The movements of the facial muscles depend, according to Darwin, on the constitution of the nervous system. We must remark, however, that Darwin took this idea from Herbert Spencer, who, a few years before Darwin published his book, had written a chapter in his 'Principles of Psychology’ entitled 'The Language of the Emotions.’ Darwin recognised the priority of Herbert Spencer, and I think it advisable to quote a passage from his book, 'The Expression of the Emotions,’ so that the reader may become acquainted with one of the most important pages published on the subject of which we are treating: 'As Mr. Herbert Spencer remarks’ (says Darwin on p. 71), 'it may be received as an “unquestionable truth that, at any moment, the existing quantity of liberated nerve-force, which in an inscrutable way produces in us the state we call feeling, _must_ expend itself in some direction--_must_ generate an equivalent manifestation of force somewhere”; so that, when the cerebro-spinal system is highly excited and nerve-force is liberated in excess, it may be expended in intense sensations, active thought, violent movements, or increased activity of the glands. Mr. Spencer further maintains that an “overflow of nerve-force, undirected by any motive, will manifestly take the most habitual routes; and, if these do not suffice, will next overflow into the less habitual ones.” Consequently, the facial and respiratory muscles, which are the most used, will be apt to be first brought into action; then those of the upper extremities, next those of the lower, and, finally, those of the whole body.’ The simplicity of this theory is seductive, but it suffices to subject it to a superficial examination in order to see that it does not quite correspond to the facts. If we find out which are the most habitual routes of nerve-force, and write them down in order one after the other, and then compare them with the movements expressing the passions, we shall see that there is not a perfect correspondence. It was perhaps for this reason that Herbert Spencer afterwards introduced the idea of the nervous lines of least resistance, in order to explain the greater facility with which certain muscles contract, compared to others. 'The molecular motion’ (says Spencer) 'disengaged in any nerve-centre by any stimulus, tends ever to flow along lines of least resistance throughout the nervous system.’ The solution of the problem was thus removed to the domain of experimental physiology. The question now is, whether in reality the muscles most commonly in use are those which have nerves offering less resistance, or whether the nervous excitement hidden in the centres is sufficiently strong to allow the resistance made by the nerves to its passage towards the muscles to be disregarded.