nd some factor in the
environment. A specific instance will make this clearer.

Groups of school children usually show an appalling percentage of
short-sightedness. Now suppose it is suggested that this is because they
are allowed to learn to read at too early an age. One can find out the
age at which any given child did learn to read, and work out the
coefficient of correlation between this age and the child's amount of
myopia. If the relation between them is very close--say .7 or .8--it
will be evident that the earlier a child learns to read, the more
short-sighted he is as he grows older. This will not prove a relation of
cause and effect, but it will at least create a great suspicion. If on
the contrary the correlation is very slight, it will be evident that
early reading has little to do with the prevalance of defective vision
among school children. If investigators similarly work out all the other
correlations that can be suggested, finding whether there is any
regular relation between myopia and overcrowding, long hours of study,
general economic conditions at home, general physical or moral
conditions of parents, the time the child spends out of doors, etc., and
if no important relation is found between these various factors and
myopia, it will be evident that no factor of the environment which one
can think of as likely to cause the trouble really accounts for the poor
eyesight of school children.

[Illustration: HEIGHT IN CORN AND MEN

FIG. 3.--An unusually short and an unusually tall man,
photographed beside extreme varieties of corn which, like the men, owe
their differences in height indisputably to heredity rather than to
environment. No imaginable environmental differences could reverse the
positions of these two men, or of these two varieties of corn, the
heredity in each case being what it is. The large one might be stunted,
but the small one could not be made much larger. Photograph from A. F.
Blakeslee.]

This has actually been done,[6] and none of the conditions enumerated
has been found to be closely related to myopia in school children.
Correlations between fifteen environmental conditions and the goodness
of children's eyesight were measured, and only in one case was the
correlation as high as .1. The mean of these correlations was about
.04--an absolutely negligible quantity when compared with the common
heredity coefficient of .51.

Does this prove that the myopia is rather due to heredity