The evolutionary scale of memory is closely connected with that of presentation; this extremely important function of the psychoplasm--the condition of all further psychic development--consists essentially in the _reproduction of presentations_. The impressions in the bioplasm, which the stimulus produced as sensations, and which became presentations in remaining, are revived by memory; they pass from potentiality to actuality. The latent potential energy of the psychoplasm is transformed into kinetic energy. We may distinguish four stages in the upward development of memory, corresponding to the four stages of presentation. I. _Cellular memory._--Thirty years ago Ewald Hering showed "memory to be a general property of organized matter" in a thoughtful work, and indicated the great significance of this function, "to which we owe almost all that we are and have." Six years later, in my work on _The Perigenesis of the Plastidule, or the Undulatory Origin of the Parts of Life: an Experiment in the Mechanical Explanation of Elementary Evolutionary Processes_, I developed these ideas, and endeavored to base them on the principles of evolution. I have attempted to show in that work that unconscious memory is a universal and very important function of all _plastidules_; that is, of those hypothetical molecules, or groups of molecules, which Naegeli has called _micellae_, others _bioplasts_, and so forth. Only _living_ plastidules, as individual molecules of the active protoplasm, are reproductive, and so gifted with memory; that is the chief difference between the organic and inorganic worlds. It might be stated thus: "Heredity is the memory of the plastidule, while variability is its comprehension." The elementary memory of the unicellular protist is made up of the molecular memory of the plastidules or _micellae_, of which its living cell-body is constructed. As regards the extraordinary performances of unconscious memory in these unicellular protists, nothing could be more instructive than the infinitely varied and regular formation of their defensive apparatus, their shells and skeletons; in particular, the diatomes and cosmaria among the protophytes, and the radiolaria and thalamophora among the protozoa, afford an abundance of most interesting illustrations. In many thousand species of these protists the specific form which is inherited is _relatively constant_, and proves the fidelity of their unconscious cellular memory. II. _Histionic memory._--Equally interesting examples of the second stage of memory, the unconscious memory of tissues, are found in the heredity of the individual organs of plants and the lower, nerveless animals (sponges, etc.). This second stage seems to be _a reproduction of the histionic presentations_, that association of cellular presentations which sets in with the formation of coenobia in the social protists.