552 



BIOLOGY AND HUMAN LIFE 



only source of energy that is constantly renewing itself at a 

 sufficiently rapid rate ; but to maintain the service of waterfalls 

 we must be sure of the steadiness of the water supply, and this 

 in turn depends upon the forest/ 



396. Soil and forests. Every year the streams and rivers 

 carry down to the sea a quantity of earth estimated to be 



Fig. 225. An eroded slope in western North Carolina 



On slopes from which the vegetation has been removed the rains and melting snows 

 produce destructive effects of great practical importance. (From photograph by 



United States Bureau of Forestry) 



worth over a billion dollars. This is not only a direct loss of 

 agricultural resource; it also interferes with the navigation of 

 streams and with the conditions of harbors. Millions of dollars 

 are spent every year dredging harbors in this country, to remove 



^ When we burn coal as fuel, we are of course again dependent upon the 

 forest (though not the forest of our own times), since all coal consists of the 

 modified remains of ancient vegetations. 



