gradually to build a neat cell, to fill this with honey, to lay an egg in it, and finally to furnish the cell with a roof of clay. Since all reflex mechanisms, and all the natural instincts of animals, contribute to the maintenance of the species, and are therefore useful, their origins must be referable to natural selection, and we have only to ask whether they must be referred to it always, and to it alone. It cannot be doubted that, in Man, and in the higher animals voluntary actions which are often repeated gradually acquire the character of instinctive actions. The individual movements pertaining to the particular action are no longer each guided by the will, but a single exercise of will is enough to liberate the whole complex action, such as writing, speaking, walking, or the playing of a whole piece of music; frequently the will-impulse may be absent altogether, and the action be set going simply by an adequate external stimulus, as in the case of sleep-walking, which is observed in fatigued children and soldiers, and in somnambulists. The external stimulus is transmitted to the proper group of muscles as unfailingly as in the case of true instincts, and this happens not only in regard to actions which, like walking, are essential to the life of the species, but also in regard to those which have arisen from chance habits or exercises. Often a short practice is sufficient to make an action in this sense instinctive, and the complexity of the instinct-mechanism gained by such practice is often astounding. Under some circumstances a person may play a piece on the piano from the score, and yet be thinking intently of other things, and be quite unconscious of what is played. In the same way it may happen that a person dominated by violent emotion, when trying to free himself from it by reading, may read a whole page, line by line, without understanding in the least what has been read. In the last case it is not directly demonstrable that the reader has made all the complex delicate eye-movements which would be liberated by the sight of the words, but in the case of playing, the listeners can perceive that the piece is correctly played, and thus that the stimulus exercised by each note on the retina of the eye is translated into the complex muscular movement of arm and finger, corresponding both to the pitch and the duration of the note, and to the simultaneousness of several notes.