I. On the mother’s side;
a. Owing to peculiarities of the fluids and tissues of the maternal body;
b. Owing to substances which penetrate the maternal organism and reach the fertilized ovum.
II. On the father’s side, owing to substances which adhere to the paternal reproductive cells, or are enclosed within these.
The number of consanguineous marriages at the present day is not less than 5½ to 6½ per 1,000; the fertility of these marriages appears to be identical with the fertility of ordinary marriages. Mayet has made a statistical investigation to determine the influence of consanguineous marriages in the pathogenesis of mental disease. He finds that the number of those congenitally affected with mental disorder is twice as great in the offspring of consanguineous marriages as in the offspring of crossed marriages; in the case of simple mental disorder, of paralytic dementia, and of epileptic dementia, the ratio is actually greater than two to one (the actual figures are 218, 257, 208 : 100). Thus we see that when there exists any cause of inheritable mental disorder, blood-relationship of the parents more than doubles the danger to the children. In the case of imbecility and idiocy the danger is less in this respect (the ratio is 150 : 100); the factor of inheritance plays a less prominent part than in the case of other psychoses.
It was remarkable that among the offspring of marriages of nephew and aunt, cases of mental disorder were almost entirely lacking. Among the offspring of marriages of uncle and niece, the inheritance of mental disorder was more prominent than among the children of first cousins. It is interesting to determine the influence of blood-relationship in cases in which the existence of inheritable predisposition could not be proved. In these cases, as regards simple insanity, paralytic dementia, and epileptic dementia, the number of cases among the offspring of consanguineous marriages was only one-half as compared with the offspring of crossed marriages; whereas in the case of imbecility and idiocy this ratio was reversed. In idiocy, where inheritance generally speaking plays a small part, the origination of the disease would often appear to depend directly on the blood-relationship of the parents; whilst as regards other forms of mental disorder, if there is no inheritable predisposition, blood relationship in the parents appears to be a positive advantage; where, however, a family predisposition to insanity exists the likelihood of actual insanity appearing in the offspring is notably enhanced by a consanguineous marriage.
The Restriction of Fertility and the Use of Means for the Prevention of Pregnancy.
As we have already pointed out, a restriction of the fertility of women occurs in the majority of marriages, to this extent, that the potential reproductive powers of the wife are not fully utilized. In recent times, however, the restriction of fertility, by the deliberate use of measures for the prevention of pregnancy, has become so widely diffused, that it appears unwise from the scientific standpoint simply to ignore the question, and it has become indispensable to study how the practice developed, and to consider what are its actual results. From our own point of view, it is the more necessary to do this, for the reason that the use of preventive measures has come to play an important part in the sexual life of woman, and therefore deserves the fullest attention, not merely from the standpoint of the sociologist, but in addition from the purely medical point of view.
In many divisions of the population, and even in entire nationalities, the prevention of pregnancy, not merely in illicit intercourse, but also in married life, has become so general a practice that the fertility of the nation as a whole has been profoundly modified. Thus, in France at the present day, the average number of children per marriage is less than two; and the two-children-system is almost universally practised in Transylvania and Norway, whilst it is very rapidly spreading in North America. In the principal towns of the whole of Europe, this system is largely on the increase among the upper classes of society. The marriages of the poor, partly owing to ignorance, and partly to indolence, are as yet comparatively little affected by this depopulative principle.
In the days of antiquity, many lawgivers endeavoured to set bounds to excessive fertility, and artificial abortion was methodically practised by those who wished to avoid an inconveniently large family. Even among savage peoples, we find that certain preventive measures are occasionally employed in sexual intercourse. Among civilized peoples, however, until the beginning of the nineteenth century, religious and moral ideas derived from the Bible continued to dominate the sexual life. It is well known that Old Testament law and Christian morality alike forbid any artificial restriction of human increase. “Increase and multiply” was the command given in Genesis to the first parents of the race; and the psalmist exclaims, “Happy is the man that hath his quiver full” of children.