4 GENERAL BIOLOGY 



have become carnivorous, and have taken to eating each 

 other. The carnivores prevent overproduction of herbivores, 

 and are themselves held in check by parasites within their 

 own ranks. Herbivores and carnivores, parasites and 

 scavengers are everywhere; for they fulfill permanent 

 functions of animal society. 



The need of shelter is another large factor in determining 

 the habits of animals, for few can afford to live in the open, 

 and most are so limited that they must find food and 

 shelter in the same haunts. For both food and shelter 

 animals are dependent primarily upon plants and secondarily 

 upon each other, and the relations that have come to exist 

 between them are so intricate they may fairly be compared 

 to a web with its threads all interwoven. 



Interdependence. — The weak are dependent on a few, 

 the strong upon many. The sturdy oak in the woods 

 seems very independent in comparison with the vine that 

 hangs upon its branches or the green mould lodged in a 

 crevice of its bark. But from leaf to root it is beset by 

 enemies and aided by friends.- There are caterpillars 

 feeding upon and within its leaves. On its twigs are aphids 

 sucking the sap out, and within them are beetles boring. 

 Other beetles and caterpillars li\'e in bark and sapwood and 

 heartwood of its trunk, and other aphids attack its roots. 



But about its roots there are friendly earthworms work- 

 ing in the soil, mixing it and making it porous; and moulds, 

 assisting in the preparation of its food. Neighboring 

 trees shade its trunk from the scorching rays of the summer 

 sun, and woodpeckers, nuthatches and warblers search its 

 bark and leaves for hidden insect enemies. There are 

 hosts of parasites also, individually insignificant, but 

 collectively, its greatest safeguards, that work wholesale 



*In Packard's Forest Insects there are listed 442 species of 

 insects affecting American oaks, and 20 additional that are found 

 in their dead stumps. 



