In ethnography, the term endogamy is used to denote a law or custom by which marriage is allowed only within the limits of a specified race, tribe, or caste; thus, in the Old Testament, Jews are forbidden to marry women of other races. The ethnographical term exogamy indicates the prohibition of marriage between persons who are more closely allied, as, for instance, the Mosaic prohibition of marriage within certain degrees of blood-relationship. Such exogamic prohibitions persist even in the legislation of the present day. In many ecclesiastical and national laws we find the marriage of first cousins and of uncle or aunt with niece or nephew forbidden; and even a prohibition of the marriage of a man with his deceased wife’s sister.
Hegar considers the danger of inbreeding to be very great in the human species; for whereas in the lower animals breeders employ a methodical and carefully considered selection of the best specimens, nothing of this kind occurs among human beings; and the health of modern civilized man is such that there are few families without a skeleton in the closet. “Not only in families, but also in villages, in small and large towns, even in classes, and in entire nations, certain peculiar qualities, morbid tendencies, and predispositions, are handed down from generation to generation. We have, for instance, the tendency of the Jews to nervous disorders and diabetes, that of the English to gout, that of the Germans to myopia.” Strahan has therefore employed the term “social consanguinity,” to indicate that by means of common customs, environment, occupation, and mode of nutrition, a similarity in type is produced, leading to a similar predisposition to disorders and diseases transmissible from father to son.
The dangers of inbreeding are believed by Hegar to be, under present-day conditions, so considerable that he would allow the marriage of near kin in exceptional cases only, and where the circumstances are peculiarly favourable—for instance, where both parties to the projected marriage are in excellent health, and where there is no great similarity between them in feature or mental type. Certain anomalies transmitted from remote ancestors, dependent on deeply-marked peculiarities of the germ cells, may be so developed by inbreeding as to become absolutely fixed characteristics. If the morbid manifestations can be traced back for several generations, if the bodily defects and disturbances of development (the so-called stigmata of degeneration), are well marked and numerous, if the functional disorders of the nervous system and of the sense organs are pronounced, leading to idiocy, insanity, epilepsy, congenital deafmutism, blindness, instinctive criminality,—there is in such cases little or no hope of the regeneration of the family. It dies out, because the members are sterile; because they are confined in prisons or asylums; or because the children, if any are born, are deficient in vitality, and fail to reach maturity.
According to the brief summary of the subject given by Hegar, the peculiarities of the offspring at the time of birth depend upon:
Factors which give rise to peculiarities of the germ-cells:
I. Germinal rudiments derived from the ancestors;
II. Influences acting on the germ-cells within the parent organism;
a. Owing to peculiarities of the fluids and tissues of the parental body;
b. Owing to substances which penetrate the parental body and reach the germ.
Germinal rudiments altered by the conjugation of the male and female reproductive cells: