132 W. II. BURT 



appeared in abundance at the beginning of tlie Tertiary. What we 

 actually mean is that their fossil remains show up in abundance 

 first at the beginning of the Tertiary period. But, they must have 

 been abundant and diversified long before the first grain of sand was 

 deposited in what we now call Tertiary beds. The paleontologist 

 knows all this, but I fear that he sometimes inadvertently conveys 

 the wrong impression by the language he uses and the charts he 

 draws, stacking one representation of a geologic age, referring to 

 the deposits containing fossils, on top of another and not indicating 

 lapses of time between them. 



Although the title of the present symposium contains the word 

 "origin" I have not used it in this paper for the reasons just given. 

 I believe it is more in keeping with the subject of the symposium to 

 call it "The history and affinities of the Recent land mammals of 

 western North America." There is precedence for this in W. B. 

 Scott's monumental work A History of Land Mammals in the 

 Western Hemisphere, published in 1913. But, whether we speak of 

 origins or histories, it is necessary first to establish space and time 

 limits, otherwise the discussion is somewhat meaningless. The area 

 about which I shall be concerned in North America is that segment 

 of the continent lying principally west of meridian 100° West and 

 north of parallel 30° North. In Asia, it is that area principally east 

 of meridian 100° East and north of parallel 30° North. In South 

 America, I consider the entire continent. These areas admittedly 

 are unequal, but each contains diversified ecological conditions, 

 and each is sufficiently large to give a good sample of the total 

 mammalian fauna. The time interval is from the beginning of the 

 Tertiary through the Recent. Bats and marine mammals are not 

 included in the following analysis. I shall be little concerned with 

 the fossil record since that was discussed in the preceding paper. 

 I should like, however, to point up a few criteria used by students 

 of Recent faunas in attempts to determine the so-called place of 

 origin of a group of animals. I would prefer to call this the area of 

 differentiation — where the group passed from one evolutionary 

 phase to another. 



1. The Present Geographic Distribution of the Group Indicates 

 Its Origin, (a) Some have thought that the central .part of the 

 area now occupied may represent the area of "origin" (differenti- 



