_Darwin’s principles have been much criticised_; in particular, the purely negative principle of antithesis has received short shrift from later writers. One of the things that he fails to account for is the imitative play of the lips. The disgusted man looks as if he were about to retch; the injured man looks bitter; the disappointed, sour; the satisfied, sweet; the mouth, in these latter cases, is set as it is when we have a bitter, sour, or sweet taste. What is the reason? We may remind ourselves that primitive language was concrete, and not abstract; that it abounded in what we should nowadays call metaphor. We may remember also that the one thing necessary in a primitive society is food, and that primitive metaphors would naturally be, to a large extent, metaphors drawn from the preparing and obtaining of food, from cooking and hunting. So we may imagine that the successful hunter, returning to camp, licked his lips, seemed already to be sucking the sweet morsel; while the unsuccessful drew his lips out sideways, as if he were trying to taste as little as possible of his sour draught. In course of time the metaphor will lapse; or, more strictly, the old concrete way of speech will give place to an abstract phrasing, and will hold its own only _as_ metaphor, as a bit of picturesque imagery; we still talk to-day of the sweets of love and revenge, of tasting success, of tainted money, of a soured disposition, of the bitter end. Meanwhile the original gesture, if only it is fitted for communication, will persist unchanged; gesture is far more conservative than language; and the look of a bitter taste will thus express the emotion of a man who is suffering, perhaps, under an unjust accusation. We may say of all such attempts at explanation what we said of the biological theory of feeling: it would be foolish to reject them outright, and yet _they are too general, too open to criticism, to satisfy the requirements of science_. We need detailed work, both upon the physiological and upon the psychological side. Consider, for example, the erection of the hair in fear and rage. This is a result of the diffused activity of the ‘sympathetic’ nervous system, the total effect of which is to _energise_ the organism; when two boys are wrestling, the friends of the weaker or less skilful shout to him to ‘get angry’; and terrified men achieve wonderful feats of leaping and running. But how precisely does the contraction of the muscles beneath the skin subserve this energising? Is it an accident, so to speak, due merely to the diffusion of the nervous activity? or has it a special physiological function? and has it, further, anything of the biological significance that Darwin attached to it? Until such questions are answered in detail, we cannot formulate general principles of the expression of emotion.