THE GENERAL ENVIRONMENT 



79 



Australia, although here it is less dramatic. 

 Another exception to the diagrammatic 

 expression of global temperature zones is 

 related to the slope of the land. Effects 

 of slope and exposure are more obvious on 

 mountains or hills than on the plains. Even 

 in level regions in the tundra, however, an 

 almost imperceptible slope toward the south 

 may make the difference between a rela- 



The world maps of rainfall or vegetation 

 show a fairly definite moisture zonation 

 superimposed on that of temperature. From 

 the equatorial regions northward, with cer- 

 tain known exceptions, the distribution 

 shows the following schematized pattern: 



1. A belt of heavy tropical rains with 

 accompanying rain forests lies near the 

 equator. 



10° N 



10° S - 



10° N 



0°S 



Fig. 3. The temperature zones become narrower near the west coast of tropical America. ( Re- 

 drawn from Agassiz. ) 



tively abundant summer biota and a sparse 

 community of hardier forms that live on a 

 similarly slight neighboring slope to the 

 north. 



The character of the soil also affects local 

 temperatures. Heavy clay soil warms up 

 much more slowly than does loose, sandy 

 loam. Alkaline soils tend to be heat ac- 

 cumulators, and warmth-limited organisms 

 which grow only on calcareous subsoil in 

 northern Germany and the British Isles are 

 not necessarily so restricted in milder 

 climates. 



2. A region of smaller annual rainfall, 

 with more marked rainy and dry seasons, 

 supports tropical savannah or tropical 

 grassland; these formations lie on both sides 

 of the tropical rain forest. 



3. Northward over much of the world, 

 there is an area of decreasing rainfall that 

 culminates in the great arid belt that con- 

 tains the Sonoran Desert of North America, 

 the Sahara, the Arabian, and Persian des- 

 erts. Their southern equivalents occur in 

 South America, Africa, and Australia. 



4. Generally, the desert gives way tc a 



