140 VV. H. BURT 



a formula based strictly on names in the literature, these relation- 

 ships are obscured and given negative instead of positive weight. 

 In my subjective system, which presupposes a knowledge of the 

 group, these seven kinds would be added to C, to give it a value of 

 34, instead of 27. Other adjustments would be made in various 

 groups, either combining or separating them. With this system, I 

 ended up with over half of the mammal fauna of western North 

 America showing affinities with that of Asia, about 57 per cent. 

 I believe this is nearer reality than anything the formulas would 

 indicate. However, the results are not in a form that is easily 

 communicable to other workers. By using this arbitrary system 

 further, about 30 per cent, instead of 45 per cent, of the western 

 North American mammals are considered as indigenous. In com- 

 paring North and South America, I arrived at 13 per cent, instead 

 of 19 per cent, of the western North American mammals showing 

 affinities with those from South America. This is fairly close. 



MOVEMENTS OF MAMMALS BETWEEN EURASIA AND 



NORTH AMERICA 



Faunal relationships of the nature just discussed, where two land 

 masses are now separated by water, indicate a movement of animals 

 from one land mass to the other in past times. That there was a 

 nearly continuous land connection from the beginning of the Ter- 

 tiary to Pleistocene time between what are now the North American 

 and Asiatic continents is well established. There is still doubt 

 concerning the directions of movements of many kinds of mammals. 

 This would be important to know, but it is not essential to the 

 present discussion (see Simpson, 1947, for a summary on these 

 connections and movements). But, a land connection is in itself 

 not sufficient for the transfer of non-flying terrestrial mammals, 

 unless it be a very short one which an animal might cross in a 

 single journey. The distance from Cape Prince of Wales to East 

 Cape, across what is now the Bering Strait, some 75 miles, might 

 conceivably have been crossed by some of the larger mammals 

 such as caribou, moose, elk, and bear, even though no vegetation 

 were present. For the smaller mammals, some of which are restricted 

 within fairly narrow limits to specific ecological conditions, I 

 think it most unlikely that they would, or could, make the crossing 



