The first thing to grasp is that, for peoples of this type, social organization is, primarily and on the face of it, identical with kinship-organization.  Before proceeding further, let us see what kinship means.  Distinguish kinship from consanguinity.  Consanguinity is a physical fact.  It depends on birth, and covers all one’s real blood-relationships, whether recognized by society or not.  Kinship, on the other hand, is a sociological fact.  It depends on the conventional system of counting descent.  Thus it may exclude real relationships; whilst, contrariwise, it may include such as are purely fictitious, as when some one is allowed by law to adopt a child as if it were his own.  Now, under civilized conditions, though there is, as we have just seen, such an institution as adoption, whilst, again, there is the case of the illegitimate child, who can claim consanguinity, but can never, in English law at least, attain to kinship, yet, on the whole, we are hardly conscious of the difference between the genuine blood-tie and the social institution that is modelled more or less closely upon it.  In primitive society, however, consanguinity tends to be wider than kinship by as much again.  In other words, in the recognition of kinship one entire side of the family is usually left clean out of account.  A man’s kin comprises either his mother’s people or his father’s people, but not both.  Remember that by the law of exogamy, the father and mother are strangers to each other.  Hence, primitive society, as it were, issues a judgment of Solomon to the effect that, since they are not prepared to halve their child, it must belong body and soul either to one party or to the other.

We may now go on to analyse this one-sided type of kinship-organization a little more fully.  There are three elementary principles that combine to produce it.  They are exogamy, lineage and totemism.  A word must be said about each in turn.

Exogamy presents no difficulty until you try to account for its origin.  It simply means marrying-out, in contrast to endogamy, or marrying-in.  Suppose there were a village composed entirely of McIntyres and McIntoshes, and suppose that fashion compelled every McIntyre to marry a McIntosh, and every McIntosh a McIntyre, whilst to marry an outsider, say a McBean, was bad form for McIntyres and McIntoshes alike; then the two clans would be exogamous in respect to each other, whereas the village as a whole would be endogamous.

Lineage is the principle of reckoning descent along one or other of two lines—­namely, the mother’s line or the father’s.  The former method is termed matrilineal, the latter patrilineal.  It sometimes, but by no means invariably, happens, when descent is counted matrilineally, that the wife stays with her people, and the husband has the status of a mere visitor and alien.  In such a case the marriage is called matrilocal; otherwise it is patrilocal.  Again, when the matrilocal type of marriage prevails,