brief and compendious expressions suitable for discussion. The
possibility of doing this is based on the constancy and continuity
with which objects of the same species are found to vary. That is to
say, we always find, after sorting any large number of such objects
in the order (let us suppose) of their lengths, beginning with the
shortest and ending with the tallest, and setting them side by 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