CHAPTER II 

 CLASSIFICATION OF ANIMAL AGGREGATIONS 



It has long been known that animals not naturally bound together 

 in organic union may aggregate into groups or clusters more or less 

 closely associated, in which physical contact may or may not occur. 

 Actual physical contact is normally found as part of the aggregation 

 phenomenon among many Protozoa, as, for example, in Paramecium; 

 in fiatworms, such as the planarians; in earthworms; in echinoderms, 

 such as starfish; in mollusks; in arthropods; and among many 

 chordates, including ascidians, fish, frogs, reptiles, birds, and mam- 

 mals. 



Among other animals similarly widely distributed through the 

 animal kingdom, collections occur in which physical contact is not 

 the rule. These may be illustrated by the jellyfish, ctenophores, or 

 copepods that may discolor the ocean for miles; by collections of 

 leeches, snails, or ostracods; by the swarms of gnats that dance to- 

 gether like particles in brownian movement; by ants, bees, schools 

 of fish, flocks of birds, herds of ungulates, and groups of various other 

 mammals, including man. The highest development of aggregations 

 not based on physical contact requires the possession of highly de- 

 veloped sense organs. 



These two t3^es of animal aggregations are not mutually exclu- 

 sive, even when reactions associated with copulation are disregard- 

 ed; for animals may be involved in first one and then the other in 

 different phases of their life-cycle or seasonal history. With many 

 birds the loose flock of the daytime may be replaced by close physi- 

 cal contact during the night roost. At times this may be due to the 

 lack of adequate perching space, and show merely toleration of close 

 proximity; but in other instances, as, for example, the Indian tree 

 swift, there is a positive movement together even in the presence of 

 abundant roosting space. Bats may show the same phenomenon 

 during their daytime sleep. 



