1913] Zoo-Geograpiiy 17 



4. An Alleghanian Zone, which inciudes, roughly speaking, 

 the northern United States. 



5. An Upper Austral Zone, covering the middle portion of 

 the United States, and divided into an eastern or humid portion 

 (the Carolinian district), and a western or arid portion (Upper 

 Sonoran). 



6. A Lower Austral Zone, comprising the southern United 

 States and the central plateau of Mexico, likewise divided into 

 an eastern (Louisianian) or humid region, and a western 

 (Lower Sonoran), or arid region. 



1. A Tropical Zone, comprising all south of the Lower 

 Austral. 



The main characteristics of these zones are as follows : 



1. The Arctic Zone is distinguished bj an entire absence of 

 trees, the vegetation consisting of stunted shrubs, low flowering 

 plants, and lichens. Among the vertebrates, reptiles and amphi- 

 bians are wholly lacking, while mammals are represented by the 

 musk-ox, barren ground caribou, arctic hare, several species of 

 lemmings, arctic fox, and near salt water and on the islands of 

 the Arctic sea, by the polar bear. On the summits of the Rocky 

 Mountains and of the Sierras where isolated patches of this 

 zone occur at high altitudes these are all absent, but pikas, 

 mountain sheep, and marmots occur at least in summer. Among 

 the breeding birds of the Arctic zone are the snow geese and 

 gyrfalcons and quite a number of shore birds. The mean tem- 

 perature of the six warmest months at the lower edge of this 

 zone is said to be about 50 F. 



2. Tlie Hudsonian Zone is a belt of more or less stunted 

 timber lying due south of the preceding. Like it, it lacks all 

 reptiles and amphibians, and also all the characteristic Arctic 

 mammals as well, this being due not alone to the higher tem- 

 perature but largely also to the forested character of the coun- 

 try, which prevents it being congenial to the animals which in- 

 habit the treeless country further north. For the same reason the 

 forest loving forms of the north, such as the wolverine, fisher^ 

 marten, Canada lynx, woodland caribou, moose, and black bear 



