142 W. H. BURT 



(Pruitt, 1957). It is misleading to consider air temperatures in 

 relation to these forms. The microclimate in which they live is 

 the important factor. 



The northern porcupine must have evolved a different physiology 

 from that of its tropical ancestors to endure the low temperatures 

 in parts of its present range, and this since Pliocene times when the 

 Panama land connection was made between North and South 

 America. Although the porcupine can den in a sheltered place, it 

 must expose itself to the elements when it is feeding. I suspect that 

 physiological evolution may proceed more rapidly than morpholog- 

 ical change, although I have no direct proof of this. However, I 

 think it not too far-fetched to postulate that physiological adjust- 

 ment to climate might have kept pace with changing temperatures 

 and that temperature per se was no direct selective mechanism 

 in the interchange of mammal kinds between Eurasia and North 

 America. In some kinds of burrowing mammals, type and depth of 

 soil might be the important selective agent (Hardy, 1945). An 

 analysis of the Recent mammalian fauna of western North America, 

 with these considerations in mind, should throw some light on the 

 general problem of what were or were not selective agents. Also, 

 we should get some idea of the relative times of the last crossings 

 made by each group. Not all mammals have evolved at the same 

 rate, but close relationships should indicate recent crossings and 

 distant relationships earlier crossings. We must first make one 

 assumption, and it seems a reasonable one, i.e., that the habits of 

 mammals have not changed markedly since the times when their 

 ancestors had the opportunity to cross the land bridge. If this 

 assumption is not valid, then we have no way of interpreting many 

 of the phenomena of the past. Also, to have had an interchange of 

 the kinds that are related and now living on the two continents we 

 demand a corridor with the following specifications: There must 

 have been soil and vegetation with a fairly continuous forest and 

 areas of open savanna country. The two types of vegetative cover 

 might or might not have been coexistent. These demands are in 

 conformity with modern interpretations of past climate and vege- 

 tation in the area of the approaches as well as of the land bridge 

 itself (see Darlington, 1957, for a summary of studies in this area). 

 In the following discussion of the various groups of mammals I 



