In Pursuit of the Gene

42 ¨ REVERSION TO THE MEAN
tionally tall men (6 feet, 5 inches) exceeded their fathers in height. For
Galton it was a foregone conclusion that the same was true for mental and
other traits:
The more exceptional the amount of the gift, the more exceptional will
be the good fortune of a parent who has a son who equals, and still more
if he has a son who overpasses him in that respect. The law is evenhanded;
it levies the same heavy succession-tax on the transmission of
badness as well as of goodness. If it discourages the extravagant expectations
of gifted parents that their children will inherit all their powers, it
no less discountenances extravagant fears that they will inherit all their
weaknesses and diseases. 48
The possibility of measuring the resemblance between parents and offspring
in any quantifiable trait was one of the great insights of Galton’s life.
By 1890 he realized that the regression coefficient was a special case of a
more general phenomenon, correlation, as he dubbed it: the degree of similarity
between any two sets of data. While the concept of correlation stood the
test of time and quickly became one of the cornerstones of the new science
of statistics, Galton had misinterpreted the biological significance of regression.
His confusion led him to far-reaching claims about heredity and evolution
that would not hold up. 49
In particular, Galton used the observed regression in human stature to
argue that evolutionary change could not possibly be the slow and continuous
process that Darwin had envisioned. The “slight and uncertain selective
influences,” he argued, were no match for the blind force of regression.
Where Darwin had gone wrong, according to Galton, was in insisting that
small continuous variants, which he called “variations,” provided the raw
fodder for natural selection. The key to understanding evolution as well as
to engineering successful race improvement, Galton argued, was the appearance
of “sports,” individuals who possessed new traits that were qualitatively
different from the traits possessed by their parents. “I feel the greatest
difficulty in accounting for the establishment of a new breed,” he explained,
“unless there has been one or more abrupt changes of type.”
Something about the nature of sports, he conjectured, allowed them to es-