, both boys and
girls, are excessively irritable, very suspicious, inordinately
selfish, hysterical, vainglorious, or in other ways show lack of
self-control and emotional stability. Later in life such conditions may
lead to intense misery. Nevertheless traits of this sort are often
combined with very fine qualities in other respects. This renders it
extremely hard to decide whether such persons are fit for marriage.

It is extremely difficult to determine whether emotional instability,
selfishness, and other undesirable traits are due to heredity or
environment. At this point we enter a field of great difficulty because
a trait may be inborn, but not hereditary. A child may be born with
serious handicaps because some ailment due to unfavorable environment
prevented its mother from nourishing it properly before it was born.
Such weakness is not truly hereditary. It will not appear in later
generations unless the mothers of those generations also suffer from
environmental conditions similar to those which prevented the first
mother from nourishing her child. It often happens that such conditions
are repeated from generation to generation. If this happens very early
in the pre-natal life of the child, the results are very likely to be
misinterpreted as hereditary.

In the last few decades the study of heredity has been so fascinating
and fruitful that biologists have given comparatively little attention
to early environmental influences. Recent work, however, suggests that
such influences are far from negligible. My own studies of season of
birth illustrate the matter. They suggest that the effect of physical
environment upon the health of the parents before a child is conceived
has an important effect upon the child's future health and achievement.

Only a hint of the chain of evidence leading to this conclusion is here
possible. Many investigations of deaths, fatigue, work, and disease, as
well as numerous carefully controlled laboratory experiments, indicate
that people feel most comfortable and vigorous, and have the best
health, when the average temperature for night and day together is about
63 deg.. Nothing is more pleasant than a day of this optimum kind in May or
June. At midday the thermometer rises to 70 deg. more or less; at night it
falls low enough so that people sleep soundly and restfully.

A study of season of birth in many countries indicates that children who
are conceived when optimum weather of thi