Of all animals, monkeys possess the greatest development of the facial muscles. This is owing principally to the circumstance that they eat everything, being half carnivorous, half herbivorous, and make use of the mouth as an organ for seizing the prey, and assisting the hands in tearing, skinning, and continually preparing the food. The countenance of the monkey is of unexampled mobility; in a few minutes one sees all expressions pass over it, from desire to contempt, from cunning to innocence, from attention to carelessness, from love to rage, from aggression to fear, from joy to sadness. [18] Darwin believed that animals show their teeth in order to let their weapons be seen, and in this way to be more feared. This explanation does not seem to me quite exact, as animals are obliged to raise the lips when they bite, so that the soft parts of the mouth covering the jaws may not be injured. It suffices to watch a dog in order to convince oneself that the showing of the teeth must be an act preparatory to that of biting. III One of the reasons why the facial muscles move more easily, is their diminutive size. It was Spencer who first clearly developed this idea, and I know of nothing more fundamental in the language of the emotions. 'Supposing,’ he says, 'a feeble wave of nervous excitement to be propagated uniformly throughout the nervous system, the part of it discharged on the muscles will show its effects most where the amount of inertia to be overcome is least. Muscles which are large, and which can show states of contraction into which they are thrown only by moving limbs or other heavy masses, will yield no signs; while small muscles, and those which can move without overcoming great resistances, will visibly respond to this feeble wave. Hence must result a certain general order in the excitation of the muscles, serving to mark the strength of the nervous discharge and of the feeling accompanying it.... It is because the muscles of the face are relatively small, and are attached to easily moved parts, that the face is so good an index of the amount of feeling.’[19] [19] _Principles of Psychology_, vol. ii., pp. 542-43. This law, however, is in my opinion insufficient to explain the expressions of the face, because we have very fine, small muscles in the ear, the skin, and elsewhere, that yet take no part in the expression, although the resistance they offer is very small. Great importance must, I think, be attached to the continual use of certain muscles, and to the different excitability of their nerves. The muscles which we most frequently put into movement are also those which most easily betray the excitement of the nerve-centres. It is so with the ear of the horse and dog, which is a faithful mirror of everything they feel, of all their emotions; while the ears of man, although possessing the same muscles, remain immovable even during the strongest emotion, and solely because we never make use of them.