As far as possible, the children of the same grade should have about the same ability; but such is seldom the case. In a recent psychological study of a school system, the author found wide differences in ability in the same grade. The distribution of abilities found in the fourth grade and in the high school are shown in Figure XIX. It will be seen that in the fourth grade pupils are found with ability equal to that of some in the high school. Of course to some extent such a condition is unavoidable, for a pupil must establish certain habits and acquire certain knowledge before passing from one grade to another. However, much of the wide variation in ability now found in the same grade of a school could be avoided if the teacher had accurate knowledge of the pupils' abilities. When a teacher learns that a child who is doing poorly in school really has ability, she is often able to get from that pupil the work of which he is capable. It has been demonstrated by experience that accurate measures of children's abilities are a great help in gradation and classification. A knowledge of mental differences is also an aid in the actual teaching of the children. The instance mentioned at the close of the last paragraph is an example. A knowledge of the differences among the mental functions of the same pupil is especially helpful. It has been pointed out that the different mental functions in the same pupil are sometimes unequally developed. Sometimes considerable differences exist in the same pupil with respect to learning capacity, the different aspects of memory, association, imagination, and attention. When a teacher knows of these differences, she can better direct the work of the pupils. For example, if a pupil have a very poor memory, the teacher can help him by aiding him to secure the advantage that comes from close and concentrated attention, frequent repetitions, logical organization, etc. On the other hand, she can help the brilliant student by preventing him from being satisfied with hastily secured, superficial knowledge, and by encouraging him to make proper use of his unusual powers in going deeper and more extensively into the school subjects than is possible for the ordinary student. In many ways a teacher can be helpful to her pupils if she has an accurate knowledge of their mental abilities.