xxxiv INTRODUCTION 



along these lines in Europe and eastern North America. But it is 

 only within recent years that the Biological Survey carried on by 

 the government has studied the old familiar generalizations in the 

 western states in detail and mapped the life zones of the United 

 States as a whole. 



The generally accepted theory that the distribution of mammals, 

 birds, reptiles, insects, and plants depends on temperature has been 

 demonstrated by Dr. Merriam as a physical law that "the northward 

 distribution of terrestrial animals and plants is governed by the sum 

 of the positive temperatures for the entire season of growth and 

 reproduction, and that the southward distribution is governed by 

 the mean temperature of a brief period during the hottest part of 

 the year."i With this as the working basis he has divided the 

 United States into three parts, having the northern (Boreal), south- 

 ern (Austral), and intermediate (Transition) climates and flora and 

 fauna. By subdivision seven zones are made, known as Alpine, 

 Hudsonian, Canadian, Transition, Upper Sonoran, Lower Sonoran^ 

 and Tropical. East of the hundredth meridian, which, broadly 

 speaking, is the dividing line between the eastern or humid and 

 western or arid sections, the Austral zone is known as Austral 

 rather than Sonoran, and divided into Alleghanian, Carolinian, and 

 Austroriparian Faunas. 



The Alpine Zone lies above the limit of trees, and is characterized 

 by dwarf shrubs and plants, the polar bear, arctic fox, reindeer, 

 the snow bunting, snow^y owl, ptarmigan, pipit, and leucosticte. 



The Hudsonian Zone is marked by dwarfed timber along "the 

 northern or higher parts of the great transcontinental coniferous 

 forest, . . . stretching from Labrador to Alaska. It is inhabited by 

 the wolverine, woodland caribou, moose, great northern shrike, pine 

 bullfinch, and wiiite-winged crossbill."- On Mt. Shasta its only trees 

 are the black alpine hemlock and white-barked pine, its character- 

 istic mammal is the cony {Ochotona), and its characteristic bird the 

 Clarke crow. It is also frequented by the sooty grouse, western 

 goshawk, Williamson sapsucker, rufous hummingbird, Oregon jay, 

 pine siskin, junco, Audubon and hermit warblers, creeper, red- 

 breasted nuthatch, kinglets, and solitaire.^ 



1 Merriam's " Laws of Temperature Control of the Geographic Distribution of Ter- 

 restrial Animals and Plants," National Geographic Magazine, vi. 229-238. 



- Merriam, C. Hart, "Life Zones and Crop Zones of the United States," Bull. No. 10, 

 Biological Survey ; " Geographic Distribution of Animals and Plants in North America," 

 YearbooJ; of the U. S. Department of Agriculture, 1894. 



3 Merriam, C. Hart, North American Fauna, No. 10, " Result of a Biological Survey 



