadow mice, and all the pocket gophers are genetically related 

 if we go back far enough on the generic tree. The relationship between 

 certain subspecific variations seems indefinite and appears to be the prod- 

 uct of the environment, even when the character seems to be nonadaptive ; 

 e.g., the white patches on the ventral surface of the pocket gophers and 

 the short tail of the meadow mouse. 



Conclusions 



The rodent fauna of the North Rim is fairly typical of that of the 

 Kaibab Plateau. All these animals show certain relationships to those 

 of the South Rim, the San Francisco Mountains, and the other highlands 

 of Arizona, but the boreal rodents of the North Rim have their nearest 

 relatives on the High Central Plateau of Utah. A chain of highlands 

 from the core of the Rocky Mountains in Colorado forms a route of 

 migration westward into Utah via the Uinta Mountains and thence 

 southward along the High Central Plateau almost to the southern border 

 of Utah. Up to this point a continuous forest of yellow pine gives a high 

 montane environment similar to that of the Kaibab Plateau, from which 

 it is separated by some fifty miles of arid Kaiparowits Canyon Lands. 

 This desert area is occupied by animals such as the ground squirrel, 

 Botta pocket gopher, and deer mouse, not greatly unlike those from the 

 arid margins of both the High Central Plateau of Utah and the Kaibab 

 Plateau of Arizona; but the montane rodents, such as the chickaree, 

 northern pocket gopher and bushy-tailed wood rat, are absent from the 

 Kaiparowits area. 



To account for the near relationship between the montane rodents 

 of the Kaibab Plateau and those of the High Central Plateau of Utah. 

 it is postulated that in the glacial periods of the Pleistocene the cold wet 

 climate allowed the yellow pine forest to grow in what is now the Kai- 

 parowits area. The montane rodents then descended to the lower levels 



