INDIVIDUAL DIFFERENTIATION AND ADAPTATION 113 



be separate individuals are organically connected and often 

 come, by the continuation of the process, to form immense 

 masses, as in the coral. Such organic associations are called 

 colonies. Colonies rarely occur in animals in which the organs 

 are highly specialized. Very often the individuals become 

 specialized for the performance of a special portion of the 

 work, and thus we get several quite differently constructed 

 individuals within the colony (polymorphism, Fig. 85). The 

 whole colony may then behave somewhat as an individual, the 

 polyps taking the place of organs (Siphonophora). Colonial 

 animals are almost always attached to fixed or floating objects. 

 These polymorphic individuals are closely adapted to each 

 other in structure and division of labor ; and the colonial habit 

 in general, even where there is no division of labor, is a suc- 

 cessful device whereby limited areas are completely occupied 

 by the members of a species (as in the case of the branching 

 corals) where the single polyps would be practically helpless. 

 The arrangement of the polyps on the common skeleton and 

 the rate of growth of the different polyps are beautifully 

 adapted to the best use of the currents of water by which the 

 food and oxygen are conveyed. 



150. Library Exercise. What phyla of the animal kingdom supply 

 instances of organic colonies? Trace different degrees of polymorphism. 

 In what different ways do the individuals occur on the common stock? 

 Show how the relative rate of growth of the differently placed individuals 

 determines the ultimate form of the colony as a whole. 



151. Social and Communal Life. Animals of the same 

 species often become associated even when there is no organic 

 connection between the individuals. The association may be 

 temporary or permanent. In its simplest form this is merely 

 a matter of gregariousness such as is seen in the schools of 

 fishes or flocks of birds, which are apparently brought together 

 at certain periods by a common instinct or by common needs. 

 A step more intimate is the banding together of predaceous 

 animals as wolves or vultures, or pelicans, for mutual help in 

 finding or capturing the prey. Corresponding to this, on the 



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