FAUNAL DIVISIONS OF EASTERN NORTH AMERICA. 211 



IV. THE GRASSLAND FAUNA. 



(a) The Prairie Type. 



(b) The Steppe Type. 



V. THE PLATEAU FAUNA. 



(a) The Cactus Desert Type. 

 (6) The Mountain Forest Type 



I. The Sub-Arctic Fauna. 



This fauna may be regarded on the whole as a remnant of the once widely 

 extended fauna of the last glacial period. Its characteristic forms arc distril)- 

 uted around the margin of the polar basin in the Eurasiatic and North American 

 land-masses and they become the distinctively Arctic species when dispersed 

 along the maritime borders of the ice-caps. 



(a) The Barren Ground or Tundra Type inhabits the circumpolar region 

 north of the tree-limit. Among mammals such forms as the Barren Ground 

 caribou (Rangifer gromlandicus) , the musk ox (Ovibos), now confined to the 

 Amrrican land mass, the Barren Ground bear (Ursus ricliardsoni) , the Arctic 

 fox (Vulpes lagopus), the two genera of lemmings (Myodes and Cunicidus), 

 Parry's spermophile (Spermophilus empetra), the red-backed mouse (Evoiomy$ 

 rutilus), and the Arctic hare (Lepus glacialis) are eminently characteristic. 



The bird-life of the region is remarkable in that during the brief season of 

 maximum sunlight certain species of plover, curlew, and other waders and 

 waterfowl invade these desolate tracts in countless thousands, rearing their 

 young and gorging themselves on the abundant food supplied by the cloudberry 

 (Rubus chamamiorus) , the crowberry (Empetrum nigrum), and other low, berry- 

 bearing plants. The longspurs (Calcarius lapponicus and C. pictus), the snow- 

 bunting (Plectrophenax) , the redpoll linnets (Acanthis), the northern horned lark 

 (Otocoris alpestris) , the pipit (Anthus rubescens) , the wheatear (Saxicola oenantfw) , 

 the willow ptarmigan (Lagopus), and the northern raven (Corvus), breed on the 

 Barren Grounds. 



(b) The Tree-limit Type. — This is an area of variable width lying between the 

 Barren Grounds or Tundra and the northern border of the fully developed 

 coniferous forest to the south. Its vegetational character is that of a stunted 

 and scattered tree-growth due, as Schimper has observed, to the chilling effects 

 of " dry winds during frosty weather." This enforces a low growth for protection 

 against freezing. 1 Similar conditions are met with on mountain ranges (timber- 

 line or alpine zone). This type is equivalent to the Hudsonian Fauna (a sub- 

 division of the Boreal Zone) of writers. It has the features of a transition tract 

 since certain forms of the typical coniferous forest range into it from the south, 

 while such species as the wolverene (Gulo luscus), the ermine (Putorius erminea), 

 and the gray wolf (Canis griseus) range across it into the Barren Grounds. It 

 limits the northward dispersal of such mammals as the black bear (Ursus ameri- 



1 Schimper, Plant Geography, English translation, Oxford, 1903, p. 168. 



