ecaution, 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.")]

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 illustr