[Illustration: NORMAL VARIABILITY CURVE FOLLOWING LAW OF CHANCE
FIG. 13.--The above photograph (from A. F. Blakeslee), shows
beans rolling down an inclined plane and accumulating in compartments at
the base which are closed in front by glass. The exposure was long
enough to cause the moving beans to appear as caterpillar-like objects
hopping along the board. Assuming that the irregularity of shape of the
beans is such that each may make jumps toward the right or toward the
left, in rolling down the board, the laws of chance lead to the
expectation that in very few cases will these jumps all be in the same
direction, as is demonstrated by the few beans collected in the
compartments at the extreme right and left. Rather the beans will tend
to jump in both right and left directions, the most probable condition
being that in which the beans make an equal number of jumps to the right
and left, as is shown by the large number accumulated in the central
compartment. If the board be tilted to one side, the curve of beans
would be altered by this one-sided influence. In like fashion a series
of factors--either of environment or of heredity--if acting equally in
both favorable and unfavorable directions, will cause a group of men to
form a similar variability curve, when classified according to their
relative height.
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