y showing Huntington's
chorea. Last generation incomplete. (Data from Hamilton.)]

The paternal ancestry of this unfortunate woman is hardly less
interesting, as may be seen from the diagram. All told, this family
history, as far as it is known, includes 59 persons; the mental
character of 12 of these is unknown; 10 died in infancy or before
their characteristics were known; of the remaining 37, 30 were
feeble-minded.

[Illustration: FIG. 21.--Family history showing deaf-mutism.
(From "Treasury of Human Inheritance.")]

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 pr