A cursory survey of the varied fields of human activity suffices to establish the existence of a fundamental rhythm, an action and reaction, an antithetical sequence as it were, by virtue of which the prevailing phase in any movement is inevitably and inexorably succeeded by its direct opposite. Manifestations of this periodicity or cyclical tendency underlying all forms of human endeavour abound on every hand. Peace follows war as night succeeds day; an age of feudalism and despotic rule gives place to one of democratic Government. The time-worn creeds and simple faiths crumble a little before the exigent demands of a higher criticism, and, even as we glow with triumph at the achievements of modern science, we feel the premonitions of new developments whose exponents care for none of those things, and speak not the language of an out-worn materialism. And here it may not be inapposite to remark that the science of medicine is by no means immune from this prevailing principle.