The necessity for sleep is not felt by all organic beings alike. The differences observed are more due to variations in habits, modes of life, and inherent organic dispositions, than to any inequality in the size of the brain, although the latter has been thought by some authors to be the cause. It has been assumed that the larger the brain the more sleep was required. Perhaps this is true as regards the individuals of any one species of animals, but it is not the case when species are compared with each other. In man, for instance, persons with large heads, as a rule, have large, well-developed brains, and consequently more cerebral action than individuals with small brains. There is accordingly a greater waste of cerebral substance and an increased necessity for repair. This is not, however, always the case, as some individuals with small brains have been remarkable for great mental activity. All animals sleep, and even plants have their periods of comparative repose. As Lelut says:[4] “No one is ignorant of the nocturnal repose of plants. I say repose and nothing else. I do not say diminution or suspension of their sensibility, for plants have no sensibility. I say diminution of their organic actions--a diminution which is evident and characteristic in all, more evident and more characteristic in some. * * * “Their interior or vital movements are lessened, the flow of the sap and of other fluids which penetrate and rise in them is retarded. Their more mobile parts--the leaves, the flowers--show by their falling, their occlusion, their inclination that their organic actions are diminished, and that a kind of repose has been initiated, which takes the place of the lying down, which, with animals, is the condition and the result of sleep.” CHAPTER II. THE CAUSES OF SLEEP. The exciting cause of natural and periodic sleep is undoubtedly to be found in the fact that the brain at stated times requires repose, in order that the cerebral substance which has been decomposed by mental and nervous action may be replaced by new material. There are other exciting causes than this, however, for sleep is not always induced by ordinary or natural influences acting periodically. There are many others, which within the strict limits of health may cause such a condition of the brain as to produce sleep. Authors, in considering sleep, have not always drawn the proper distinction between the exciting and the immediate cause. Thus Macario,[5] in alluding to the alleged causes of sleep, says: