250 THE SOCIAL LIFE OF ANIMALS 



convenience that we interrupt the connecting chain 

 of events at some comparatively conspicuous link 

 and arbitrarily make this the dividing point, when 

 one is needed, between the more and the less social. 

 It must be recognized that any such division is a 

 matter of convenience rather than a natural break in 

 the development from mass or simple group behavior 

 to highly evolved social life. 



For our purpose in the present account it is suffi- 

 cient to recognize that the well-integrated social 

 systems of man and other mammals, of bird flocks 

 and of insect colonies, exhibit among them the 

 highest expressions of social abilities that have 

 evolved. In the range of social development shown 

 in these animals we find attributes that are truly 

 social in the most exclusive use of the word. But 

 these highest expressions of social living have their 

 roots in tendencies that in the form of unconscious 

 co-operation accompany animal aggregations extend- 

 ing throughout the whole animal world, as well as 

 to some extent among plants. Conceding then the 

 difficulties in the way of making any exact definition 

 of social behavior, I wish to present some of the 

 social implications of mass physiology, particularly 

 among well-integrated societies of animals. 



One of the characteristics of social life among the 

 insects is the presence of castes (121) which perform 



