63 GEOGEAPHICAL DISTRIBUTION. 



and the Caucasus— the polar bear (Thalassarctos), and the beaver 

 (Castor), all of which, comprising in each case but a single species, 

 appear to be, with the possible exception of the bison, specifically 

 identical with North American forms. 



North American, or Nearctic Division.— The dominant feat- 

 ures of the North American mammalian fauna are preeminently 

 those which also stamp the character of the Eurasiatic fauna. 

 Thus, among the commoner animals we have the deer, moose or 

 elk, reindeer, bison (possibly identical with, or at least very closely 

 allied to, the European aurochs), cats, lynxes, weasels, bears, wolves, 

 foxes, the beaver, hares, squirrels, and marmots. Many of the forms 

 embraced in these types, moreover, are, as has already been stated, 

 speciflcally identical with their Eiu-asiatic congeners. But, while 

 there are such striking resemblances between the two faunas — re- 

 semblances that penetrate to almost all parts of the regions that are 

 under consideration — it cannot be denied that there are also a num- 

 ber of almost equally well-marked differences ; but these are neither 

 sufficiently numerous, nor sufficiently important, to invalidate the 

 claims carried by the positive characters for uniting the two trans- 

 Atlantic divisions into one region, the Holarctic. The prepon- 

 derating element in the North American mammalian fauna (as, 

 indeed, also in the Eurasiatic) is furnished by the group of the 

 rodents, which here comprise nearly, or fully, one-half of all the 

 recognised mammalian forms. Of about twenty-six genera repre- 

 sented, nearly one-half are restricted, or are peculiar, to this re- 

 gion ; but the actual number of specific forms embraced in these 

 peculiar genera scarcely numbers one-fourth of the total number 

 of species. The most distinctively North American families are 

 the HaploodontidsB, a very limited group (two species) of beaver- 

 like animals inhabiting the west coast, and the Saccomyidse, or 

 pouched-rats and gophers (Saccomys, Geomys, Thomomys, &c.), 

 animals characteristic of the fauna of the Western plains and ele- 

 vated mountain regions. Among the rats and mice (Muridse) we 

 meet with, in addition to certain peculiar North American forms, 

 the genus Arvicola, the field-mouse, or vole, which has an extensive 

 representation throughout the temperate portions of the Eastern 

 Hemisphere as well ; along with this animal we find the lemming 

 (Myodes), another Eurasiatic form. It is a singular fact, to be 

 noted in this connection, that the typical genus Mus, which in- 



