12 ANIMAL LIFE AND SOCIAL GROWTH 



food supply which keeps those that hatch from 

 starvation. 



The multiplication of each of these food classes 

 acts as a check on the one preceding it. The 

 development of Protozoa and algae is arrested 

 and sent below normal by the swarm of minute 

 crustaceans; the latter are met and checked 

 by the vast swarm of minnows, which in turn are 

 checked in their increase by the rise in numbers of 

 predaceous fishes and by fish-eating birds at- 

 tracted by the good fishing. In this way a grad- 

 ual readjustment of the numbers of animals 

 in each of the food classes will occur; but usually, 

 long before a new balance is reached, a new dis- 

 turbance in the water level results from the reces- 

 sion of the flood waters to their more usual 

 channels. 



As the lakes grow smaller and the teeming life 

 they enclose is daily restricted within narrower 

 and narrower bounds, a fearful slaughter ensues. 

 The predaceous fishes thrive for a time, since 

 their food is more easily caught; but finally they 

 too are thinned out by the lack of food and space. 



Year after year and century after century in 

 such lakes, there is a continuous ebb and flow in 

 the amount of animal and plant life present, but 

 with all and in the long run, a fairly definite bal- 

 ance is maintained. Over a term of years the 



