The great progress which psychology has made, with the assistance of evolution, in the latter half of the century culminates in the recognition of _the psychological unity of the organic world_. Comparative psychology, in co-operation with the ontogeny and phylogeny of the _psyche_, has enforced the conviction that organic life in all its stages, from the simplest unicellular protozoon up to man, springs from the same elementary forces of nature, from the physiological functions of sensation and movement. The future task of scientific psychology, therefore, is not, as it once was, the exclusively subjective and introspective analysis of the highly developed mind of a philosopher, but the objective, comparative study of the long gradation by which man has slowly arisen through a vast series of lower animal conditions. This great task of separating the different steps in the psychological ladder, and proving their unbroken phylogenetic connection, has only been seriously attempted during the last ten years, especially in the splendid work of Romanes. We must confine ourselves here to a brief discussion of a few of the general questions which that gradation has suggested. All the phenomena of the psychic life are, without exception, bound up with certain material changes in the living substance of the body, the _protoplasm_. We have given to that part of the protoplasm which seems to be the indispensable substratum of psychic life the name of _psychoplasm_ (the "soul-substance," in the monistic sense); in other words, we do not attribute any peculiar "essence" to it, but we consider the _psyche_ to be merely _a collective idea of all the psychic functions of protoplasm_. In this sense the "soul" is merely a physiological abstraction like "assimilation" or "generation." In man and the higher animals, in accordance with the division of labor of the organs and tissues, the psychoplasm is a differentiated part of the nervous system, the _neuroplasm_ of the ganglionic cells and their fibres. In the lower animals, however, which have no special nerves and organs of sense, and in the plants, the psychoplasm has not yet reached an independent differentiation. Finally, in the unicellular protists, the psychoplasm is identified either with the whole of the living protoplasm of the simple cell or with a portion of it. In all cases, in the lowest as well as the highest stages of the psychological hierarchy, a certain chemical composition and a certain physical activity of the psychoplasm are indispensable before the "soul" can function or act. That is equally true of the elementary psychic function of the plasmatic sensation and movement of the protozoa, and of the complex functions of the sense-organs and the brain in the higher animals and man. The activity of the psychoplasm, which we call the "soul," is always connected with metabolism.