286 ELEMENTS OF GENERAL SCIENCE 



But the balance of life is maintained by complex relations 

 between many kinds of living things. When a grasshopper 

 lays its eggs, the animals that can use grasshopper eggs as food 

 begin to destroy them if they find them. If the eggs hatch, the 

 young grasshoppers begin at once to eat the tender leaves of 

 grass and other vegetation. When there is a large number 

 of grasshoppers, there is danger of destruction of the kinds of 

 vegetation that are used as food, and in any case the vegeta- 

 tion may be held in check. But birds feed upon grasshoppers, 

 a single bird often eating in one day as many as the entire 

 brood of one mother insect. In California a meadow lark was 

 known to eat 314 grasshoppers in one feeding, and flickers 

 have been known to devour over 5000 ants in one day. Obvi- 

 ously, the insects that survive are the ones which, while get- 

 ting their own food, escaped the birds and also the diseases 

 and climatic dangers that surrounded them. 



The birds that devour the insects are themselves the food 

 for other animals; they too are in danger of disease, cold, 

 heat, and drought, and but few of their kind survive. Every 

 natural region is to be looked upon as a society of living 

 things, each one of which bears certain definite relations to 

 some or many of the others. The squirrels in the woods thrive 

 fairly well, and in undisturbed nature their number remains 

 fairly constant. If they increase too rapidly for a while, their 

 food supply becomes too small or their enemies thus find a 

 larger supply of food and likewise thrive, and in so doing 

 reduce the number of squirrels. If the squirrels are removed 

 from their normal regions, where they play a part in the bal- 

 ance of life, they may fail completely or may find new and 

 unusual conditions well suited to them. How closely living 

 things are related to their surroundings may be shown by 

 supposing a change in living places between a wolf of the 

 Western plains, a squirrel of the Central States forest, and a 

 fish of the Gulf of Mexico. Neither would survive if placed 

 in the region in which the others live. 



