132 THE THEORY OF EVOLUTION 



verenes, sheep, antelopes, bison, the wapiti (erro- 

 neously called "elk") the moose, the caribous and 

 many others, and these continue to inhabit the 

 northern half of the continent and have given to it 

 that Old World stamp which is recognized by in- 

 cluding this boreal area in the great Holarctic region. 

 Many species, which are now extinct, accompanied 

 this Pleistocene invasion, the most conspicuous of 

 which was the Siberian mammoth, or hairy elephant, 

 which ranged from the south of France to Kamchatka 

 and, on the American side, from Alaska to New 

 England. Systematic naturalists still debate the 

 question as to whether these immigrants have suffi- 

 ciently changed since their arrival in America to 

 deserve to rank as species distinct from their Old 

 World relatives. Whether the American and Euro- 

 pean bison, the Scandinavian and American moose, 

 the wapiti of America, that of Asia and the European 

 red stag, the caribou and the reindeer, are valid 

 species or merely geographical races of the same 

 species, need not be discussed here. Whichever view 

 we take, the Old World character of the northern 

 half of our continent is not affected. 



Between this "boreal zone" and the Sonoran 

 region, there is a 'transition zone' 1 where the two 

 faunas mingle. The Sonoran region, which, roughly 

 speaking, includes the United States, except the 

 high mountain ranges, which carry the northern 

 fauna far to the South, and the plateau of Mexico, 

 is the characteristically North American region. Its 



