AGGREGATIONS OF ANIMALS 105 



up and wander off only to come together again 

 later; or, if the disturbance is not too strong, the 

 animals may be stimulated enough to move a 

 tiny distance and then settle more closely together 

 than they were before. After such a collection is 

 formed, mere inertia may prevent its being dis- 

 rupted by stimuli which would set less quiet indi- 

 viduals off into rapid motion. In such collections 

 as these, and many others that might be named, 

 the animals seem to be substituting each other 

 for missing elements in their normal environment. 



As the social appetite grows, animals may move 

 about until by directed movement, more fre- 

 quently, by trial and error, they find other mem- 

 bers of their species with which they can aggre- 

 gate. In the case of the common fresh-water 

 fish known as the black catfish or black bullhead, 

 the adult male or female stands guard over the 

 eggs and the newly hatched fishes. After the 

 adult fish has abandoned the young school, they 

 still continue to aggregate into dense masses 

 which mill about continually during the day but 

 break up at night to come together again with 

 early dawn. Field and laboratory studies show 

 that these fishes will move towards any object 

 they can see of about the same size and color as 

 themselves. 



When two such fishes move together on sighting 



