(5) Turning to factors that have in general received less explicit recognition at the hands of the authorities who have written on the subject, we may note first that as regards the most fundamental type of incest as revealed by psycho-analytic study--that of parent and child--there is involved a sort of biological absurdity, which may well have been to some extent instrumental, through the agency of natural selection, of bringing about that inhibition of the incestuous tendencies for which we have to account. Parents and children necessarily differ considerably in age, though less so in most primitive communities than in the civilised societies of to-day, where marriage is so often postponed till relatively late in life. Even among primitive peoples however the difference is very appreciable, especially in view of the fact (which must be borne in mind in this connection) that with such peoples, owing to the harder conditions of existence, life is often shorter than in civilised communities, while the enfeeblement that accompanies advancing age comes on proportionately sooner. If men were to follow blindly the impulses manifested in the primitive and fundamental forms of incest tendency, sons would cohabit with their mothers, daughters with their fathers. In such unions one of the partners would be relatively aged, and the offspring would in consequence very probably be lacking in that degree of vitality or health normally possessed by the children of parents of more equal age; they would moreover fail, in the majority of cases, to enjoy that degree of provision and protection which could be afforded where _both_ parents are still youthful. Even if such unions were to occur, they could not (in the case of one sex at any rate) be continued in the following generation; for if a son were to cohabit with his mother and if a male child resulted from their union, this child could not in turn fruitfully unite with his mother (who would also be his grandmother), as she would be now definitely past the reproductive age. Further, such unions would come into opposition with the almost universal tendency to find sexual attractiveness in youthful rather than in aged persons--a tendency which, like the appreciation of beauty in the opposite sex in general, we may suppose has been shaped largely, if not wholly, by the operation of natural selection, which has ensured that men and women should in the main be attracted to those who are most likely to produce strong and healthy children.