Applied Eugenics | Page 8

great as between
ordinary children of the same age and sex, brought up under similar
environment. There seems to be no reason, except heredity, why twins
should be more alike. The data showed (3) that the twins were no more
alike in traits subject to much training than in traits subject to little or
no training. Their achievement in these traits was determined by their
heredity; training did not measurably alter these hereditary
potentialities.
"The facts," Professor Thorndike wrote, "are easily, simply and
completely explained by one simple hypothesis; namely, that the nature
of the germ-cells--the conditions of conception--cause whatever
similarities and differences exist in the original natures of men, that
these conditions influence mind and body equally, and that in life the
differences in modification of mind and body produced by such
differences as obtain between the environments of present-day New
York City public school children are slight."
"The inferences," he says, "with respect to the enormous importance of

original nature in determining the behavior and achievements of any
man in comparison with his fellows of the same period of civilization
and conditions of life are obvious. All theories of human life must
accept as a first principle the fact that human beings at birth differ
enormously in mental capacities and that these differences are largely
due to similar differences in their ancestry. All attempts to change
human nature must accept as their most important condition the limits
set by original nature to each individual."
Meantime other investigators, principally followers of Karl Pearson in
England, were working out correlation coefficients in other lines of
research for hundreds of different traits. As we show in more detail in
Chapter IV
, it was found, no matter what physical or mental trait was measured,
that the coefficient of correlation between parent and child was a little
less than .5 and that the coefficient between brother and brother, or
sister and sister, or brother and sister, was a little more than .5. On the
average of many cases the mean "nature" value, the coefficient of direct
heredity, was placed at .51. This gave another means of measuring
nurture, for it was also possible to measure the relation between any
trait in the child and 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? It would, by a
process of exclusion, if every conceivable environmental factor had
been measured and found wanting. That