VARIATIONS OF RODENTS 



255 



and their distribution was continuous from the High Central Plateau 

 to the Kaibab Plateau. During the warmer, arid, interglacial periods 

 of the Pleistocene, the intervening forest disappeared and the montane 

 rodents were forced to ascend to the higher mountains. Thus the con- 

 tinuity of population was periodically broken and the Kaibab animals 

 were isolated, as they are now in the Recent epoch. This isolation by the 

 Grand Canyon of the Colorado River and by the adjacent deserts has 

 been so effective since the last Ice Age that the montane rodents of the 

 Kaibab Plateau are ideally stranded for speciation. 



It seems possible that during the Ice Ages the yellow pine forests and 

 the montane rodents were continuous from Utah southward into Ari- 

 zona (e.g., on the San Francisco Mountains, Mogollon Plateau and 

 other highlands of east-central Arizona). Because of their more southerly 

 latitudes and the rapid development of the canyon of the Colorado River, 

 the highlands of central Arizona must have been cut oliF from the Kaibab 

 Plateau at an earlier period than the Kaibab Plateau was cut off from 

 the High Central Plateau of Utah. The result has been the later and 

 closer genetic connection between the boreal rodents of the Kaibab Pla- 

 teau and those of the mountains of southern Utah. 



LITERATURE CITED 



Bailey, V. 



1900. Revision of American Voles of the Genus Microtus. North Araer. Fauna. 



17:1-88. 

 1915. Revision of the Pocket Gophers of the Genus Tlwmomys. North Araer. 



Fauna. 39:1-136. 

 1931. Mammals of New Mexico. North Amer. Fauna. 53 :1-412. 

 1935. Mammals of the Grand Canyon Region. Grand Canyon Nat. Hist. 



Assoc, Nat. Hist. Bull. 1:1-42. 



Burt, W. B. 



1934. The Mammals of Southern Nevada. Trans. San Diego Soc. Nat. Hist. 

 7:375-427. 



Davis, W. B. 



1939. The Recent Mammals of Idaho. Caxton Printers Ltd., Caldvpell, Idaho. 

 400 pp. 



Durham, F. E. 



1952. A new Pocket Gopher from the North Rim of the Grand Canyon, 

 Arizona. Jour. Mammal. 33:498-499. 



DURRANT, S. D. 



1952. Mammals of Utah. Kansas Univ. Pubs. Mus. Nat. Hist. 6:1-549. 



Goldman, E. A. 



1938. New Pocket Gophers of the Genus Thomomys from Arizona and Utah. 



Jour. Washington Acad. Sci. 28:333-343. 

 1947. The Pocket Gophers (Genus Thomomys) of Arizona. North Amer. 

 Fauna. 59:1-39. 



