Among the characteristics which we agree with Kraepelin in regarding as the essential mental features of the personality, varying degrees of capacity for acquiring skill by practice and of liability to fatigue are by far the most important. The effects of practice and fatigue on the course of mental work are such as absolutely to determine the amount that can be done and the mental capacity for work. They differ, however, in their relation to the amount of work done, not only in the antagonistic direction of their effects, but also in other features which clearly show their kinship with the corresponding pheno mena on the physiological side. While the effect of practice hardly ever extends beyond the sphere of the work that has been practised, and acts on other functions, even of a simular nature, only in a limited degree, fatigue has a far more exten sive effect and reduces the general mental capacity for work. There is much in the common experience of daily life which seems to contradict this statement. Thus, change of work seems to have a favourable influence on our working capacity when we are fatigued. This appearance has led even eminent physiologists to assume that fatigue acts within the same narrow limits as practice. Thus, Mosso says, in his book on Fatigue, “Apparently fatigue is localised in a particular region of the brain, for we often see that people who have become incapable of thinking over a certain subject, or considering a particular piece of business, find relief in thinking about some thing else, or free themselves from the sense of dulness in their heads by fixing their attention intently on other and different things-for instance, on a game of chess.” Richter, too, ascribes a restorative effect to change of work in education, and thinks that he has thus discovered why school children do not show more fatigue.