Ontogeny is really a new science in our century, introduced by Von Leeuwenhock, de Graaf, and others. It was founded in 1827, by von Baer. The differences of opinion and the discovery of different theories are very far from being ended. Purely scientific points of view are being combined with social, physiological, or ethical ones. It is maintained that by changing the diet of the mother the sex of the child can be determined. Attempts have been made to show that about three fifths of all men of genius were first-born children. People are studying what influence the age of parents has on the child; extreme youth of parents seems unfavourable for the offspring as well as extreme age. The first child of a too youthful mother is often weak, and besides ordinarily the joys of motherhood are not desired, because she feels that physically and psychically a child is too great a burden to her, who herself is only a child. The conditions of a strong, well-nourished offspring require the postponement of the marriage age for women. In northern countries it should be established, if not by law at least by custom, at about twenty years. This is all the more necessary because then the young woman can have behind her some years of careless youthful joy, an undisturbed self-development, and will also have reached the physical development necessary for motherhood. While twenty years should be regarded as the earliest period of marriage it should actually be often postponed some years still for the well-being of the woman, the man, and the children, and married life as a whole, in which most conflicts arise because women have decided about their fate before their personality was definitely formed, before their heart was able to find its choice. The love of the man chooses and the young girl often confuses the happiness of being loved with the happiness of loving, an experience which later on is gone through in a tragic way. To the many questions which are related to heredity and natural selection, belongs one which notices the significance of nature's purpose to cause strong opposites to exert upon one another the strongest attraction. This attraction often during married life changes into antipathy; it almost results in impatience against the characteristics which originally had so deep an attraction. Nature in this case seems to wish to reach its end with the greatest lack of consideration for the happiness of the individual. So often the contradictions of parents seem really to be moulded in full in the child. Occasionally these contradictions are expressed as a deep discord, but in both cases there often arises an exceptional being. To attain correct results in this case, belongs to the numerous still open possibilities.