tep or
break. It would be an absolutely _continuous_ distribution.

If then, the results of all the tests that have been made on all mental
traits be studied, it will be found that human mental ability as shown
in at least 95% of all the traits that have been measured, is
distributed throughout the race in various degrees, in accordance with
the law of chance, and that if one could measure all the members of the
species and plot a curve for these measurements, in any trait, he would
get this smooth, continuous curve. In other words, human beings are not
sharply divided into classes, but the differences between them shade off
into each other, although between the best and the worst, in any
respect, there is a great gulf.

If this statement applies to simple traits, such as memory for numbers,
it must also apply to combinations of simple traits in complex mental
processes. For practical purposes, we are therefore justified in saying
that in respect of any mental quality,--ability, industry, efficiency,
persistence, attentiveness, neatness, honesty, anything you like,--in
any large group of people, such as the white inhabitants of the United
States, some individuals will be found who show the character in
question in a very low degree, some who show it in a very high degree;
and there will be found every possible degree in between.

[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 fac