in the highest order, but most people are
grouped about the average ability.

=Detecting Mental Differences.= It has already been said that mind has
many different aspects and that people differ with respect to these
aspects. Now let us ask how we can measure the degree of development of
these aspects or functions of mind. We measure them just as we measured
muscular speed as described in the first chapter. Each mental function
means ability to do something--to learn, to remember, to form images,
to reason, etc. To measure these different capacities or functions we
have but to require that the person under consideration _do_ something,
as learn, remember, etc., and measure how well and how fast he does it,
just as we would measure how far he can jump, how fast he can run, etc.

In such measurements, the question of practice is always involved. If we
measure running ability, we find that some are in practice while others
are not. Those in practice can run at very nearly their ultimate
capacity. Those who are not in practice can be trained to run much
faster than they do. To get a true measure of running capacity, we
should practice the persons to be measured till each runs up to the
limit of his capacity, and then measure each one's speed. The same thing
is true, to some extent, when we come to measure mental functions
proper. However, the life that children live gives exercise to all
fundamental functions of the mind, and unless some of the children
tested have had experience which would tend to develop some mental
functions in a special way, tests of the various aspects of learning
capacity, memory, association, imagination, etc., are a fairly good
measure of original, inherited tendencies.

Of course, it must be admitted that there are measurable differences in
the influence of environment on children, and when these differences are
extreme, no doubt the influence is shown in the development of the
child's mind. A child reared in a home where all the influences favor
its mental development, ought to show a measurable difference in such
development when compared with a child reared in a home where all the
influences are unfavorable. It is difficult to know to what extent this
is true, for the hereditary and environmental influences are usually in
harmony, the child of good hereditary stock having good environmental
influences, and vice versa. When this is not the case, _i.e._ when a
child of good stock is reared und