[4] For instance, the correlation between the
right and left femur in man's legs is .98.
Professor Thorndike found in the New York City schools fifty pairs of
twins of about the same age and measured the closeness of their
resemblance in eight physical characters, and also in six mental
characters, the latter being measured by the proficiency with which the
subjects performed various tests. Then children of the same age and sex,
picked at random from the same schools, were measured in the same way.
It was thus possible to tell how much more alike twins were than
ordinary children in the same environment.[5]
[Illustration: THE EFFECT OF NURTURE IN CHANGING NATURE
FIG. 2.--Corn of a single variety (Leaming Dent) grown in two
plots: at the left spaced far apart in hills, at the right crowded. The
former grows to its full potential height, the latter is stunted. The
size differences in the two plots are due to differences in environment,
the heredity in both cases being the same. Plants are much more
susceptible to nutritional influences on size than are mammals, but to a
less degree nutrition has a similar effect on man. Photograph from A. F.
Blakeslee.]
"If now these resemblances are due to the fact that the two members of
any twin pair are treated alike at home, have the same parental models,
attend the same school and are subject in general to closely similar
environmental conditions, then (1) twins should, up to the age of
leaving home, grow more and more alike, and in our measurements the
twins 13 and 14 years old should be much more alike than those 9 and 10
years old.
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