The fact that the emotions of crowds are apt to be very violent has long been recognised, and the popular mind, in seeking to account for it, has commonly postulated very special and even supernatural causes. The negro author of a most interesting book[17] has given the following description of the religious frenzy of a crowd of Christian negroes: “An air of intense excitement possessed the mass of black folk. A suppressed terror hung in the air and seemed to seize us,—a pythian madness, a demoniac possession, that lent terrible reality to song and word. The massive form of the preacher swayed and quivered as the words crowded to his lips. The people moaned and fluttered and then a gaunt brown woman suddenly leaped into the air and shrieked like a lost soul, while round about came wail and groan and outcry, a scene of human passion such as I had never even imagined.” The author goes on to say that this frenzy is attributed by the black folk to the direct influence of the Spirit of the Lord, making mad the worshippers with supernatural joy, and that this belief is one of the leading features of their religion. Similar practices, depending upon the tendency of collective emotion to rise to an extreme intensity, have been common to the peoples of many lands in all ages; and similar supernatural explanations have been commonly devised and accepted. I need only remind the reader of the Dionysiac orgies of ancient Greece. The facts are so striking that for the popular mind they remain unaccountable, and not to be mentioned without some vague reference to magnetism, electricity, hypnotism, or some mysterious contagion; and even modern scientific writers have been led to adopt somewhat extravagant hypotheses to account for them. Thus Dr Le Bon[18] speaks of “the magnetic influence given out by the crowd” and says that, owing to this influence, “or from some other cause of which we are ignorant, an individual immerged for some length of time in a crowd in action soon finds himself in a special state, which much resembles the state of fascination in which the hypnotised individual finds himself in the hands of the hypnotiser.” He goes on to say that in the hypnotised subject the conscious personality disappears and that his actions are the outcome of the unconscious activities of the spinal cord. Now, crowds undoubtedly display great suggestibility, but great suggestibility does not necessarily imply hypnosis; and there is no ground for supposing that the members of a crowd are thrown into any such condition, save possibly in very rare instances. There are however two hypotheses, sometimes invoked for the explanation of the peculiarities of collective mental life, which demand serious consideration and which we may with advantage consider at this point.