During the last forty years a great number of works on comparative animal psychology have appeared, principally occasioned by the great impulse which Darwin gave in 1859 by his work on _The Origin of Species_, and by the application of the idea of evolution to the province of psychology. The more important of these works we owe to Romanes and Sir J. Lubbock, in England; to W. Wundt, L. Büchner, G. Schneider, Fritz Schultze, and Karl Groos, in Germany; to Alfred Espinas and E. Jourdan, in France; and to Tito Vignoli, in Italy. In Germany, Wilhelm Wundt, of Leipzig, is considered to be the ablest living psychologist; he has the inestimable advantage over most other philosophers of a thorough zoological, anatomical, and physiological education. Formerly assistant and pupil of Helmholtz, Wundt had early accustomed himself to follow the application of the laws of physics and chemistry through the whole field of physiology, and, consequently, in the sense of Johannes Müller, in _psychology_, as a subsection of the latter. Starting from this point of view, Wundt published his valuable "Lectures on human and animal psychology" in 1863. He proved, as he himself tells us in the preface, that the theatre of the most important psychic processes is in the "unconscious soul," and he affords us "a view of the mechanism which, in the unconscious background of the soul, manipulates the impressions which arise from the external stimuli." What seems to me, however, of special importance and value in Wundt's work is that he "extends the law of the persistence of force for the first time to the psychic world, and makes use of a series of facts of electro-physiology by way of demonstration."