In any case the natural tendency in a small group would be to marry within the generation, and this might readily become crystallised in the kinship terms.
The eight-class system, as we have seen, resulted from the distinction between elder and younger sister. What is the meaning of this and what analogies do we find to it?
Widely extended also are the systems of age-grades. In all parts of the world the men, and sometimes the women, are or have been divided into associations, to which reference was made in Chapter I, which begin by being co-extensive with the tribe for all practical purposes, since all pass through the initiation ceremonies. The various initiation ceremonies during what may be termed the involuntary stage of these associations, no less than in their later form of secret societies, determine the rights and duties of the individuals who undergo them. The period at which they take place is determined, broadly speaking, by the age of the individual. It is therefore clear that for the peoples in the lower stage of culture considerations of age are of the highest importance.
We find that in practice the elder brother has much authority, both over the younger brother and the sister. In Victoria he decides whom they are to marry. As we have seen in the tables of terms, the Wathi-Wathi man distinguishes both elder and younger of either sex by special terms, which points to their having special rights or duties.
If therefore we cannot see why primitive man should have enacted that the elder rather than the younger, or the daughter of the elder rather than the daughter of the younger, should be preferred, it is at any rate of a piece with his other customs.
From the terms of kinship tabulated above various conclusions have been drawn. It will be seen that a man applies to all the women in the other phratry on the level of his generation the same term as he applies to his actual wife. On this basis it has been argued that at one time all the men in one phratry were united in marriage with all the women in the other within the limits of the generation. Before this again a stage of absolute promiscuity is supposed to have existed. This alternative explanation of the kinship organisations demands to be considered.
 J.A.I. XIV, 354; N. Queensl. Eth. Bull. VI, 6; Spencer and Gillen, Northern Tribes, p. 90.
 Morgan, in Smithsonian Contr. vol. XVII; Globus, LXIX, 3; Nat. Tribes, pp. 88-9.
 For lists of tribes where this distinction is found see Mathew, Eaglehawk, p. 223-4.
TYPES OF SEXUAL UNIONS.
Terminology of Sociology. Marriage. Classification
of Types. Hypothetical
and existing forms.