14 Life-histories of Northern Animals 



and that the southward distribution is governed by the mean tem- 

 perature of a brief period during the hottest part of the year." * 



But other factors enter the problem of distribution. Of 

 these humidity is probably most important. In North Amer- 

 ica it makes divisional lines and cuts each of the temperate 

 zones in two or three segments called Faunas. This is a 

 theoretical limitation. As a matter of practice the boundaries 

 of the Faunas were arrived at by actual observation, thus: 



When a great many of the well-marked life-forms called 

 species agree in recognizing a common limit, the fact is ac- 

 cepted as evidence that within that area is a set of conditions 

 necessary to create a Fauna (or Flora). 



A Faunal area may be divided into several subfaunas. 

 These are of course less pronounced. When a species is found 

 ranging over several subfaunas it is usually represented in each 

 by a geographical race. 



The Arctic Region is divided into five Faunas: 



Alaskan^ characterized by the Fur-seal, Northern Sea-lion, 

 Banded-seal, Pacific Walrus, Grant Caribou, yellow wagtail, 

 Emperor goose, Steller eider. Nelson gull. 



Barren-ground, characterized in its main area by Muskox, 

 Parry Ground-squirrel, Lemmings, blue-goose, snow-goose, 

 willow ptarmigan, etc. {Ungava and certain Polar Islands 

 should probably be ranked as subfaunas of this.) 



Greenland, characterized by Greenland Caribou, Ward 

 Muskox, Greenland redpoll linnets, Reinhardt ptarmigan, and 

 various European species. 



Alpine, which is the top of each mountain that rises above 

 timber line anywhere in North America, and characterized by 

 white-tailed ptarmigan, pipit. Calling-hare, etc. 



Aleutian Fauna,^ comprising the Aleutian Peninsula and 

 contiguous Islands, a treeless coast region characterized 

 chiefly by peculiar species of Voles; also of birds, such as 



* Life Zones and Crop Zones, etc., 1898, p. 54. 



" In moving this from the Temperate to the Arctic Realm I follow Osgood, N. A. 

 Fauna, No. 24, 1904, p. 24. 



