380 ELEMENTS OF GENERAL SCIENCE 



392. The balance of life. The grass of the field is eaten by 

 herbivorous animals, and herbivorous animals are eaten by 

 carnivorous animals. It might seem, therefore, that the num- 

 ber of carnivorous animals is limited by the available grass 

 for grass-eating animals. 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 

 the 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 vegetation 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 grass- 

 hoppers in one feeding, and flickers have been known to 

 devour over 5000 ants in one day. Obviously, the insects 

 that survive are the ones which, while getting 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 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 num- 

 ber of squirrels. If the squirrels are removed from their 

 normal regions, where they play a part in the balance of life, 

 they may fail completely or may find new and unusual 

 conditions well suited to them. How closely living things 



