122 ZOOLOGY 



colony {polymorphism, Fig. 87). 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 indi- 

 viduals are closely adapted to each other in structure and divi- 

 sion of labor; and the colonial habit in general, even where there 

 is no division of labor, is a successful 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. 



154. Library and Museum Exercise. — What phyla of the animal kingdom 

 supply instances of colonies? Trace di£Eerent 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 very much as bud arrangement determines 

 the character of a tree. 



155. 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. The bond in these cases is not phys- 

 ical, but instinctive and psychical. 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. 

 Undoubtedly sex attraction is an early bond and a powerful one 

 in bringing individuals together in pairs or in small groups. A 

 step more intimate than mere gregariousness is the banding 

 together of predaceous animals as wolves or vultures, or pelicans, 

 for mutual help in flnding or capturing the prey. Correspond- 

 ing to this, on the part of their victims, we find the herding of 

 the bison, of deer, and their allies for protection, whether by 

 fighting together or by the stationing of sentinels to give notice 

 to the feeding herd of the approach of danger. In still other 

 forms, notably among such insects as the bees and ants, there 

 is a very intimate and permanent union in social life. This is 



