Fig. 16.—Two family histories showing split foot. (From "Treasury of Human Inheritance.")
We may consider next the hereditary history of some forms of nervous defect, the exact nature of the causes of which can be less definitely stated than in all of the preceding instances of defect. Fig. 20 gives a brief history of the heredity of Huntington's chorea—a form of insanity which here resulted in the death of all but one of the affected persons in the first four generations; the fifth generation is the present and is incomplete. Although the matings were with normals in every case, yet in four of the eight marriages all of the offspring were affected. From one affected male 23 affected persons descended in four generations and their multiplication is still going on. There can be no doubt as to the unfitness of marriage into such a family.
Fig. 18.—Family history showing a form of night blindness. Character of matings incompletely known. (Data from Bordley.)
A very complete family history showing deaf-mutism is given in Fig. 21. It cannot be said that in every case here the defect is innate, i. e., hereditary, and it is not known that the cause of the defect was the same in every family concerned, for deaf-mutism may result from several different causes. In most cases in this history, however, the defect behaves like a Mendelian dominant. In certain other cases it is clearly known to follow the Mendelian formula. Such pedigrees as this show how dangerous it is to marry into a family in which this defect exists.
Goddard has recently published several family histories showing feeble-mindedness. One of the most significant of these—significant both socially and eugenically—is summarized here in Fig. 22. Of this Goddard writes: "Here we have a feeble-minded woman [IV, 3] who has had three husbands (including one 'who was not her husband'), and the result has been nothing but feeble-minded children. The story may be told as follows:
