166 OUTLINES OF FORESTRY. 



The most extended water areas are situated in 

 the equatorial regions, the greater part of the land 

 areas being situated either in the temperate or in 

 the polar zones. 



At the equator, therefore, where the sun's heat 

 is greatest, there exists the greatest expanse of 

 water. Here are three readily-movable elements, 

 the air, the water, and vapor, each of which can 

 take in considerable heat without growing very 

 hot. The differences between the temperature of 

 the equatorial and polar regions produce vast cur- 

 rents, both in the atmosphere and in the ocean, 

 which effect an interchange between the excessive 

 heat at the equator and excessive cold at the poles. 



Even a comparatively small change in the dis- 

 tribution of the land and water areas of the earth 

 would produce marked changes in its life. 



If, for example, most of the earth's surface in 

 the equatorial regions was composed of land, an 

 excessive temperature would be thereby produced 

 that would render the equatorial regions abso- 

 lutely uninhabitable by any of the present races of 

 man. Consider, for example, tropical Africa. The 

 equator by no means crosses this continent at its 

 greatest breadth, and yet, notwithstanding the fact 

 that nearly all the continent is considerably more 



