VERTEBRATES II 



high, dry Great Plains, and the arid and mountainous Great Basin. 

 Climatic conditions vary from the tropical heat and constant tempera- 

 ture of southern Florida and the Mexican border to the Arctic cold and 

 violent and sudden changes of the Canadian border and the high moun- 

 tain regions, and conditions of moisture from the excessively wet 

 western slopes of the Cascades which look towards the Pacific to the 

 deserts of the Great Basin and the extreme southwest. 



The actual extent of territory occupied by any species of animal 

 is determined by several factors, of which the most important are 

 probably temperature and amount of moisture, both of which tend to 

 check the migration of species and to confine them within certain more 

 or less definite boundaries. The United States Biological Survey, 

 has formulated the following two fundamental Laws of Temperature 

 Control of Distribution: First, the northward distribution of terres- 

 trial animals and plants is governed by the total quantity of heat 

 during the season of growth and reproduction; Second, the southward 

 distribution is governed by the mean temperature of a brief period 

 during the hottest part of the year. The meaning of these laws is that 

 each species requires a certain minimum total quantity of heat during 

 the period of the year when growth and reproduction are going on, which 

 is the summer season in temperate regions, and that this factor estab- 

 lishes the northern limit of its distribution: also, that excess of heat 

 through a sufficiently long portion of this period tends to check or stop 

 growth and reproduction, and consequently the mean temperature 

 of the mid-summer weeks, when the heat is the greatest, is the factor 

 which establishes the southern limit of distribution of a species. 



On the basis of these laws the North American continent has been 

 divided into three Life Regions, the Boreal, Austral and Tropical, the 

 first two of which have been subdivided each into three Life zones. 

 The Boreal Region is the portion of the continent north of the Canadian 

 boundary, and is composed of the Arctic, Hudsonian and Canadian 

 zones, each transcontinental in extent. The Arctic zone is the circum- 

 polar region north of the northern limit of forests; it is the home of the 

 muskox, polar bear, Arctic fox, wolf and hare and barren-ground caribou. 

 The Hudsonian zone is the region of firs and spruces which extends as a 

 wide belt between Labrador and Alaska, and southwards along the 

 main ridge of the Rockies into the United States, and occurs also in 

 limited areas in several high mountain regions in the Rockies and 

 Cascades; it is the home, in Canada, of the woodland caribou, moose and 

 wolverine. The Canadian zone is the region of the great coniferous 

 forests and occupies southern Canada, extending extensively into the 



