DISTRIBUTION OF ANIMALS 507 



Africa, as well as many characteristic birds, such as cuckoos, pheasants, 

 babbling thrushes, and broadbills. 



The Palearctic and Nearctic regions have not such rich faunas as the 

 regions already mentioned and not so many characteristic forms. Com- 

 mon to these regions are deer, bisons, bears, wolves, beavers, and marmots. 

 There are remains of the mammoth in Siberia and the hairy rhinoceros in 

 Europe. Distinguishing the Palearctic region are certain wild sheep, 

 the ibex, the chamois, wild horses and asses, camels, and the tiger. The 

 Nearctic region, on the other hand, has a relative scarcity of hollow- 

 horned ruminants, which are represented by the bighorn, the American 

 bison, and the mountain goat and also possesses badgers, prairie dogs, and 

 certain pouched rats. 



There are some interesting analogies between regions in the exist- 

 ence of corresponding but unrelated forms. Such forms are the humming 

 birds, the greatest number of which are in South America with some 

 species extending into North America but which are entirely absent 

 from the Old World, where their place is taken by the sunbirds. Other 

 forms that illustrate such an analogy are the large-billed toucans of South 

 America, to which the hornbills in Africa and southern Asia correspond. 



578. North American Life Zones. — This continent has been divided 

 by Merriam into regions and life zones based upon temperature, forming 

 bands crossing the country from east to west, and carried southward along 

 the mountains and northward along the central valleys by the effect 

 of altitude. In the western part of the United States, where great 

 differences of elevation, soil, and climate exist, there is great irregularity 

 in distribution. Three regions were recognized by Merriam — the boreal 

 region, which included most of Canada; the austral region, which included 

 roughly the United States and northern and central Mexico; and the 

 tropical region, which covered the southern tip of Florida, the West 

 Indies, and the coasts of Mexico north to about 25 degrees latitude. 

 This mapping of the continent has been severely criticized recently 

 because of too great dependence upon temperature and the failure to 

 recognize vegetation regions. 



The vegetation regions of North America (Fig. 311) may be roughly 

 outlined as follows: South of the frozen arctic region is a treeless area 

 referred to in a general way as tundra. In the northern part of this 

 area the ground thaws to a depth of but a few inches in summer and the 

 surface supports a growth only of lichens and mosses. As one proceeds 

 southward the depth to the frost line increases and grasses, low her- 

 baceous vegetation, and even low shrubs appear. Finally the limit of 

 trees is reached and one enters the northern coniferous forest, which is 

 represented on the Pacific coast by western and northwest coniferous 

 forests, extending down into California and along the Rocky Mountains 

 into Arizona and New Mexico. Central United States in the east is 



