HISTORY AND NATURAL HISTORY 43 



there, if favorable niches are somewhat rare, dense 

 aggregations may result, like New England coral on 

 a suitably hard bottom, or the animals found on a 

 wharf piling. 



These accidental animal groupings may persist 

 only as long as the physical forces which brought 

 them together continue to act. Usually, however, 

 they last somewhat longer, as a result of a slightly 

 positive social inertia which tends to keep animals 

 concentrated in whatever place they happen to be 

 found. If the groupings are to have much perma- 

 nence this quality of social inertia, the tendency of 

 animals to continue repeating the same action in 

 the same place, must be reinforced by another 

 quality: the social force of toleration for the pres- 

 ence of others in a limited space. The densely packed 

 communities of animals on a wharf piling can per- 

 sist only if toleration for crowding is well developed. 



Other dense collections may be brought about by 

 forced movements of animals in response to some 

 orienting influence in their environment. These 

 oriented, compelled reactions are frequently called 

 tropisms. They are shown by the moths or June 

 beetles or may-flies that collect about lights. Such 

 aggregations are a result of the inherited, internal 

 organization of the animals; and the irresistible at- 

 traction of the may-fly to the light is joined with 



