There is also great difference between the general mental make-up of children--a difference in type. There is the child who excels in dealing with abstract ideas. He usually has power also in dealing with the concrete, but his chief interest is in the abstract. He is the one who does splendid work in mathematics, formal grammar, the abstract phases of the sciences. Then there is the child who is a thinker too, but his best work is done when he is dealing with a concrete situation. Unusual or involved applications of principles disturb him. So long as his work is couched in terms of the concrete, he can succeed, but if that is replaced by the _x, y, z_ elements, he is prone to fail. There is another type of child--the one who has the executive ability, the child of action. True, he thinks, too, but his forte is in control of people and of things. He is the one who manages the athletic team, runs the school paper, takes charge of the elections, and so on. For principles to be grasped he must be able to put them into practice. The fourth type is the feeling type, the child who excels in appreciative power. As has been urged so many times before, these types have boundaries that are hazy and ill defined; they overlap in many cases. Some children are of a well-defined mixed type, and most children have something of each of the four abilities characteristic of the types. Still it is true that in looking over a class of children these types emerge, not pure, but controlled by the dominant characteristics mentioned. The same variation is found among any group of children if they are tested along one line, such as memory. Some have desultory, some rote, some logical memories; some have immediate memories, others the permanent type. In imagery, some have principally productive imagination, others the matter-of-fact reproductive; some deal largely with object images that are vivid and clear-cut, others fail almost entirely with this type, but use word images with great facility. In conduct, some are hesitating and uncertain, others just the reverse; some very open to suggestions, others scarcely touched at all by it; some can act in accordance with principle, others only in terms of particular associations with a definite situation. So one might run the whole gamut of human traits, and in each one any group of individuals will vary: in attention, in thinking, in ideals, in habits, in interests, in sense discrimination, in emotions, and so on. This is one of the greatest contributions of experimental psychology of the past ten years, the tremendous differences between people along all lines, physical as well as mental.