Applied Eugenics | Page 7

The external influences have been
identical; they have never been separated."
(6) "The two sisters are very different in ability and disposition. The
one is retiring, but firm and determined; she has no taste for music or
drawing. The other is of an active, excitable temperament; she displays
an unusual amount of quickness and talent, and is passionately fond of
music and drawing. From infancy, they have been rarely separated even
at school, and as children visiting their friends, they always went
together."
And so on. Not a single case was found in which originally dissimilar
characters became assimilated, although submitted to exactly the same
influences. Reviewing the evidence in his usual cautious way, Galton
declared, "There is no escape from the conclusion that nature prevails
enormously over nurture, when the differences of nurture do not exceed
what is commonly to be found among persons of the same rank in
society and in the same country."
This kind of evidence was a good start for eugenics but as the science
grew, it outgrew such evidence. It no longer wanted to be told, no
matter how minute the details, that "nature prevails enormously over
nurture." It wanted to know exactly how much. It refused to be satisfied
with the statement that a certain quantity was large; it demanded that it

be measured or weighed. So Galton, Karl Pearson and other
mathematicians devised means of doing this, and then Professor
Edward L. Thorndike of Columbia University took up Galton's problem
again, with more refined methods.
The tool used by Professor Thorndike was the coefficient of correlation,
which shows the amount of resemblance or association between any
two things that are capable of measurement, and is expressed in the
form of a decimal fraction somewhere between 0 and the unit 1. Zero
shows that there is no constant resemblance at all between the two
things concerned,--that they are wholly independent of each other,
while 1 shows that they are completely dependent on each other, a
condition that rarely exists, of course.[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. Again (2) if similarity in training is the cause of similarity
in mental traits, ordinary fraternal pairs not over four or five years apart
in age should show a resemblance somewhat nearly as great as twin
pairs, for the home and school condition of a pair of the former will not
be much less similar than those of a pair of the latter. Again, (3) if
training is the cause, twins should show greater resemblance in the case
of traits much subject to training, such as ability in addition or
multiplication, than in traits less subject to training, such as quickness
in marking off the A's on a sheet of printed capitals, or in writing the
opposites of words."
The data were elaborately analyzed from many points of view. They
showed (1) that the twins 12-14 years old were not any more alike than
the twins 9-11 years old, although they ought to have been, if
environment has great power to mold the character during these
so-called "plastic years of childhood." They showed (2) that the
resemblance between twins was two or three times as