Claparède has opposed what he calls an “instinctive” theory to the toxic theory of sleep. According to this theory, sleep is the manifestation of an instinct “the object of which is to arrest activity; we do not sleep because we are intoxicated or exhausted, but to prevent ourselves from falling into such a condition.” However, in order to bring this narcotic instinct into play, certain conditions are necessary, one of which certainly would be the intoxication of the nerve centres. M. Claparède supposes that sleep is an active phenomenon, induced when waste matter begins to accumulate in the organism. “To bring about sleep, the nerve centres must be influenced by waste matter, and this influence can readily be regarded as a kind of intoxication.” Hunger is an instinctive sensation as much as sleepiness, but it does not appear until our tissues are in a condition of exhaustion, the exact nature of which cannot as yet be indicated. There is no real contradiction between the toxic and instinctive theories of sleep. The two theories represent different sides of a special condition of the organism. The analogy between sleep and natural death is in favour of the supposition that the latter, also, is due to an intoxication much more profound and serious than that which results in sleep. Therefore, as natural death in human beings has been studied only very superficially, it is impossible to do more than frame theories regarding it. It would be natural if, just as in sleep there is an instinctive desire for rest, so also the natural death of man were preceded by an instinctive wish for it. As I have already discussed this subject in the “Nature of Man” (chap. xi) I need not deal with it at length here. I should like, however, to add some information which I have recently obtained.