The second stage of psychic activity, compound reflex action, begins with the cenobitic protists (v.g., the volvox and the carchesium). The innumerable social cells, which make up this cell-community or cœnobium, are always more or less connected, often directly connected by filamentous bridges of protoplasm. A stimulus that alights on one or more cells of the community is communicated to the rest by means of the connecting fibres, and may produce a general contraction. This connection is found, also, in the tissues of the multicellular animals and plants. It was erroneously believed at one time that the cells of vegetal tissue were completely isolated from each other, but we have now discovered fine filaments of protoplasm throughout, which penetrate the thick membranes of the cells, and maintain a material and psychological communication between their living plasmic contents. That is the explanation of the mimosa: when the tread of the passer-by shakes the root of the plant, the stimulus is immediately conveyed to all the cells, and causes a general contraction of its tender leaves and a drooping of the stems.
An important and universal feature of all reflex phenomena is the absence of consciousness. For reasons which we shall give in the [tenth chapter] we only admit the presence of consciousness in man and the higher animals, not in plants, the lower animals, and the protists; consequently all stimulated movements in the latter must be regarded as reflex—that is, all movements which are not spontaneous, not the outcome of internal causes (impulsive and automatic movements).[14] It is different with the higher animals which have developed a centralized nervous system and elaborate sense-organs. In these cases consciousness has been gradually evolved from the psychic reflex activity, and now conscious, voluntary action appears, in opposition to the still continuing reflex action below. However, we must distinguish two different processes, as we did in the question of instinct—primary and secondary reflex action. Primary reflex actions are those which have never reached the stage of consciousness in phyletic development, and thus preserve the primitive character (by heredity from lower animal forms). Secondary reflex actions are those which were conscious, voluntary actions in our ancestors, but which afterwards became unconscious from habit or the lapse of consciousness. It is impossible to draw a hard and fast line in such cases between conscious and unconscious psychic function.
Older psychologists (Herbart, for instance) considered “presentation” to be the fundamental psychic phenomenon, from which all the others are derived. Modern comparative psychology endorses this view in so far as it relates to the idea of unconscious presentation; but it considers conscious presentation to be a secondary phenomenon of mental life, which is entirely wanting in plants and the lower animals, and is only developed in the higher animals. Among the many contradictory definitions which psychologists have given of “presentation,” we think the best is that which makes it consist in an internal picture of the external object which is given us in sensation—an “idea,” in the broader sense. We may distinguish the following four stages in the rising scale of presentative function:
I. Cellular presentation.—At the lowest stages we find presentation to be a general physiological property of psychoplasm; even in the simplest unicellular protist sensations may leave a permanent trace in the psychoplasm, and these may be reproduced by memory. In more than four thousand kinds of radiolaria, which I have described, every single species is distinguished by special, hereditary skeletal structure. The construction of this specific, and often highly elaborate, skeleton by a cell of the simplest description (generally globular) is only intelligible when we attribute the faculty of presentation, and, indeed, of a special reproduction of the plastic “feeling of distance,” to the constructive protoplasm—as I have pointed out in my Psychology of the Radiolaria.[15]
II. Histionic presentation.—In the cœnobia or cell-colonies of the social protists, and still better in the tissues of plants and lower, nerveless animals (sponges, polyps, etc.), we find the second stage of unconscious presentation, which consists of the common psychic activity of a number of closely connected cells. If a single stimulus may, instead of simply spending itself in the reflex movement of an organ (the leaf of a plant, for instance, or the arm of a polyp), leave a permanent impression, which can be spontaneously reproduced later on, we are bound to assume, in explaining the phenomenon, a histionic presentation, dependent on the psychoplasm of the associated tissue-cells.
III. Unconscious presentation in the ganglionic cells.—This third and higher stage of presentation is the commonest form the function takes in the animal world; it seems to be a localization of presentation in definite “soul-cells.” In its simplest form it appears at the sixth stage of reflex action, when the tricellular reflex organ arises: the seat of presentation is then the intermediate psychic cell, which is interposed between the sensitive cell and the muscular cell. With the increasing development of the animal nervous system and its progressive differentiation and integration, this unconscious presentation also rises to higher stages.
IV. Conscious presentation in the cerebral cells.—With the highest stage of development of the animal organization consciousness arises, as a special function of a certain central organ of the nervous system. As the presentations are conscious, and as special parts of the brain arise for the association of these conscious presentations, the organism is qualified for those highest psychic functions which we call thought and reflection, intellect and reason. Although the tracing of the phyletic barrier between the older, unconscious, and the younger, conscious, presentation is extremely difficult, we can affirm, with some degree of probability, that the evolution of the latter from the former was polyphyletic; because we find conscious and rational thought, not only in the highest forms of the vertebrate stem (man, mammals, birds, and a part of the lower vertebrates), but also in the most highly developed representatives of other animal groups (ants and other insects, spiders and the higher crabs among the articulata, cephalopods among the mollusca).
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 cœnobia in the social protists.