724 TEXTBOOK OF ZOOLOGY 



the case of North America may be seen by a comparison with Fig. 

 380. As will be seen later, the animals of a climatic region are 

 as distinctive as the plants. The soil, which is the result of inter- 

 action between the climate, the plants and animals, and the rock 

 substratum, also is characteristic of each climate. Thus, in the 

 area influenced by each climatic type, there is a definite biotic com- 

 munity or formation, characterized by plants and animals whose 

 relations to their environment are similar or equivalent. 



The Frincipal Biotic Formations 



A rough classification of the climatic formations of North America 

 includes the following: 



1. The Tundra Formation 



2. The Coniferous (Evergreen) Forest Formation 



3. The Deciduous Forest Formation 



4. The Grassland Formation 



5. The Sagebrush Formation 



6. The Desert Formation 



In addition there are transitional communities diflBcult of repre- 

 sentation on a map because of their discontinuous distribution or 

 small area. Some of these may be referred to, provisionally, as 

 Woodland, Desert Scrub, Chaparral, and Swamp communities. 



1. The Tundra Formation (Sedge-Musk Ox Biome). — This is the 

 community of the arctic "barren grounds," beyond the northern 

 limit of trees and between it and the polar region of perpetual snow 

 and ice. Included, also, is the region above timber line on high 

 mountains, where climatic conditions are similar to those of the 

 far north. The vegetation of the tundra is composed mainly of 

 grasses, sedges, and lichens (e.g., reijideer moss), although there 

 are also patches of dwarf willows and other woody plants from a 

 fraction of an inch to a few feet in height. Typical animals of the 

 Arctic tundra are the musk ox and the arctic caribou which feed 

 on the low vegetation. Here, also, are found great hosts of rodents, 

 including lemmings, whose enormous increases in numbers and 

 periodic migrations in Europe have been known for many hundreds 

 of years. Arctic hares and arctic ptarmigan, with those animals 

 previously mentioned, furnish food for the arctic fox and the arctic 



