V. _Cellular theory of consciousness._--It is a vital property of every cell. The application of the cellular theory to every branch of biology involved its extension to psychology. Just as we take the living cell to be the "elementary organism" in anatomy and physiology, and derive the whole system of the multicellular animal or plant from it, so, with equal right, we may consider the "cell-soul" to be the psychological unit, and the complex psychic activity of the higher organism to be the result of the combination of the psychic activity of the cells which compose it. I gave the outlines of this _cellular psychology_ in my _General Morphology_ in 1866, and entered more fully into the subject in my paper on "Cell-Souls and Soul-Cells." I was led to a deeper study of this "elementary psychology" by my protracted research into the unicellular forms of life. Many of these tiny (generally microscopic) protists show similar expressions of sensation and will, and similar instincts and movements, to those of higher animals; that is especially true of the very sensitive and lively infusoria. In the relation of these sensitive cell-organisms to their environment, and in many other of their vital expressions (for instance, in the wonderful architecture of the rhizopods, the thalamophoræ, and the infusoria), we seemed to have clear indications of conscious psychic action. If, then, we accept the biological theory of consciousness (No. IV.), and credit every psychic function with a share of that faculty, we shall be compelled to ascribe it to each independent protist cell. In that case its material basis would be either the entire protoplasm of the cell, or its nucleus, or a portion of it. In the "psychade theory" of Fritz Schultze the elementary consciousness of the _psychade_ would have the same relation to the individual cells as personal consciousness has to the multicellular organism of the personality in the higher animals and man. It is impossible definitively to disprove this theory, which I held at one time. Still, I now feel compelled to agree with Max Verworn, in his belief that none of the protists have a developed self-consciousness, but that their sensations and movements are of an unconscious character.