34 AGRICULTURE. 



geological effect: (i) that of disintegration, and 

 (2) that of accumulation. 



(i) Disintegration. — In the sea even the 

 hardest rocks are made to crumble by marine 

 animals borincr into them. In like manner 

 many animals burrow and bore through the soil. 



^\\^ prairie dog of the western United States 

 digs a deep burrow in the earth, and casts up 

 a mound at its entrance. There are whole vil- 

 lages of these mounds, which in some localities 

 cover many acres. Muskrats, crayfish, moles, 

 woodchucks, and gophers in countless numbers 

 are performing similar operations. 



Ants, especially in tropical countries, bring 

 up sand grains from their underground tunnels, 

 and form multitudes of ant-hills sometimes afoot 

 or more in hight. Myriads of other insects, 

 or their larvae, pulverize the soil particles or 

 enrich them with their excreta and decayed 

 bodies. 



But the most important of these animal 

 agencies in stirring up, pulverizing, mixing, and 

 ventilating the soil is that of the common earth- 

 worm. Darwin, in his investigations upon the 

 earthworm, estimated that in many parts of 

 England "more than ten tons of earth annually 

 pass through their bodies and is brought to the 

 surface on each acre of land." In this way the 

 whole superficial bed of soil would pass through 

 their bodies in a few years. The specific action 



