ates 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.

[Illustration: 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.

[Illustration: FIG. 26.--Family history showing ability.
(From Whetham.)]

Figs. 26 and 27 illustrate two such pedigrees of ability. In each of
these histories there is also a