§ 79. =Sleep and Dream.=—The profound sleep that comes to us every night, and that we take entirely as a matter of course, rests without any doubt upon an instinctive tendency; but there can be little doubt, either, that the instinct has been modified in the course of human evolution. _It seems probable, indeed, that profound sleep, the lapse of all but the vegetative organic functions, has been developed from the same fundamental tendency as hypnosis_, so that natural sleep and artificial hypnosis represent two branches which spring from a single stem. This original and instinctive tendency is toward what we may call, in biological phrase, a =partial or defensive sleep=, a rest enjoyed while the animal is still partly on guard. It underlies the sleep of the mother, who is roused at once by the movement of her infant child; the sleep of the nurse, who is awaked by the restlessness of her patient; the sleep of the tired horseman or driver, who keeps the saddle or holds the reins, and remains alive to any sign of uneasiness on the part of his horse. It shows also in the ability of the wearied surgeon to rouse himself and perform an operation, though he falls asleep once more the moment it is over and has no remembrance of it at his normal waking. Such a partial rest, persisting only thus occasionally in the life of civilised man, is all that an animal surrounded by dangers can afford; if sight and smell and taste may be allowed to lapse, still touch and hearing must keep awake,—must keep awake, at any rate, to the kind of stimulus that spells danger. We are speaking now in figurative terms; the history and nervous mechanism of the sleep-tendency offer a problem to science, and must be scientifically worked out; but it is enough here if you get a general notion of the way in which sleep began. In process of time, as dangers grow less or as the nightly care of the community is put into the hands of watchmen whose special duty it is to signal their approach, _sleep becomes total and profound_. Even our own protected sleep, however, is not always undisturbed. We resign ourselves to it with a full sense of security; and we go to sleep in a dark and quiet room, we rid ourselves of the friction of clothes, we keep a constant temperature in our bedroom, we lie down. Sleep, nevertheless, is interrupted, more or less often according to age and constitution, by a =dream=, by a series of experiences like those of the waking life; and sometimes the dream is accompanied by muscular activity; we talk or walk in our sleep.