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- 
