I n the three chapters which make up Part I of this mono graph it is my aim to discuss certain aspects of the psychology and sociology of property in the animal world. As I have suggested in Chapter I, this aim can only be achieved through a consideration of two related problems. First, we must ask, what are the facts as to the existence or not of property among animals? These we may determine only through the use of an objective criterion. The most suitable criterion by which to judge relevant behaviour situations is that possession shown by defence against aggression. Accordingly, then, our first object is to determine the nature and extent of property among animals as shown by exclusive possession and active defence of such property objects against the aggression and attempted spoliation by indi viduals of the same related species or of entirely different species. This exclusive possession and control, which is the mark of interest in property, will have variable time limits. Among the solitary wasps and bees, for example, the nest is defended only during the activity of the reproductive cycle. While the female is actively engaged in completing and provisioning her nests, they will be defended. Later, egg laid and nest sealed, the wasp will be entirely indifferent to foes, animal or human. At the other end of the insect scale, however, one may see an ant colony flourish for many years and maintain its nest intact during this period against the attacks of marauding species. Whether ownership is shown, therefore, for three hours, three months or three years, is immaterial to our criterion. So long as a study of each case reveals exclusive possession reinforced by a dis position to defend against the interference of others, we are justified in assuming that we have before us an example of a primitive property relationship.