n a given direction in the
first group is accompanied in the second group by a deviation in the
opposite direction. If we imagine that as one measurement increased
above its average a second related measurement decreased below its
average the correlation in such a case would be negative. For
instance, if we measured the relation between the number of berry
pickers employed and the quantity of berries remaining unpicked, in a
number of different fields we would get a negative correlation
coefficient. Some organisms are formed in such a way that increase in
one dimension, such as length, is associated with decrease in another,
such as breadth; measurement of the relatedness of these dimensions
would give a coefficient of correlation that might be very high,
indicating a considerable relation in the deviations, but it would be
negative. In an instance of negative correlation the relation is that
of "the more the fewer." As we shall see presently, a negative
correlation may be just as important and significant as a positive
correlation.

The application of the principles of heredity to our subject of
Eugenics is of such great importance that it is reserved for separate
consideration in the next chapter. We may, therefore, devote the
remainder of this chapter to the consideration of data of another
kind, which are commonly treated by this same method of determining
correlation coefficients between two sets of varying phenomena in
order to determine whether there is any actual relation between them
or not. This will serve to illustrate the use of this method.

We shall turn then to the subject of differential or selective
fertility in human beings and consider its relation to Eugenics. As a
starting point we may take the self-evident statement that a group of
organisms will tend to maintain constant characteristics through
successive generations only when all parts of the group are equally
fertile. If exceptional fertility is associated with the presence or
absence of any characteristic the number of individuals with or
without that trait will either increase or diminish in successive
generations, and the character of the distribution of the group as a
whole will gradually become altered, the average moving in the
direction of the more fertile group. Or if infertility is so
associated, then the average of the whole group moves away from that
condition. Eugenically, then, we should ask whether in human society
there is at p