slow but sure; good-tempered, but disposed to be sulky when
provoked;--the other is quick, vivacious, forward, acquiring easily and
forgetting soon; quick-tempered and choleric, but quickly forgiving and
forgetting. They have been educated together and never separated."

(5) "They were never alike either in mind or body, and their
dissimilarity increases daily. 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 bet