2. The loss of reputation has prevented the marriage of the desired mates. This is not at all uncommon. 3. Venereal infection has led to the abandonment of marriage. This is especially common. 4. Illicit experiences may have been so disillusioning, owing to the disaffecting nature of the consorts, that an attitude of pessimism and misanthropy or misogyny is built up. Such an attitude prevents marriage not only directly, but also indirectly, since persons with such an outlook are thereby less attractive to the opposite sex. 5. A taste for sexual variety is built up so that the individual is unwilling to commit himself to a monogamous union. 6. Occasionally, threat of blackmail by a jilted paramour prevents marriage by the inability to escape these importunities. We consider next the relative birth-rate of the married and the incontinent unmarried. There can not be the slightest doubt that this is vastly greater in the case of the married. The unmarried have not only all the incentives of the married to keep down their birth-rate but also the obvious and powerful incentive of concealment as well. Passing to the relative death-rate of the illegitimate and legitimate progeny, the actual data invariably indicate a decided advantage of the legitimately born. The reasons are too obvious to be retailed. Now, then, knowing that the racial contribution of the sexually moral is greater than that of the sexually immoral, we may compare the quality of the sexually moral and immoral, to get the evolutionary effect. For this purpose a distinction must be made between the individual who has been chaste till the normal time of marriage and whose sexual life is truly monogamous, and that abnormal group who remain chaste and celibate to an advanced age. These last are not moral in the last analysis, if they have valuable and needed traits and are fertile, because in the long run their failure to reproduce affects adversely the welfare of their group. While the race suffers through the failure of many of these individuals to contribute progeny, probably this does not happen, so far as males are concerned, as much as might be supposed, for such individuals are often innately defective in their instincts or, in the case of disappointed lovers, have a badly proportioned emotional equipment, since it leads them into a position so obviously opposed to race interests. But, to pass to the essential comparison, that between the sexually immoral and the sexually moral as limited above, it is necessary first of all to decide whether monogamy is a desirable and presumably permanent feature of human society. We conclude that it is: 1. Because it is spreading at the expense of polygamy even where not favored by legal interference. The change is most evident in China. 2. In monogamy, sexual selection puts a premium on valuable traits of character, rather than on mere personal beauty or ability to acquire wealth; and