If this conclusion be accepted, it will follow that the more widely distributed varieties of the classificatory system of relationship are associated with a social structure which has the exogamous social group as its essential unit. This position has only to be stated for it to become apparent how all the main features of the classificatory system are such as would follow directly from such a social structure. Wherever the classificatory system is found in association with a system of exogamous social groups, the terms of relationship do not apply merely to relatives with whom it is possible to trace genealogical relationship, but to all the members of a clan of a given generation, even if no such relationship with them can be traced. Thus, a man will not only apply the term “father” to all the brothers of his father, to all the sons’ sons of his father’s father, and to all the sons’ sons’ sons of his father’s father’s father, to all the husbands of his mother’s sisters and of his mother’s mother’s granddaughters, etc., but he will also apply the term to all the members of his father’s clan of the same generation as his father and to all the husbands of the women of the mother’s clan of the same generation as the mother, even when it is quite impossible to show any genealogical relationship with them. All these and the other main features of the classificatory system become at once natural and intelligible if this system had its origin in a social structure in which exogamous social groups, such as the clan or moiety, were even more completely and essentially the social units than we know them to be to-day among the peoples whose social systems have been carefully studied. If you are dissatisfied with the word “classificatory” as a term for the system of relationship which is found in America, Africa, India, Australia and Oceania, you would be perfectly safe in calling it the “clan” system, and in inferring the ancient presence of a social structure based on the exogamous clan even if this structure were no longer present. Not only is the general character of the classificatory system exactly such as would be the consequence of its origin in a social structure founded on the exogamous social group, but many details of these systems point in the same direction. Thus, the rigorous distinctions between father’s brother and mother’s brother, and between father’s sister and mother’s sister, which are characteristic of the usual forms of the classificatory system, are the obvious consequence of the principle of exogamy. If this principle be in action, these relatives must always belong to different social groups, so that it would be natural to distinguish them in nomenclature.