270 READINGS IN BIOLOGICAL SCIENCE 



year after year. Without the influence of man there is seldom any over- 

 grazing or famine. 



Various animals have attempted to insure a stable food supply by storing 

 food or raising crops. Certain ants, squirrels, beavers and other animals 

 in times of plenty regularly gather appropriate foods which they store 

 away for unfavorable seasons. The ancestors of ants appear to have been 

 generally carnivorous, but certain desert species have gradually developed 

 habits of gathering, husking, drying and storing the seeds of certain grasses. 

 Other desert ants gather honey from flowers and store it away for use dur- 

 ing the dry season in certain peculiar individuals of their worker caste. 

 These remain within the nest, become greatly distended, and serve as 

 living bottles for their fellows. Some species of termites make httle beds 

 of wood, the dead bodies of their comrades, and excrement. On these 

 they plant and rear the fungi which they use as food. The leaf-cutter ants 

 have progressed to an even more specialized type of agriculture. They go 

 out along beaten paths and bring home bits of leaves. Some of these they 

 use to thatch over their nest, but others are chewed up and arranged in 

 beds on ^^■hich a peculiar type of fungus is grown. This is not eaten in 

 its natural state, but is first carefully pruned with the mandibles of the 

 ants. In this way pecuHar growths, which were called "kohl-rabi clumps" 

 by MuUer, are produced. These growths constitute the sole food of leaf- 

 cutters. When a young queen is ready to start a new nest she takes a little 

 of the material from an old fungus bed and places it in a little pouch at the 

 back of her mouth. She then makes her nuptial flight, breaks off her wings, 

 digs a burrow, cuts a few leaves and prepares a little bed. Then she takes 

 the fungus from the back of her mouth and plants it. She lays eggs and 

 carefully rears a few young. When these mature, they begin to care for 

 her and she then has nothing to do for the rest of her hfe but eat and lay 

 eggs. Aside from a few insects, man is the only animal which has practised 

 agriculture systematically. The assurance of a continual and adequate food 

 supply among insects and men has been important in making the develop- 

 ment of social life possible. 



Plants are important factors in regulating climate and in making condi- 

 tions in any locality suitable for animals. They cover the bare soil and thus 

 conserve moisture. When there is a growth of plants over an area, winds 

 cannot dry out the soil so readily and deep-growing roots bring water to 

 the surface. Water is continually lost through the leaves of plants and by its 

 presence in the atmosphere tends to make temperatures more equable. 

 During the heat of the day water evaporates and thus cools the air. At 

 night its condensation gives up heat and its presence as vapor serves as a 

 blanket which prevents radiation of heat from the earth. Thus when plants 

 cover the soil they help to furnish water to keep air temperatures more 

 uniform. 



Plants continually shed leaves which fall to the surface of the soil below 



