A man from a dark-skinned race would become
little darker by living under the desert sun, while a white man would
take on a good deal of tan.
The limited effect of nurture in changing nature is in some fields a
matter of common observation. The man who works in the gymnasium knows
that exercise increases the strength of a given group of muscles for a
while, but not indefinitely. There comes a time when the limit of a
man's hereditary potentiality is reached, and no amount of exercise will
add another millimeter to the circumference of his arm. Similarly the
handball or tennis player some day reaches his highest point, as do
runners or race horses. A trainer could bring Arthur Duffy in a few
years to the point of running a hundred yards in 9-3/5 seconds, but no
amount of training after that could clip off another fifth of a second.
A parallel case is found in the students who take a college examination.
Half a dozen of them may have devoted the same amount of time to it--may
have crammed to the limit--but they will still receive widely different
marks. These commonplace cases show that nurture has seemingly some
power to mold the individual, by giving his inborn possibilities a
chance to express themselves, but that nature says the first and last
word.
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