REACTIONS ON LAND 



83 



quently built into towers often several inches high by the larger 

 forms; castings are also piled in subterranean chambers or cracks. 

 Two minor reactions of interest have to do with the piling of tiny 

 pebbles about the mouth of the burrow and with their use as gizzard 

 stones. Finally, important results spring from the two chief food 

 coactions, namely, the consumption of plant parts and animal mate- 



FiG. 17. — A vertical section of a harvester-ant nest {Pogonomyrniex occidcntalis 



Cres.) in western Kansas. It is nearly 6 feet (2 meters) from the top of the 



mound to the bottom of the excavation. (Photo by Prof. G. A. Dean.) 



rial and the utilization of organic matter in rich soils (cf. Jacot, 1936, 

 a and h). 



It is manifest that earthworms are in essence cultivators of the 

 soil of moist regions, increasing its fertility, rendering it more uniform 

 in texture, and improving its water relations. They also deepen the 

 fertile dark horizon, especially when the penetration is several feet, 

 and frequently tend to counteract acidity by bringing up considerable 

 amounts of lime from the zone of concentration. This process may be 



