574 HOW DOES MAN CONSERVE HIS RESOURCES? 



necessarily been disturbed by man in his ever increasing demand 

 for food and other supplies. In consequence, he must learn how 

 to conserve the supplies which he now has and which are so nec- 

 essary to him. 



Although in biology we are not directly concerned with methods 

 of conservation which deal with our mines or our fuels, we are 

 indirectly studying the conservation of water supplies when we 

 deal with the problem of the protection of our forests. This 

 unit is also concerned with the conservation of useful plants and 

 animals. There are two general methods to consider : first, how 

 to protect our useful plants and animals ; second, how to eliminate 

 harmful ones. 



Our personal health is the largest problem of conservation with 

 which we have to deal. The study of present-day statistics shows 

 us the imperative need of the conservation of human health and 

 human life. Communities are doing much more for the health of 

 the people than they did twenty or thirty years ago, but there are 

 still too many careless individuals who do not do all they can to 

 care for themselves properly and who forget the rights of others 

 in a democracy. Conservation means protection or care of the 

 natural gifts that nature has given us. 



PROBLEM I. WHAT ARE THE VALUES OF TREES? 



Prevention of erosion by covering of organic soil. Streams 

 unprotected by forests may dig out soil and carry it far from its 

 original source. Results of the carrying-power of streams may 

 be seen in the deltas formed at the mouths of great rivers. The 

 forest prevents the erosion of soil by holding back the water and 

 letting it out gradually. This it does by covering the inorganic 

 soil with humus or decayed organic material which, like a big 

 sponge over the forest floor, holds water through long periods of 

 drought. It is estimated that the forest floor can absorb and hold 

 for some time a rainfall of four or five inches. The roots of the 

 trees, too, help hold the soil in place and prevent erosion. 



Regulation of rainfall and water supply. The gradual evapora- 

 tion of water through the stomata of the leaves cools the atmos- 



