112 ANIMAL LIFE AND SOCIAL GROWTH 



At the low level of social development of 

 many of these aggregations, the appearance of a 

 social appetite is an intermittent phenomenon 

 which may be awakened by the physiological 

 changes that lead to the breeding season or by 

 climatic changes that induce hibernation or aesti- 

 vation. In many cases these exhibitions of a 

 more than usually strong social appetite may be 

 given only in yearly cycles. Some of the breed- 

 ing aggregations of marine worms show monthly 

 or, better, lunar rhythms, and the outbreaks of 

 social appetites that lead to slumber aggregations, 

 are awakened by the daily approach of nightfall. 

 Such inconstant social appetites resemble in this 

 respect the spasmodic appearance of sexual or 

 food appetites. In more closely knit and better 

 developed social communities, the action of the 

 social appetite is more steady and therefore less 

 spectacular than in its initial stages. 



These aggregations of animals may be knit 

 into co-operative units by various sorts of stimuli. 

 One of the simplest methods is that of contact 

 integration whereby a stimulus received by one 

 animal causes it to start to move; its move- 

 ment is sensed by a closely pressed neighbor and 

 so the stimulus to motion is passed throughout the 

 entire group. With earthworms, ants and other 

 animals the integration by touch is aided by 



