146 W. H. BURT 



Squirrels (Family Sciuridae). This is a diversified group with 

 many closely related kinds on the two land masses today. Partic- 

 ularly close, in the two areas, are the marmots (Marmota), ground 

 squirrels (Cilellus), chipmunks {Eutamias), tree squirrels (Sciurus), 

 and flying squirrels {Glaucomys in North America and Pteromys in 

 Eurasia). For the passage of the marmots and ground squirrels, 

 open savanna is required, but for the tree squirrels and flying 

 squirrels, there must have been a fairly continuous forest. Climate 

 and soil would have been influencing factors only as they affected 

 the vegetation. There must have been several crossings up to and 

 including the last land connections. 



One group in this family of rodents, the prairie dogs (Cynomys), 

 apparently had their entire evolutionary history in North America. 

 They are inhabitants of short-grass areas and require deep soil 

 for their burrows. These conditions apparently did not prevail to 

 the northward and they never reached the land bridge. 



Beaver (Family Castoridae). A forest, or cover of shrubs, and 

 fresh water would seem to be required here. Fossil evidence would 

 indicate an early Tertiary crossing. Present day relationships and 

 distributions indicate a late crossing also. It is possible that there were 

 several interchanges. Temperature would not have been a factor. 



New World mice and voles (Family Cricetidae). This group is so 

 diversified that any type of vegetative cover would have sufficed. 

 The subfamily Cricetinae, long-tailed representatives, probably had a 

 fairly early ancestral crossing. Evidence of this is found in their 

 present distant relationships and in the fossil record. A Pleistocene 

 crossing is not indicated. However, the subfamily Microtinae now 

 has close relatives on the two continents (in some genera the same 

 species) so crossings must have persisted to the end of the last 

 land bridge. Climatic conditions would not afTect this group directly. 

 The vegetative cover postulated would suffice, be it forest or 

 savanna. Why the North American cricetids or the Old World 

 murids did not make the crossing is difiicult to explain. Some would 

 argue that because they are ecological homologues, competition 

 would keep the two groups separated. I am not convinced that 

 this is the answer. It is possible that each group had its evolutionary 

 history far to the south, fairly late, and that time was not sufficient 

 for them to reach the bridge before separation of the continents. 

 However, there is no good evidence of this. 



Mountain beaver (Family Aplodontidae). The little information 



