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.
We cannot speak of heredity in connection with diseases to which all are susceptible and incapable of developing immunity. The presence or absence of such a disease is determined solely by the presence or absence of infection. Many physical and mental defects result from infection as the primary cause. If the infection is one to which all exposed are susceptible and incapable of developing immunity we cannot speak of the defect as in any way hereditary; if the infection is one to which some are susceptible, others not, to which some can develop immunity, others cannot, then we may speak of the defect as hereditary. Thus certain forms of blindness or insanity are due primarily to gonorrheal or syphilitic infection, insusceptibility to which is rare or unknown. Such defects cannot be considered as affording evidence of heredity though they reappear in successive generations.
In general the subject of the heredity of immunity and susceptibility forms one of the most important eugenic aspects of this whole subject. In a few cases it is known that immunity or insusceptibility to specific forms of infection is a unit character which follows Mendelian laws in heredity. It can be added to races or subtracted from them and pure bred immune races built up. So far this has not been demonstrated for man. There is some circumstantial evidence that immunity to specific forms of infection has been a great, although hitherto neglected, factor in man's evolution, and even in the history of his civilization and conquest. It is at once obvious that here is a great field for the common labor of the students of heredity and of medicine and of Eugenics.
Fig. 25 illustrates a family history of infertility. This is apparently hereditary, but before that could be asserted definitely to be so here or in any similar case, we should know that the infertility were not the result of an infection to which immunity is rare or unknown. That infertility is really hereditary in this instance is indicated, first, by the fact that the person marked A later, by a second marriage into fertile stock, had a large family, and second, by the fact that the individual B and his child by marriage into fertile stocks produced in the last generation again a large family and so saved this whole family from extinction.
Fig. 25.—Family history showing infertility. (From Whetham.)
Before leaving the subject of the heredity of the kinds of traits we have been using as illustrations, we should add just a word. It is often objected that one cannot properly speak of the heredity of such general things as "insanity" or "deaf-mutism" or "blindness" or "heart disease," because each of these includes a great variety of specific forms of these disorders which cannot strictly, medically, be compared. But the student of heredity replies that when he speaks of the heredity of insanity or heart disease, that is often just what he means. He means that often no particular form of these defects is necessarily strictly heritable as such, but that in a family there may be a general instability of nervous system or circulatory system, which may take any one of several possible specific forms, the form actually appearing depending upon particular conditions which are frequently environmental and beyond determination. In some cases specific forms of disorder are actually heritable as such.
Such an inclusive thing as "ability" may depend upon many different specific conditions. Yet there are families in which persons of exceptional ability are unusually frequent. The fact that persons of ability are more frequent in certain families than in the general population of the same social class and with about the same opportunity for the demonstration of inherent ability, gives evidence of its heredity, although we may not be able to summarize the facts under any particular law but must adhere to their statistical expression.