RECENT LAND MAMMALS OF WESTERN NORTH AMERICA 139 



America and Asia. However, I am of the opinion that the number 

 of common genera is near reaHty. The great variables, then, will 

 be found in iV — C for each fauna, in any of the formulas used here. 

 But, regardless of the formula used, the trend is the same for these 

 three faunas, and all seem to be indicative of the relationships. In 

 Simpson's formula, (C/Ni) X 100, if we compare western North 

 America with Asia and South America, N2 could vary from 69 to 

 infinity and the measure would be the same, 40 for western North 

 America and Asia; 1 1 for western North America and South America. 

 What his formula actually gives is the percentage of common 

 kinds in the smaller of the two faunas, regardless of the size of the 

 larger one. This formula is usable as far as it goes, but it is my 

 opinion that it should be applied in both directions to give the true 

 picture. This gives two measures and makes it more cumbersome to 

 use than a formula that takes into account the total of the faunas 

 to be compared and gives but one measure. Such a formula is the 

 middle one in Fig. 1, [C/(iVi + N^ - Q] X 100. In this formula, 

 C appears in both numerator and denominator, and the measure 

 obtained is the percentage of common kinds in the total of two 

 faunas. This formula works best if the two faunas being compared 

 are equal, or nearly so, in size. If they are very unequal in size, 

 and the smaller fauna is mostly common to the larger one, the result 

 obtained may be misleading. 



The top formula, [2C/(iVi + N2)] X 100, where C appears only 

 in the numerator, gives a measure, different from the others, but 

 shows the same trend in the faunas here compared. 



In the figure, you will note that twenty-seven genera are common 

 to North America and Asia (C = 27). Several genera have close 

 relatives on the two continents, but, because of a different name in 

 the lists, they will appear in that part of the fauna which is consid- 

 ered indigenous. Some of these for western North America are: 

 Scapanus (mole), Taxidea (badger), Tamiasciurus (red squirrel), 

 Glaucomys (flying squirrel), Phenacomys (vole), Sylvilagus (cotton- 

 tail), and Odocoileus (deer). These genera are given the same value, 

 on the indigenous side for North America, as Dipodomys (kangaroo 

 rat), Perognathus (pocket mouse), Thomomys (pocket gopher), 

 and Antilocapra (pronghorn). But, they are related, and fairly 

 closely, to their Asiatic counterparts, whereas kangaroo rats, 

 pocket mice, pocket gophers, and pronghorns are not. If we use 



