Turning now to defects of other kinds, an interesting history is illustrated in Fig. 23. Here a single individual fatally affected with angio-neurotic oedema gave rise, in four completed generations, to 113 persons, 43 of whom were affected. In 11 this disease was the direct cause of death. The Mendelian character of the heredity here can be neither asserted nor denied. In generations II-V matings between normal and affected gave 42 affected and 35 unaffected offspring. Fig. 24 gives a brief family history showing pulmonary tuberculosis. In the history given susceptibility to this disease behaves as a Mendelian dominant. We cannot as yet say whether this is or is not a general rule. In describing the heredity of diseases primarily due to infection, one or two important cautions must be observed. Of course the source of the infection cannot be "hereditary," and apparently it is only in comparatively few instances that infection occurs during fetal life. To some infections certain persons are susceptible, others are not; some when susceptible are capable of developing immunity, others are not. When an infection is of such character and prevalence that practically all persons in approximately similar environments of a given character are infected, susceptibility or the power of developing immunity will determine whether or not an individual will exhibit the disease caused by the infective agent. Practically all persons living in the denser communities are infected with tuberculosis; those who are susceptible and incapable of developing immunity succumb, the insusceptible and those developing immunity do not. These conditions are heritable; but in speaking of the heredity of such a disease as tuberculosis it should be clear that the heredity concerned is really that of susceptibility and the power of developing immunity. Yet the person who is really susceptible can, by taking sufficient precaution, escape serious infection, and thus the result for that person would be the same as if he were insusceptible, but his offspring would have to take similar precautions if they were to escape the disease. [Illustration: FIG. 22. Family history showing feeble-mindedness. Data from Goddard. _A_, alcoholic; _d.i._, died in infancy; _E_, epileptic; _ill._, illegitimate; _in._, incest; *, same individual as _III_, 6; _n.m._, not married; _S_, sexual pervert; _T_, tuberculous.] [Illustration: FIG. 23.--Family history showing angio-neurotic oedema. (From "Treasury of Human Inheritance.")] [Illustration: FIG. 24.--Family history showing tuberculosis. (Data from Klebs, after Whetham in "Treasury of Human Inheritance.")]