It is clear that the same difficulties apply when we try to work out the development on the hypothesis that a group of mothers existed.  We are therefore reduced to the supposition that the term brother denoted originally a person born within a given period of time, and that this period was the same for whole sections of the community; in other words that the name brother was given to all males born between, let us say, B.C. 10,000 and B.C. 9,990.  This is of course equivalent to the establishment of age grades and is in itself not unthinkable; age grades are of course perfectly well known among primitive peoples; but the establishment of age grades implies a degree of social organisation; and, what is more important, this hypothesis makes the term brother quite meaningless as a kinship term; for at the present day a common term of address for members of an age grade does not imply any degree of consanguinity, and unless it be proved that age grades are a product of the period of “group marriage” it cannot be argued that they ever did imply kinship.

It is sufficiently clear from these examples that Morgan entirely failed to work out the process by which the transition from pure to regulated promiscuity came about.  But if the process is uncertain the causes are equally obscure.  In Mr Morgan’s view, or at any rate in one of the theories on which he accounted for the change, it was due to “movements which resulted in unconscious reformation”; these movements were, he supposes, worked out by natural selection.  These words, it is true, apply primarily to the origin of the “tribal” or “gentile” organisation, as Mr Morgan terms totemism, but they probably apply to the original passage from promiscuity to “communal marriage,” and I propose to examine how far such a theory has any solid basis.

Natural selection is a blessed phrase, but in the present case it is difficult to see in what way it is supposed to act.  The variation postulated by Mr Morgan as a basis for the operation of natural selection is one of ideas, not physical or mental powers.  Now under ordinary circumstances we mean by natural selection the weeding out of the unfit by reason of inferiorities, physical or psychical, which handicap them in the struggle for existence.  But it cannot be said that the tendency to marry or practice of marrying outside one’s own generation is such a handicap to the parents.  How far is it injurious to the children of such unions?  Or rather, how far have children who are the offspring of brothers and sisters or of cousins a better chance of surviving than the offspring of unions between relatives of different generations?