The woman was the social nucleus, the point to which he returned from his wanderings. In this primitive stage of society, however, the bond between woman and child was altogether more immediate and constraining than the bond between woman and man. The maternal instinct is reinforced by necessary and constant association with the child. We can hardly find a parallel for the intimacy of association between mother and child during the period of lactation; and, in the absence of domesticated animals or suitable foods, and also, apparently, from simple neglect formally to wean the child, this connection is greatly prolonged. The child is frequently suckled from four to five years, and occasionally from ten to twelve.[95] In consequence we find society literally growing up about the woman. The mother and her children, and her children's children, and so on indefinitely in the female line, form a group. But the men were not so completely incorporated in this group as the women, not only because parentage was uncertain and naming of children consequently on the female side, but because the man was neither by necessity nor disposition so much a home-keeper as the women and their children. The tangential disposition of the male is expressed in the system of exogamy so characteristic of tribal life. The movement toward exogamy doubtless originates in the restlessness of the male, the tendency to make new co-ordinations, the stimulus to seek more unfamiliar women, and the emotional interest in making unfamiliar sexual alliances. But, quite aside from its origin, exogamy is an energetic expression of the male nature. Natural selection favors the process by sparing the groups which by breeding out have heightened their physical vigor.[96] There results from this a social condition which, from the standpoint of modern ideas, is very curious. The man makes, and, by force of convention, finally must make, his matrimonial alliances only with women of other groups; but the woman still remains in her own group, and the children are members of her group, while the husband remains a member of his own clan, and is received, or may be received, as a guest in the clan of his wife. Upon his death his property is not shared by his children, nor by his wife, since these are not members of his clan; but it falls to the nearest of kin within his clan--usually to his sister's children. The maternal system of descent is found in all parts of the world where social advance stands at a certain level, and the evidence warrants the assumption that every group which advances to a culture state passes through this stage. Morgan gives an account of this system among the Iroquois: