THE EARTH FOR MANKIND 



541 



efforts to increase his production by making improvements 

 along every possible line. The chemistry of soil enables him to 

 modify the soil in accordance with the needs of special crops. 

 The use of machinery enables one man to plow a hundred 



Fig. 223. Relics of man in the Stone Age 



Remains of ancient times show that man made tools long before he discovered the 

 use of metals. At first these were convenient sticks and stones, roughly shaped to 

 fit the hand; later they were selected materials carefully worked over to suit definite 

 purposes. /, hatchet; 2, hammerhead; 3, ax; 4, 5, arrowheads; 6, fishhooks. (/, 2, 



and 4, after Tyler; 3, 5, and 6, original) 



acres in the time formerly needed for plowing one. It en- 

 ables him to break the earth up more thoroughly, to cultivate 

 more quickly, to harvest and thrash his grain with a fraction 

 of the effort formerly required. The use of biology as applied to 

 heredity enables him to increase the output of his acres and of 



