152 W. H. BURT 



find its way to North America until after the Panama land connec- 

 tion with South America was established. It has now penetrated to 

 the far northern forests and no doubt would have made the crossing 

 had it not arrived too late. Time, I suspect, was the important 

 factor for the porcupine. 



Pronghorn (Family Antilocapridae). As far as known, this is 

 strictly a North American product. Any intercontinental exchange 

 must have been by pre-pronghorn ancestors. Why did not this 

 animal reach Asia when other artiodactyls did? I suspect that the 

 answer is to be found in the ecology of the pronghorn. It is an 

 inhabitant of short-grass, semi-arid country. It is tolerant of low 

 as well as high temperatures. If the ancestors of pronghorns had 

 similar habitat preferences, we may assume that these short-grass 

 semi-arid conditions did not form a pathway on the approaches or 

 on the bridge itself. In this case, ecological conditions would serve 

 as a selective agent to prevent movement of the pronghorn. 



Except for the coati and ringtail, these groups that apparently 

 did not cross the land bridge now inhabit areas of lower winter 

 temperatures than those that prevailed when the continents were 

 connected. General ecological conditions, soil and vegetation, I 

 suspect, were more important in restricting their northward move- 

 ments than was temperature. For the porcupine it was probably 

 the time element. The coati and ringtail conceivably could have been 

 restricted by low temperatures. 



SUMMARY AND CONCLUSIONS 



It is suggested that the term "origin" as applied generally to 

 faunas or taxa be replaced by the term "evolutionary phase." 

 Origin implies a fairly definite time and place for the beginning of a 

 fauna or taxon. But the evolutionary process is a continuum, and 

 a phase may represent a transition in time and space. 



The difficulties in evaluating mammalian faunas in order to 

 designate affinities are pointed out. After a somewhat subjective 

 analysis, it is concluded that over one-half (57 per cent) of the 

 mammalian fauna of western North America shows affinities with 

 the fauna of Asia, about 13 per cent with that of South America, 

 and the remainder (30 per cent) may be considered as indigenous. 



Present relationships indicate an exchange of faunas in the past. 

 Further, they indicate suitable habitats on land connections for 



