avoidance on the wife’s mother and the daughter’s husband, with exactly the same psychological correlate, it is, to my mind, rash to decree without attempt to produce evidence that this custom must, in each case, have developed from entirely distinct motives. To be sure, this particular usage has not yet, in my opinion, been satisfactorily accounted for. Nevertheless, in contradistinction to some of my colleagues and to the position I myself once shared, I now believe that it is pusillanimous to shirk the real problem involved, and that in so far as any explanation admits the problem, any explanation is preferable to the flaunting of fine phrases about the unique character of cultural phenomena. When, however, we ask what sort of explanation could be given, we find that it is by necessity a _cultural_ explanation. Tylor, _e.g._, thinks that the custom is correlated with the social rule that the husband takes up his abode with the wife’s relatives and that the taboo merely marks the difference between him and the rest of the family. We have here clearly one cultural phenomenon as the determinant of another. It is not so difficult as might at first appear to harmonize the principle that a cultural phenomenon is explicable only by a unique combination of antecedent circumstances with the principle that like phenomena are the product of like antecedents. The essential point is that in either case we have past history as the determinant. It is not necessary that certain things should happen; but if they do happen, then there is at least a considerable likelihood that certain other things will also happen. Diversity occurs where the particular thing of importance, say the wheel, has been discovered or conceived in one region but not in another. Parallelism tends to occur when the same significant phenomenon is shared by distinct cultures. It remains true that in culture history we are generally wise after the event. _A priori_, who would not expect that milking must follow from the domestication of cattle?