318 



BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



As is well known, and almost universally admitted, the animal and 

 plant life of the Arctic lands is nearly everywhere the same, many of the 

 species having a circumpolar range, while the genera are mainly, and 

 the families almost entirely, the same throughout. Especially is this the 

 case with mammals. To show how gradual is the change from almost 

 absolute uniformity in the Arctic regions to the ultimate diversity met 

 with in the intertropical latitudes it is only necessary to divide latitud- 

 inally the so-called "Nearctic" and "PalaBarctic" regions into several 

 minor areas, and to tabulate and compare the genera found in each. 

 Adopting as our first division the region approximately bounded south- 

 ward by the isotherm of 36 F., and hence embracing the Arctic, Sub- 

 Arctic, and Cold Tern perate lands of the northern hemisphere, we find that 

 of the fifty-four commonly recognized genera of non-pelagic mammals 

 occurring north of this boundary, five are subcosmopolitan; twenty- 

 seven, or more than one half, are strictly circumpolar, being represented 

 throughout the greater part of the region north of this boundary ; that 

 five more are found on both shores of the Atlantic, and that five others 

 are common to both shores of the Pacific. This leaves only twelve 

 less than one-fourth that are peculiar to either the northern portion of 

 North America or to the corresponding portion of the Old World, of 

 which eight are restricted to America and four to the EuropaBO-Asiatic 

 continent. These genera and their distribution are approximately shown 

 in the subjoined table. 



Gtnera of mammals of the Arctic and Cold Temperate portions of the northern hemisphere (the 

 region north of the mean annual of 35 F.). 



