It must be reiterated that this is a statistical, not a biological, law;
and that even Galton probably goes a little too far in applying it to
individuals. It will hold good for a whole population, but not
necessarily for only one family. Further, we can afford to reemphasize
the fact that it in no way prevents the improvement of a race by
selection and assortative mating.
Stature is the character which Dr. Galton used to get an exact
measurement of the amount of regression. More recent studies have
changed the value he found, without invalidating his method. When large
numbers are taken it is now abundantly proved that if parents exceed the
average stature of their race by a certain amount their offspring will,
in general, exceed the racial average by only one-half as much as their
parents did. This is due, as Galton said, to the "drag" of the more
remote ancestry, which when considered as a whole must represent very
nearly mediocrity, statistically speaking.
The general amount of regression in heredity, then, is one-half. If it
be expressed as a decimal, .5, the reader will at once note its identity
with the coefficient of correlation which we have so often cited in this
book as a measure of heredity.
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