34 galton.org
34
Inquiries into Human Faculty
side like a long row of park palings between the same limits, their upper
outline will be identical. Moreover, it will run smoothly and not in
irregular steps. The theoretical interpretation of the smoothness of outline
is that the individual differences in the objects are caused by different
combinations of a large number of minute influences; and as the
difference between any two adjacent objects in a long row must depend
on the absence in one of them of some single influence, or of only a few
such, that were present in the other, the amount of difference will be
insensible. Whenever we find on trial that the outline of the row is not a
flowing curve, the presumption is that the objects are not all of the same
species, but that part are affected by some large influence from which the
others are free; consequently there is a confusion of curves. This
presumption is never found to be belied.
It is unfortunate for the peace of mind of the statistician that the
influences by which the magnitudes, etc., of the objects are determined
can seldom if ever be roundly classed into large and small, without
intermediates. He is tantalised by the hope of getting hold of sub-groups
of sufficient size that shall contain no individuals except those belonging
strictly to the same species, and he is almost constantly baffled. In the end
he is obliged to exercise his judgment as to the limit at which he should
cease to subdivide. If he subdivides very frequently, the groups become
too small to have statistical value; if less frequently, the groups will be
less truly specific.
A species may be defined as a group of objects whose individual
differences are wholly due to different combinations of the same set of
minute causes, no one of which is so powerful as to be able by itself to
make any sensible difference in the result. A well-known mathematical
consequence flows from this, which is also universally observed as a fact,
namely, that in all species the number of individuals who differ from the
average value, up to any given amount, is much greater than the number
who differ more than that amount, and up to the double of it. In short, if
an assorted series be represented by upright lines arranged side by side
along a horizontal base at equal distances apart, and of lengths
proportionate to the magnitude of the
