As to quantities of sleep, there is the curious fact that the amount habitually taken in the civilised communities of this part of the world differs at different ages. Babies sleep a good part of the twenty-four hours, and probably schoolboys and schoolgirls (under our present conditions of life and work) ought to be given ten hours or more. Whilst adult men sleep from six to eight or nine hours, it is a curious fact that old people—not very old people, but those of sixty-five or thereabouts—often find themselves unable to sleep more than four hours at night, and take an hour or two in the daytime to make up for the deficiency. I remember hearing Mr. Darwin state this as to himself to his physician, Sir Andrew Clarke, who said it was very usual at his age, and difficult to explain, since at a greater age, when a man is called “very old,” a more or less continuous somnolent condition sets in. The father of a great judicial dignitary of these days, himself a barrister in large practice, when he was sixty years old would snatch fifteen or twenty minutes’ sleep at any and every opportunity throughout the day, even at the midday meal sometimes, so as altogether to disconcert those who were with him, and he told me that he never slept more than four hours at night, but got up and commenced work at four in the morning. The cessation in early old age of the desire for more than half the amount of sleep taken by younger men suggests that the regulating cause of the number of hours which are needed for sleep may be simply and directly the actual amount of work done by body and mind. This imperceptibly becomes less as men grow older, and so less recuperative sleep is necessary, though what work they do may be more effective and better adjusted to its purpose when they have arrived at the condition which is called “old age.” We have seen that sleep in its widest sense comprises the simple condition of quiescence brought about in even the minutest living things by the recurring night, as well as the strangely elaborated varieties of cessation of activity in the whole or parts of the brain of man and of his body. Some of these cessations of activity naturally and spontaneously occur in unsophisticated mankind, when darkness falls on the earth at each succeeding evening. And it is hardly possible to doubt that a tendency to periodic sleep has become fixed in the substance of living things by the alternation of night and day—as well as in some cases by the change of the seasons.