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? 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.
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