PHYSIOLOGICAL EFFECTS 117 



onstrated again and again to have ill effects on 

 crowded animals. Thus snails and many other 

 animals grow more slowly and produce stunted 

 individuals if crowded. Fruit flies and common 

 hens reproduce less rapidly if crowded than if 

 given plenty of space, and many animals includ- 

 ing Protozoa, certain beetles, small crustaceans, 

 mice, rabbits and men have a higher death rate 

 under crowded conditions. The warning from 

 these accumulated experiences is needed; but the 

 work of the last decade and a half has demon- 

 strated that this is not the whole effect of crowd- 

 ing; some of this newer evidence furnishes the 

 subject matter for this chapter. 



Many different animals have been found in 

 the laboratory to use less oxygen per individual 

 if they are present in small numbers than if iso- 

 lated. Goldfishes do not form schools but when 

 four of these are placed together in about a liter 

 of water, they consume less oxygen per fish than 

 if the same four are isolated and each put into the 

 same amount of water. These relations hold 

 whether the fishes are in quiet or in flowing water. 

 Similarly groups of serpent starfishes in the early 

 stages of such experiments, groups of water fleas, 

 isopods and frog tadpoles have the same sort of 

 effect upon the rate of oxygen consumption of the 

 individuals that compose the groups. It is as 



