In any case before my conjecture is definitely laid aside as erroneous, it may, let us hope, be considered desirable to await fuller evidence as to the extent of the operation, in actual savage life, of that particular custom of ‘Avoidance’ which I consider, in its inception, and as the earliest law, to have been a ‘vera causa’ of widest operation in primitive social evolution. ‘Avoidance’ is, however, to-day, a mere faint image of a remote past, and its genetic significance has utterly faded from among even those people who yet, with strange conservatism, still blindly yield an everyday obedience to it, in form at least. Belonging to a class of savage habits presenting features so extraordinary, ‘Avoidance between brother and sister’ has ever been a puzzle to inquirers. This Avoidance is only the most obscure of all the numerous cases of the strange habit, but it is also that which, up to the present, seems least to have attracted the notice of anthropologists. In this class of custom, the Avoidance of which most frequent mention has been made in literature, is avoidance between mother-in-law and son-in-law, whereas that between brother and sister is to my knowledge but rarely mentioned.[4] And yet, as far as my own experience goes (and it extends over more than a quarter of a century among primitive peoples in the South Seas), Avoidance of brother and sister is not only as common as, but infinitely more strict and severe in action than, the Avoidance of ‘Mothers-in-law.’ It is indeed probable that the very severity of observance has led to its being so little noticed. For by the action of this law, a brother and a sister, after childhood, are kept so far apart from one another, that only those who have actually lived long amidst natives can be expected to have had a chance of being aware of the restraints to intercourse between them. Even then it would be from some such casual occurrence as the accidental rencontre of the two, placing them thus in sudden and unavoidable proximity to each other, which would lead to an observation, by an European, of their extraordinary attitude and behaviour under such circumstances.