ANIMAL AND ITS ENVIRONMENT 



461 



frigid zones express rather well the comparative temperature con- 

 ditions on a continent. Many factors, however, operate in the con- 

 trol of the amount of precipitation and its distribution. In general, 

 the distribution of available rainfall on an ''ideal" top-shaped conti- 

 nent without mountain ranges might be expected to follow the plan 

 of Fig. 198, in which the darker areas indicate maximum rainfall 

 and the unshaded areas represent very arid regiqns. Each climatic 

 type makes possible the occupancy of the region concerned by a 



[m P<zppdua\ snom or ice 

 DE Tundpa 

 GH) Evergreen ioresi 

 1=] Deciduous forest 



■^ Grassland 



dl] Scrub Csagebrush) 



EM Deszpf 



O Tropical forests savanm 



Fig-. 199. — Hypothetical distribution of biotic communities on an ideal continent. 



definite type of biotic community which can be most easily desig- 

 nated by the character of the conspicuous or dominant vegetation. 

 Taking into consideration the seasonal distribution of available 

 moisture and the annual variations in temperature, the ''ideal con- 

 tinent" might be expected to present an aspect something like that 

 represented in Fig. 199. It will be seen at once that in the regions 

 of deficient temperature heat is the determining factor, and that in 

 warmer regions the amount of moisture is the major influence. How 

 closely this expectation is realized in the case of North America 

 may be seen by a comparison with Fig. 200. As will be seen later, 



