206 SOCIAL BEHAVIOUR 
And it is through such division of labour that the social com- 
munity reaches its highest expression. 
It is a somewhat remarkable fact that in man, where we 
find the social division of labour brought to a high pitch of 
perfection, and carried out with great nicety of accommoda- 
tion to those circumstances which civilization has rendered 
extremely complicated, there is no organic differentiation of 
structure among the co-operating individuals ; whereas, so 
low down in the scale of life as the colonial polype, Hydrac- 
tinia, which is often found growing on the shells occupied by 
hermit crabs, there are at least three kinds of differentiated 
individuals: nutritive polypes with mouth and tentacles; 
mouthless sensitive members ; and others whose sole office is 
reproduction. But these differentiated individuals in the 
colonial zoophytes are connected at their bases by a common 
flesh ; and the division of labour is a product of organic 
evolution, and is probably not in any degree determined or 
guided by consciousness. We may say, then, that the division 
of labour in the zoophyte is wholly physical, whereas in man 
it is chiefly conscious or psychical ; as is also the bond of 
union between the several members of the colony. Inter- 
mediate between these extremes stand the social insects. In 
them there is no physical bond of union, for each individual 
is distinct and separate ; the social linkage is in some degree 
conscious under the conditions of their nurture; and the 
division of labour is partly conscious, though probably in 
large degree based on instinctive foundations, and partly the 
outcome of an organic differentiation of structure seen in the 
reproductive members and in the sterile workers, as exempli- 
fied in the common wood ant (Fig. 24). In some cases the 
workers themselves may be divided into different castes. 
So much has been written—and well written—on the 
social life of insect communities, that it will here suffice to 
indicate some of the problems which arise when we endeavour 
to interpret the modes of behaviour which have been carefully 
observed. In the honey-bee we have the well-known differen- 
tiation of structure into drones or effective males, queens 
