170 ANIMAL LIFE 



all-important as to gain for him the name of " the social 

 animal," there is no differentiation of individuals adapted 

 only for certain kinds of work. Among these highest 

 examples of social animals, the presence of an advanced 

 mental endowment, the specialization of the mental power, 

 the power of reason, have taken the place of and made 

 unnecessary the structural differentiation of individuals. 

 The honey-bee workers do different kinds of work : some 

 gather food, some care for the young, and some make wax 

 and build cells, but the individuals are interchangeable ; 

 each one knows enough to do these various things. There 

 is a structural differentiation in the matter of only one 

 special work or function, that of reproduction. 



With the ants there is, in some cases, a considerable 

 structural divergence among individuals, as in the genus 

 Atta of South America with six kinds of individuals 

 namely, winged males, winged females, wingless soldiers, 

 and wingless workers of three distinct sizes. In the case 

 of other kinds with quite as highly organized a communal 

 life there are but three kinds of individuals, the winged 

 males and females and the wingless workers. The workers 

 gather food, build the nest, guard the " cattle " (aphids), 

 make war, and care for the young. Each one knows enough 

 to do all these various distinct things. Its body is not so 

 modified that it can do but one kind of thing, which thing 

 it must always do. 



The increase of intelligence, the development of the 

 power of reasoning, is the most potent factor in the devel- 

 opment of a highly specialized social life. Man is the 

 example of the highest development of this sort in the ani- 

 mal kingdom, but the highest form of social development 

 is not by any means the most perfectly communal. 



89. Advantages of communal life. The advantages of 

 communal or social life, of co-operation and mutual aid, are 

 real. The animals that have adopted such a life are among 

 the most successful of all animals in the struggle for exist- 



