THE study of morbid heredity is full of interest, because the knowledge of its laws may assist us in finding preventive measures against it, and because it may thereby be a means of comforting persons who are under those laws. In seeking a definition of morbid heredity, we first take Sanson's definition of biological heredity as the transmission from ascendants to descendants, by sexual generation, of natural or acquired properties. With acquired properties we may include morbid ones. Heredity of morbid properties seems to obey the same law as heredity of natural properties, for which we may accept Darwin's formulas of—1. The law of direct and immediate heredity, under which parents tend to transmit their physical and moral characteristics to their descendants. 2. The law of predominance of direct heredity, under which the character of one of the two progenitors is predominant in the product. 3. The law of heredity in reversion, racial heredity, which is applicable to the often-observed facts of atavism, or the reappearance in descendants of the characteristics of a more or less remote ancestor; and 4. The law of homochronous heredity, or the reappearance of hereditary characteristics at the same periods of life in ascendants and descendants.
Morbid heredity does not inevitably obey the laws of direct heredity. It is a well-known fact that diseases in morbid families are not usually transmitted with a perfect likeness. The homologous or similar heredity, which is observed chiefly as to mental diseases, is rare as to other diseases. Usually the disease is modified in descent. A diabetic patient produces an ataxic son, or a hysterical daughter, or an epileptic child. John Hunter seems to have anticipated these variations when he maintained the existence not of hereditary diseases proper, but of a hereditary disposition to contract them—a hypothesis which, though somewhat vague, may account for dissimilar heredity and also for the frequent happy absence of heredity. The probability of morbid heredity manifesting itself is increased when both the parents are attainted with the same defect. Consanguineous marriages, which have been charged with being an important factor in the genesis of neuropathy, of deaf-mutism, and of degeneration in general, really are of effect only through the accumulation of heredity. Consanguinity favors the heredity both of good and of bad family qualities. In healthy families it is desirable; in morbid families it should be shunned.
Pathological selection of nervous parties, who seem to be attracted to one another by invincible sympathies, involves the