240 



STUDIES, SCIENTIFIC AND SOCIAL 



CHAP. 



nivora there is a much smaller proportion of peculiar 

 genera, but this is to some extent counterbalanced by each 

 region possessing two families absent from the other. 

 Ungulata, again, show considerable diversity, the Palse- 

 arctic Region possessing ten genera and the Nearctic five 

 which are not found in the other. Only five genera are 

 common to both, and of these five, two, Rangifer and 

 Alces, are arctic, while two others, Ovis and Bos, have one 

 species each in the Nearctic Region against about thirteen 

 between them in the Palsearctic. The Rodents, again, have 

 only eleven genera common to both regions, while the 

 Palgearctic has sixteen and the Nearctic seventeen which 

 are not found in the other region. 



The following summary will enable us to see the total 

 amount of similarity and difference : — 



SUMMARY OF DISTRIBUTION. 



Here we see that in both the regions the number of 

 genera not found in the other largely exceed those 

 common to both, the proportion of the total genera thus 

 limited being 58 per cent, in the Palgearctic, and 56 per 

 cent, in the Nearctic Region ; while in each case out of a 

 total of 27 families no less than 8 are so limited. If 

 we compare this amount of diversity with that between 

 the Ethiopian and Oriental Regions, we shall find that, 

 while as regards genera it is somewhat less, as regards 

 families it is considerably greater. In the Oriental Re- 

 gion out of a total of about 72 families of mammals only 8 



