AGGREGATIONS OF ANIMALS 113 



chemical stimulation which can be perceived 

 only or mainly when the animals are in contact 

 with the material that gives the chemical stimu- 

 lus. With ants, for example, it is impossible to 

 separate purely touch from purely chemical 

 stimulation since the two are closely bound 

 together and with earthworms, there is doubt 

 whether they can perceive chemical stimulation 

 of many sorts unless they are in contact with 

 the stimulating agent. 



Sight plays an important role in the integration 

 of many aggregations which range from herds of 

 mammals, flocks of birds and schools of fishes to 

 the breeding collections of many frogs, where the 

 males will attempt to clasp any moving object 

 of about a frog's size which they can see. Simi- 

 larly the well-established flashing in unison of 

 fireflies appears to be set off by the synchronous 

 flashing of some pace-setting individual. 



Many of the activities of animal groups are 

 regulated by low frequency vibrations which are 

 perceived through the substratum while other 

 groups are bound into working units by sounds 

 carried through the air. Beebe has found that 

 there is a close correlation between the develop- 

 ment of vocal powers of tropical birds and the hab- 

 itats in which they live. Relatively solitary 

 birds that live in the open country where the view 



