THE ADAPTATION Of ANIMALS TO THE ENVIRONMENT ygQ 



such as oceans, mountains, deserts and large rivers and tacilitated by 

 "highways" such as land connections between continents. The present 

 distribution of the species of animals is determined by the barriers and 

 highways that exist and have existed in the geologic past. The biogeo- 

 graphic realms, discussed on page 735, are regions made up of whole 

 continents, or of large parts of a continent, separated by major geo- 

 graphic barriers, and characterized by the presence of certain unique 

 animals and plants. Within these biogeographic realms, and established 

 by a complex interaction of climate, other physical factors and biotic 

 factors, are large, distinct, easily differentiated community units called 

 biomes. In each biome the kind of climax vegetation is uniform— grasses, 

 conifers, deciduous trees— but the particular species of plant may vary 

 in different parts of the biome. The kind of climax vegetation depends 

 upon the physical environment and the two together determine the 

 kinds of animals present. The definition of a biome includes not only the 

 actual climax community of a region but also the several intermediate 

 serai communities that precede the climax community. 



358. Terrestrial Life Zones 



Some of the biomes recognized by ecologists are tundra, coniferous 

 forest, deciduous forest, broad-leaved evergreen subtropical forest, 

 grassland, desert, chaparral and tropical rain forest. These biomes are 

 distributed, though somewhat irregularly, as belts around the earth 

 (Fig. 38.5), and as one travels from the equator to the pole he may tra- 

 verse tropical rain forest, grassland, desert, deciduous forest, coniferous 

 forest, and finally reach the tundra in Northern Canada, Alaska or 

 Siberia. Since climatic conditions at higher altitudes are in many ways 

 similar to those at higher latitudes, there is a similar succession of biomes 

 on the slopes of high mountains (Fig. 38.6). As one ascends from the 



Snow t ICC 



Vf-fJ^T— S«OW LINC 



MOSSES t tlCMCNS 

 >^^^^\ LOW HERBACEOUS VE&CTATIOM 



-IREC U~t 



Figure 38.6. The correspondence of the life zones at high altitudes and at high 

 latitudes. (Allee et al.: Principles of Animal Ecology.) 



