efect
usually), certain forms of deaf mutism and cataract, imbecility,
Huntington's chorea (a form of dementia).

In observing Mendelian heredity we should bear in mind that a given
character may be due either to the presence or to the absence of a
"determiner" in the germ. Long hair such as is characteristic of many
"Angora" varieties of the guinea pig and cat, for example, is believed
to be due to the absence of a determiner which stops its growth. Blue
eyes are due to the absence of a brown pigment determiner, _et
cetera_. The presence or absence in the offspring of such characters
as we know do Mendelize can be predicted when we know the parental
history for two generations.

Turning now to the inheritance of mental traits and including, of
course, moral traits here as well, we find that we are almost entirely
limited to the statistical statement of results. Pearson found upon
examining data from a large number of school children, brothers and
sisters, that the coefficients of heredity between them were the same
as for their physical traits. His results are summarized in Figure 12.
The physical traits measured were, in the order plotted in the
figure--health, eye color, hair color, hair curliness, cephalic index
(ratio between breadth and length of cranium), head length, head
breadth, head height. These gave an average of .54 in brothers, .53 in
sisters, and .51 in brothers and sisters. The psychical traits in
order were--vivacity, assertiveness, introspection, popularity,
conscientiousness, temper, ability, handwriting. The corresponding
averages were .52, .51, .52.

[Illustration: FIG. 12.--Coefficients of heredity of physical
and psychical characters in school children. Characters
enumerated in text. (From Pearson.)]

Galton's pioneer works on "Hereditary Genius," "English Men of
Science," and "Natural Inheritance" showed with great clearness the
fact of mental and moral heredity. Wood's recent extensive study of
"Mental and Moral Heredity in Royalty" shows the same thing, although
not all the results of these investigations are given in mathematical
form. Little can be said regarding Mendelian heredity of mental traits
because the psychologist has not yet told us how to analyze even the
common and simpler psychic characters into their fundamental units;
since we do not know what the mental hereditary units are, obviously
we cannot work with them. Much of our knowledge in this field does not
per