As subjects for study he recommends children because they permit forms of expression to appear vigorously and without constraint; lunatics, because they are subject to strong passions without control; galvanized persons, in order to facilitate the muscles involved, and finally, to establish the identity of expression among all races of men and beasts. Of these objects only children are important for our purpose. The others either are far removed from our sphere of activity, or have only theoretic value. I should, however, like to add to the subjects of observation another, viz., the simple unstudied persons, peasants and such otherwise unspoiled individuals whom we may believe innocent of all intention to play a comedy with us. We can learn much from such people and from children. And it is to be believed that in studying them we are studying not a special class but are establishing a generally valid paradigm of the whole of mankind. Children have the same features as adults, only clearer and simpler. For, suppose we consider any one of Darwin’s dicta,--e.g., that in the expression of anger and indignation the eyes shine, respiration becomes more rapid and intense, the nostrils are somewhat raised, the look misses the opponent,--these so intensely characteristic indices occur equally in the child and the adult. Neither shows more or fewer, and once we have defined them in the child we have done it for the adult also. Once the physiognomy of children and simple people has been studied, the further study of different kinds of people is no longer difficult; there is only the intentional and customary masking of expression to look out for; for the rest, the already acquired principles, mutandis mutatis, are to be used. Darwin posits three general principles on which most expressions and gestures are to be explained. They are briefly: I. The principle of purposeful associated habits. II. The principle of contradication. III. The principle of the direct activity of the nervous system. With regard to the first. When, in the course of a long series of generations, any desire, experience, or disinclination, etc., has led to some voluntary action, then, as often as the same or any analogous associated experience is undergone, there will arise a tendency to the realization of a similar action. This action may no longer have any use but is inherited and generally becomes a mere reflex.