CLASSIFICATION OF ANIMAL AGGREGATIONS 13 



There are abundant examples of animals that lead wholly or par- 

 tially solitary lives during part of their seasonal- or life-cycle but at 

 another period come together into flocks or in actual physical con- 

 tact. This is true of the cowbirds, reared singly from eggs surrepti- 

 tiously laid singly in the nests of other species of birds. The young 

 cowbirds develop quite out of touch with other members of their own 

 kind and yet collect into definite flocks when adult. Another aspect 

 of the same kind of behavior is shown by the grackles, which nest 

 fairly separately but join in large flocks before the fall migration; by 

 deer, which summer separately or in partial family groups but winter 

 in herds; by frogs, which remain practically sohtary during the year 

 except for possible hibernation groups and then aggregate during 

 the breeding season; by solitary bees or wasps, which for the greater 

 part of the year are out of physical contact with their fellows and 

 yet during the summer may form overnight aggregations in closest 

 physical proximity; or, to give one more of many possible examples, 

 by land isopods, which congregate into dense bunches when their 

 habitat becomes dry. 



The aggregations of the physical-contact type are, of necessity, 

 transitory in character in motile organisms; but in sessile animals, 

 such as the ascidians, or the marine mussel Mytilus, this may well be 

 the normal way of living. The physical-contact type of aggregation 

 finds its most complete expression among the sessile colonial organ- 

 isms that grow in dense stands of many individuals, which are physi- 

 cally connected with each other throughout Hfe. Obelia hydroids 

 represent this growth form. 



Collections without physical contact, such as the flock or the 

 herd, may be constant and normal for some species; and the animals 

 in these are usually said to exhibit the social habit. This social habit 

 finds its best development in the insects, such as the ants and ter- 

 mites, among whom division of labor is carried out to its logical end, 

 in that polymorphic forms have evolved of which some do not com- 

 plete their sexual development while others specialize upon repro- 

 duction. These have been well described by Wheeler and Forel. 



Animal aggregations may be classified on many other bases be- 

 side that of the degree of physical contact. Deegener (1918) has 



