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.” 1 With this as the working basis he has divided the 
United States into three parts, having the northern (Boreal), south 
era (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, snowy 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 white-winged crossbill.” 2 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. 8 
1 Merriam’s “ Laws of Temperature Control of the Geographic Distribution of Ter¬ 
restrial Animals and Plants,” National Geographic Magazine, vi. 229-238. 
2 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,” 
Yearbook of the U. S. Department of Agriculture , 1894. 
8 Merriam, C. Hart, North American Fauna , No. 16, “ Result of a Biological Survey 
