It would be interesting to know, if it could be ascertained, as throwing light on this question with respect to man, what occurs with the higher anthropomorphous apes - whether the young males and females soon wander away from their parents, or whether the old males become jealous of their sons and expel them, or whether any inherited instinctive feeling, from being beneficial, has been generated, leading the young males and females of the same family to prefer pairing with distinct families, and to dislike pairing with each other. A considerable body of evidence has already been advanced, showing that the offspring from parents which are not related are more vigorous and fertile than those from parents which are closely related; hence any slight feeling, arising from the sexual excitement of novelty or other cause, which led to the former rather than the latter unions, would be augmented through natural selection, and thus might become instinctive; for those individuals which had an innate preference of this kind would increase in number. It seems more probable that degraded savages should thus unconsciously have acquired their dislike and even abhorrence of incestuous marriages, rather than that they should have discovered by observation and reasoning the evil results. The abhorrence occasionally failing is no valid argument against the feeling being instinctive, for any instinct may occasionally fail or become vitiated, as sometimes occurs with parental love and the social sympathies. In the case of man, the question whether evil follows from close interbreeding will probably never be answered by direct evidence, as he propagates his kind so slowly and cannot be subject to experiment; but the almost universal practice of all races at all times of avoiding closely-related marriages is an argument of considerable weight; and whatever conclusion we arrive at in regard to the higher animals may safely be extended to man (1868, vol.2, pp.123-4)

It has been clearly shown by Mr Huth that there is no instinctive feeling against incest in man any more than in gregarious animals. We know also how readily any prejudice or feeling may give rise to abhorrence, as shown by Hindus in regard to objects causing defilement. Although there seems to be no strong inherited feeling in mankind against incest, it seems possible that men during primeval times may have been more excited by strange females than by those with whom they habitually lived; in the same manner as according to Mr Cupples, male deerhounds are inclined towards strange females, while the females prefer dogs with whom they have associated. If any such feelings formerly existed in man, this would have led to a preference for marriages beyond the nearest kin, and might have been strengthened by the offspring of such marriages surviving in greater numbers, as analogy would lead us to believe would have occurred (1875, vol.2, pp.103-4)

The breach of a rule held sacred by the tribe, will thus, as it seems, give rise to the deepest feelings, - and this quite apart from the social instincts, excepting in so far as the rule is grounded on the judgement of the community. How so many strange superstitions have arisen throughout the world we know not; nor can we tell how some great and real crimes, such as incest, have come to be held in abhorrence (which is not however quite universal) by the lowest savages. It is even doubtful whether in some tribes incest would be looked on with greater horror, than would the marriage of a man with a woman bearing the same name, though not a relation....[quotes examples from E. B. Tylor] We may therefore reject the belief, lately insisted on by some writers, that the abhorrence of incest is due to our possessing a special God-implanted conscience.(1874, p.176)

To this I can only answer by remarking, that a new mode of attack with new weapons might be a great advantage, as shown by the case of the Ovis cycloceros [described earlier] who thus conquered a domestic ram famous for his fighting power. Though the branched antlers of the stag are well adapted for fighting with his rivals, and though it might be an advantage to the prong-horned variety slowly to acquire long and branched horns, if he had to fight only with others of the same kind, yet it by no means follows that branched horns would be the best fitted for conquering a foe differently armed. In the foregoing case of the Oryx leucoryx, it is almost certain that the victory would rest with an antelope having short horns, and who therefore did not have to kneel down, though an oryx might profit by having still longer horns, if he fought only with his proper rivals. (1874, p.783)

As everyone knows, male deer have branched antlers. During the breeding season, two stags fight by lowering their heads so that their antlers interlock. Each then attempts to force the other backwards, until at last the weakest is forced to break away and flee. Because of the branching structure of the antlers it is rare for a stag to be pierced by its opponent's antlers. Occasionally, however, a stag grows antlers without branches; such a stag may wound and kill its adversary.... Why should natural selection have favoured a device - the branching of antlers - which appears to reduce the chances a stag may have of winning fights? (1972, p.8)