A child that excels most of his colleagues in one or a few points is said to have marked ability in that direction--as the exceptional athlete, or the child with exceptional literary or moral feeling. On the other hand, a child that seems to measure well up to the average in most points, and even to excel in a few, may fall far short in some matters,--that is, may be deficient. Thus a perfectly good child in every other way may be unable to master the ordinary requirements in arithmetic, or a child may have an entirely satisfactory development in every way and be deficient in musical discrimination. Another kind of difference is to be found in what may be called general capacity. Some children show higher capacity than the average along nearly every line that can be measured or tested, without showing a preponderance in any one direction. Such children are said to be of high grade, or of high "vitality." In the same way many children are below the average in nearly every line, without being particularly defective along any one line. They can do one thing about as well as another, just as the high-grade boys and girls can do one thing about as well as another; but in the former there is a limit to the possible development which is exceeded in the latter. Among both classes of children the full development depends upon suitable environment, but what is suitable for one may not be suitable for the other. From a consideration of these differences in degree and difference in kind we may see that there is no course of training or treatment, no method of instruction, no trick for the mother or for the teacher that will be usable for all children under all circumstances, to make them all come up to some preconceived uniform standard. On the other hand, if we consider the differences as worth developing, and even emphasizing, it must be obvious that the training and the treatment should be adapted to the individual child so far as possible. Starting out with essentially different human beings, uniform treatment will not make them all alike, nor will _any_ treatment make them all alike. But starting out with a particular human being, we can learn to treat him in such a way as to make him develop into a more desirable person than he would become if he were neglected or if he were treated differently. And that is the main problem, after all.