=Sexual selection is certainly true of mankind.=--Before adducing the outlines of the evidence in favour of marriage as an instrument of selection, it may be well to point out that here we are really discussing what Darwin called “sexual selection,” modified by the psychology and peculiar characters of mankind. We must protect ourselves from the critics who will remind us that sexual selection is very largely discredited to-day, rather more than a generation after Darwin's enunciation of it in _The Descent of Man_ (1871). The controversy regarding sexual selection as the producer of feathers and markings and song, and so forth, amongst the lower animals, is fortunately quite irrelevant to our present discussion, which is concerned with mankind. We can afford to note with equanimity the observation that, in lower species, no mature female goes unmated, for instance; the fact remains that in the case of mankind a very considerable percentage of women remain unmarried. The case is similar as regards the male sex. In short, one may declare that, whether or not sexual selection is possible, or occurs, or accomplishes anything, in the case of the lower animals, it palpably and patently is possible, and does occur, amongst mankind, and especially amongst civilised peoples, in the form of selection by or for marriage--which, as we have seen, is in effect selection for parenthood. Let us first note the statistical evidence regarding marriage-selection of health and energy. =Spencer on marital longevity.=--We are all aware that married people live longer, on the average, than unmarried people, the conclusion being, “of course,” that marriage is good for the health. But some are taken and others left in this respect, and if, for any conceivable reason, health is a factor making for selection by marriage, that may be a real explanation, in whole or in part, of the longer life of married people. Considering the risks to life involved in motherhood, the superior longevity of married as compared with unmarried women would be incomprehensible except on some such assumption. Yet it is the fact, so imperfect still is the entry of the idea of selection into the popular and even the expert mind, that the superior longevity of married people is still constantly asserted to mean that marriage makes for long life; every year, when the statistics are printed, this argument may be seen in the newspapers, and I remember encountering it in the _Encyclopædia Britannica_, to my utter astonishment.