All living organisms, without exception, are sensitive; they are influenced by the condition of their environment, and react thereon by certain modifications in their own structure. Light and heat, gravity and electricity, mechanical processes and chemical action in the environment, act as _stimuli_ on the sensitive psychoplasm, and effect changes in its molecular composition. We may distinguish the following five chief stages of this sensibility: I. At the lowest stage of organization the _whole psychoplasm_, as such, is sensitive, and reacts on the stimuli from without; that is the case with the lowest protists, with many plants, and with some of the most rudimentary animals. II. At the second stage very simple and undiscriminating _sense-organs_ begin to appear on the surface of the organism, in the form of protoplasmic filaments and pigment spots, the forerunners of the nerves of touch and the eyes; these are found in some of the higher protists and in many of the lower animals and plants. III. At the third stage _specific organs_ of sense, each with a peculiar adaptation, have arisen by differentiation out of these rudimentary processes: there are the chemical instruments of smell and taste, and the physical organs of touch, temperature, hearing, and sight. The "specific energy" of these sense-organs is not an original inherent property of theirs, but has been gained by functional adaptation and progressive heredity. IV. The fourth stage is characterized by the _centralization_ or integration of the _nervous system_, and, consequently, of sensation; by the association of the previously isolated or localized sensations presentations arise, though they still remain unconscious. That is the condition of many both of the lower and the higher animals. V. Finally, at the fifth stage, the highest psychic function, _conscious perception_, is developed by the mirroring of the sensations in a central part of the nervous system, as we find in man and the higher vertebrates, and probably in some of the higher invertebrates, notably the articulata. All living organisms without exception have the faculty of _spontaneous movement_, in contradistinction to the rigidity and inertia of unorganized substances (_e.g._, crystals); in other words, certain changes of place of the particles occur in the living psychoplasm from internal causes, which have their source in its own chemical composition. These active vital movements are partly discovered by direct observation and partly only known indirectly, by inference from their effects. We may distinguish five stages of them. I. At the lowest stage of organic life, in the chromacea, and many protophyta and lower metaphyta, we perceive only those _movements of growth_ which are common to all organisms. They are usually so slow that they cannot be directly observed; they have to be inferred from their results--from the change in size and form of the growing organism.