I. _The cell-soul_ (_or cytopsyche_): first stage of phyletic psychogenesis.--The earliest ancestors of man and all other animals were unicellular protozoa. This fundamental hypothesis of rational phylogeny is based, in virtue of the phylogenetic law, on the familiar embryological fact that every man, like every other metazoon (_i.e._, every multicellular organism with tissues), begins his personal existence as a simple cell, the stem-cell (_cytula_), or the impregnated egg-cell (see p. 63). As this cell has a "soul" from the commencement, so had also the corresponding unicellular _ancestral forms_, which were represented in the oldest series of man's ancestors by a number of different protozoa. We learn the character of the psychic activity of these unicellular organisms from the comparative physiology of the protists of to-day. Close observation and careful experiment have opened out to us in this respect, in the second half of the nineteenth century, a new world of the most interesting phenomena. The best description of them was given by Max Verworn in his thoughtful work, based on original research, _Psycho-physiological Studies of the Protists_. The work includes also the few earlier observations of the "psychic life of the protist." Verworn came to the firm conclusion that the psychic processes are unconscious in all the protists, that the phenomena of sensation and movement coincide with the molecular vital processes in their protoplasm, and that their ultimate causes are to be sought in the properties of the protoplasmic molecules (the _plastidules_). "Hence the psychic phenomena of the protists form a bridge that connects the chemical processes of the inorganic world with the psychic life of the highest animals; they represent the germ of the highest psychic phenomena of the metazoa and of man."