However, it seems that abilities that are very much specialized, such as musical ability, artistic ability, etc., may exist in high order while other mental functions may be only mediocre. It is a common thing for a musical person to be of rather poor ability otherwise. To the extent that special abilities require specialized differences in the structure of brain, nervous system, or sense organ, they can exist in some degree of independence of other functions. Musical ability to some extent does require some such differences and may therefore be found either with a high or a low degree of ability in other characteristics.
It is doubtless true that at maturity the unequal power of mental functions in the same person may be partly due to the fact that one function has been exercised and others neglected. A person having very strong musical tendencies is likely to have such a great interest in music that he will think other activities are not worth while, and will consequently neglect these other activities. It will therefore turn out that at maturity the great differences in mental functions in such a person are in part due to exercise of one function and neglect of others. But there can be no doubt that in many cases there are large original, inherited differences, the individual being poor in one aspect of mind and good in others. Feeble-minded people are usually poor in all important aspects of mind. However, one sometimes finds a feeble-minded person having musical or artistic ability, and often such a person has a good rote memory, sometimes a good verbal memory. However, the so-called higher mental functions—logical memory, controlled association, and constructive imagination—are all poor in a feeble-minded person.
Each mental function may be looked upon as in some measure independent; each is found existing in people in varying degrees from zero ability up to what might be called genius ability. The frequency curves in Fig. VI show this. Take rote memory for example. Idiots are found with practically zero ability in rote memory. At the other extreme, we find mathematical prodigies who, after watching a long freight train pass and noting the numbers of the cars, can repeat correctly the number of each car. Rote memory abilities can be found representing every step between these two extremes. This principle of distribution holds true in the case of all mental functions. We find persons practically without them, and others possessing them 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 under poor environmental influences, or when a child of poor stock is reared under good influences, the results seem to show that the differences in environment have little effect on mental development, as far as the fundamental functions are concerned, except in the most extreme cases.
Each mental function is capable of some development. It can be brought up to the limit of its possibilities. But recent experiments indicate that such development is not very great in the case of the elementary, fundamental functions. Training, however, has a much greater effect on complex mental activities that involve several functions. Rote memory is rather simple; it cannot be much affected by training. The memory for ideas is more complex; it can be considerably affected by training. The original and fundamental functions of the mind depend upon the nature of the nervous system which is bequeathed to us by heredity. This cannot be much changed. However, training has considerable effect on the coördinations and combinations of mental functions. Therefore, the more complex the mental activities which we are testing, the more likely they are to have been affected by differences in experience and training.
If we should designate the logical memory capacity of one person by 10, and that of another by 15, by practice we might bring the first up to 15 and the second to 22½, but we could not equalize them. We could never make the memory of the one equal to that of the other. In an extreme case, we might find one child whose experience had been such that his logical memory was working up to the limit of its capacity, while the other had had little practice in logical memory and was therefore far below his real capacity. In such a case, a test would not show the native difference, it would show only the present difference in functioning capacity.
Fairly adequate tests for the most important mental functions have been worked out. A series of group tests with directions and norms follow. The members of the class can use these tests in studying the individual differences in other people. The teacher will find other tests in the author’s Examination of School Children, and in Whipple’s Manual of Mental and Physical Tests.