When an affirmation has been sufficiently repeated and there is unanimity in this repetition—as has occurred in the case of certain famous financial undertakings rich enough to purchase every assistance— what is called a current of opinion is formed and the powerful mechanism of contagion intervenes. Ideas, sentiments, emotions, and beliefs possess in crowds a contagious power as intense as that of microbes. This phenomenon is very natural,since it is observed even in animals when they are together in number. Should a horse in a stable take to biting his manger the other horses in the stable will imitate him. A panic that has seized on a few sheep will soon extend to the whole fiock. In the case of men collected in a crowd all emotions are very rapidly contagious, which explains the suddenness of panics. Brain disorders, like madness, are themselves contagious. The frequency of madness among doctors who are specialists for the mad is notorious.Indeed, forms of madness have recently been cited—agoraphobia, for instance—which are communicable from men to animals.