The sporadic occurrence of gregariousness at widely separated points of the animal field—in man and sheep, in ant and elephant—inclines one to suppose that multicellularity must have arisen also at multiple points, and that the metazoa did not arise from the protozoa by a single line of descent. It {104} suggests also that there is some inherent property in mobile living organisms that makes combination of individuals into larger units a more or less inevitable course of development under certain circumstances and without any gross variation being necessary to initiate it. The complex evolution which multicellularity made possible, and perhaps enforced, can scarcely fail to make one wonder whether the gregarious animal has not entered upon a path which must of necessity lead to increasing complexity and co-ordination, to a more and more stringent intensity of integration or to extinction. The varying degrees to which the social habit has developed among different animals provide a very interesting branch of study. The class of insects is remarkable in furnishing an almost inexhaustible variety of stages to which the instinct is developed. Of these that reached by the humble bee, with its small, weak families, is a familiar example of a low grade; that of the wasp, with its colonies large and strong, but unable to survive the winter, is another of more developed type; while that of the honey bee represents a very high grade of development in which the instinct seems to have completed its cycle and yielded to the hive the maximum advantages of which it is capable. In the honey bee, then, the social instinct may be said to be complete. It is necessary to examine somewhat closely into what is denoted by the completeness or otherwise of the social habit in a given species.