RECENT LAND MAMMALS OF WESTERN NORTH AMERICA 135 



anything about more ancient times." But he does use present 

 situations to explain past events. It is my opinion that the only 

 indisputable evidence comes from the fossil record, but we can get 

 leads from present distributions and relationships. However, I 

 shall not dwell further on this subject here, but shall go directly 

 to my principal topic, that of the present affinities of our Recent 

 mammalian fauna. 



When one attempts to evaluate, or designate, affinities of two 

 or more faunas, he must first indicate the criteria to be followed. 

 It might seem simplest to count the species or genera, or whatever 

 category is to be employed, apply a formula, and come up with a 

 measure indicating degree of taxonomic resemblance between two 

 or more faunas. This will give a quantitative measure and make it 

 possible for us to communicate our results to our fellow workers 

 without elaborate descriptive material. But, the source data are 

 subject to errors, as I shall point out later, and the errors, if they 

 exist, will influence the result. As in dealing with so many biological 

 problems, a certain amount of subjectivity must enter the picture, 

 and to reduce everything to numerical terms may be misleading — 

 or downright wrong. Many non-taxonomists, and some taxonomists, 

 I am sorry to report, think of species and subspecies of mammals 

 as discrete entities. When they look at a check list, or faunal list, 

 they assume that each name represents a distinct unit, and that all 

 names in the same category represent units of equal value. Nothing 

 could be farther from the truth. All the categories of mammals are 

 to some extent subjective. In addition, they have changed with 

 time. What was once a species may now be a genus or a family, 

 either through evolution of the animals, if enough time is involved, 

 or through changes in man's concepts. Let me give a couple of 

 examples that I cited in a previous paper (1954). These have noth- 

 ing to do with the evolution of mammals, but they do show the 

 evolution of man's ideas. In 1909, when Osgood revised the genus 

 Peromyscus, some twenty-eight named species in the literature 

 ended up as one (maniculatus) , when he had finished his study. 

 More recently, in 1951, Hall reduced what were listed as twenty-five 

 species of weasels in the literature to three. I think it is apparent 

 that if one were using numbers of species per se, in comparing 

 faunas, the results would be quite different if data were taken from 

 the 1908 literature or that of 1956. Yet, many biologists continue 



