86 ANIMAL AGGREGATIONS 



corresponding increase in the rapidly breeding, non-predaceous 

 fishes; and at last the game fishes which derive their principal food 

 from the non-predaceous fishes also increase in numbers. Evidently 

 the multiplication of each of these 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 entomostracans; the latter are 

 met and checked by the vast swarm of minnows, which are in turn 

 checked by the increase in predaceous fishes. In this way a gradual 

 readjustment of the conditions will occur; but usually, long before 

 this new equilibrium is reached, a new disturbance of the water level 

 results in the recession of the water. As the lakes grow smaller and 

 the teeming life they inclose 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 of space. 



Year after year in such lakes and in other animal communities 

 there is a fairly steady balance of organic life. The community re- 

 mains in dynamic equilibrium. The rate of reproduction about 

 equals the death-rate. Every species must fight its way from hatch- 

 ing to maturity. Adults are as rare as human centenarians; yet no 

 species is exterminated, and each is maintained at the average num- 

 ber, for which we have reason to think there is sufficient food year 

 after year. Two ideas explain the order that is evolved in such com- 

 munities. First, there is the background of common interests among 

 all elements of the community. New evidence concerning the nature 

 of some of these common interests will be presented shortly. Second, 

 there is the struggle for existence and the elimination not only of the 

 less fortunate but, at times, of the less fit animals. 



Upon such a foundation as this, modern comparative sociology is 

 built in part, and must be built in entirety if it is to be sohdly ground- 

 ed. With this conception of the type of integration existing in eco- 

 logical animal communities, and with the realization that even such 

 loosely knit communities can be regarded as constituting a unit, we 

 are better prepared to search for integrations in animal aggregations 

 and to evaluate those found. 



