THE ANIMAL AND ITS ENVIRONMENT 



723 



perate, aiid frigid zones express rather well the comparative tem- 

 perature conditions on a continent. Many factors, however, operate 

 in the control of the amount of precipitation and its distribution. 

 In general, the distribution of available rainfall on an "ideal" top- 

 shaped continent without mountain ranges might be expected to 

 follow the plan of Fig. 378 in which the darker areas indicate 

 maximum rainfall and the unshaded areas represent very arid re- 

 gions. Each climatic type makes possible the occupancy of the 

 region concerned by a definite type of biotic community which can 



(m Peppstual snouj or ice 

 [m Tundra 

 HlD Evergreen hrest 

 IM] Deciduous forest 



Em Grassland 



Scrub (sagebrush) 

 M] Deserf 



Tropical forest <l savanna 



Fig. 379. — Hypothetical distribution of biotic communities on an "ideal continent. 



be most easily designated by the character of the conspicuous or 

 dominant vegetation. Taking into consideration the seasonal dis- 

 tribution of available moisture and the annual variations in tem- 

 perature, the "ideal continent" might be expected to present an 

 aspect something like that represented in Fig. 379. It will be seen 

 at once that in the regions of deficient temperature heat is the de- 

 termining factor, and that in warmer regions the amount of moisture 

 is the major influence. How closely this expectation is realized in 



