? It would, by
a process of exclusion, if every conceivable environmental factor had
been measured and found wanting. That point in the investigation can
never be reached, but a tremendously strong suspicion is at least
justified. Now if the degree of resemblance between the prevalence of
myopia in parents and that in children be directly measured, and if it
be found that when the parent has eye trouble the child also has it,
then it seems that a general knowledge of heredity should lead to the
belief that the difficulty lies there, and that an environmental cause
for the poor vision of the school child was being sought, when it was
all the time due almost entirely to heredity. This final step has not
yet been completed in an adequate way,[7] but the evidence, partly
analogical, gives every reason to believe in the soundness of the
conclusion stated, that in most cases the schoolboy must wear glasses
because of his heredity, not because of overstudy or any neglect on the
part of his parents to care for his eyes properly during his childhood.

[Illustration: WHY MEN GROW SHORT OR TALL

FIG. 4.--Pedigree charts of the two men shown in the preceding
illustration. Squares represent men and circles women; figures
underlined denote measurement in stocking feet. It is obvious from a
comparison of the ancestry of the two men that the short one comes from
a predominantly short family, while the tall one gains his height
likewise from heredity. The shortest individual in the right-hand chart
would have been accounted tall in the family represented on the left.
After A. F. Blakeslee.]

The extent to which the intelligence of school children is dependent on
defective physique and unfavorable home environment is an important
practical question, which David Heron of London attacked by the methods
we have outlined. He wanted to find out whether the healthy children
were the most intelligent. One is constantly hearing stories of how the
intelligence of school children has been improved by some treatment
which improved their general health, but these stories are rarely
presented in such a way as to contribute evidence of scientific value.
It was desirable to know what exact measurement would show. The
intelligence of all the children in fourteen schools was measured in its
correlation with weight and height, conditions of clothing and teeth,
state of nutrition, cleanliness, good hearing, and the condition of the
cervical glands, tons