Apart from human societies the most noteworthy social communities of animals are found among insects, especially in ants, bees, wasps, and termites. It is true that in the mammalia we find such communities as the troop of apes, the herd of cattle, the pack of wolves, the school of porpoises, the so-called “rookeries” of seals, and the colonies of “prairie dogs” and of beavers; and that among birds there are analogous communities. Undoubtedly the temporary or permanent association of many individuals is in such cases an advantage to the race, and confers mutual benefits on the associates. But in none of these cases is division of labour carried to such a high degree as among the social insects. And it is through such division of labour that the social community 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 accommodation 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, _Hydractinia_, 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. Intermediate 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 exemplified in the common wood ant (Fig. 24). In some cases the workers themselves may be divided into different castes. [Illustration: FIG. 24.--Wood ant. 1, Queen; 2, male; 3, worker (from Shipley).]