According to Duchenne de Boulogne all passions have a special muscle at their service, which enters into activity in order to express the feelings of the mind; benevolence, joy, laughter, sadness, attention, reflection, lasciviousness, irony, contempt, fright, cruelty, pain, weeping, appear on the human countenance each through the medium of a muscle possessing the privilege of representing a particular emotion. [24] G. B. Duchenne de Boulogne: _Mécanisme de la Physionomie Humaine_ (Paris, 1862), p. 32. Duchenne de Boulogne certainly went rather far with his theory of localisations. He treated the face as Gall did the skull and brain. Classifications of mental faculties are too artificial, being derived from an abstraction based on facts and phenomena neither distinct nor definable. Gall wished to localise, so to speak, the metaphysical and theological faculties of the mind in the different parts of the brain, and invented the phrenology which made him famous. But in none of his writings is there any penetration to the real origin of facts. He let his imagination and, more than all, the force of his own eloquence, carry him away, and his phrenology, then called the science of the future and the reforming doctrine of society, has now fallen into oblivion in spite of the prophecies of his followers. II Darwin reduced the principles on which the expression of the emotions depends to the following three: I. The principle of serviceable associated habits. II. That of antithesis. III. That of actions due to the constitution of the nervous system, independently from the first of the will and to a certain extent of habit. According to Darwin the expression of pain depends essentially on the first and third of these principles. He assumes, indeed, that in all animals, in the series of innumerable generations, intense pain has produced the most violent and diverse movements in order to escape its cause. As the muscles of the thorax and of the vocal organs are those most habitually used, they were called more particularly into action, and animals began to make sounds, to howl and to screech. Darwin believed that vocal sounds were useful to animals, particularly to the young and to those living in community, because in case of danger the cries serve to call the parents or to warn the other animals. These opinions of Charles Darwin open up a wide field for discussion, but for the present I intend to speak only of the expression of the face, so as to confine my subject within reasonable limits.