Man’s psychic life runs the same evolution—upward progress, full maturity, and downward degeneration—as every other vital activity in his organization.
[CHAPTER IX]
THE PHYLOGENY OF THE SOUL
Gradual Historical Evolution of the Human Soul from the Animal Soul—Methods of Phylogenetic Psychology—Four Chief Stages in the Phylogeny of the Soul: I. The Cell-Soul (Cytopsyche) of the Protist (Infusoria, Ova, etc.): Cellular Psychology; II. The Soul of a Colony of Cells, or the Cenobitic Soul (Cœnopsyche): Psychology of the Morula and Blastula; III. The Soul of the Tissue (Histopsyche): Its Twofold Nature: The Soul of the Plant: The Soul of the Lower, Nerveless Animal: Double Soul of the Siphonophora (Personal and Kormal Soul); IV. The Nerve-Soul (Neuropsyche) of the Higher Animal—Three Sections of its Psychic Apparatus: Sense-Organs, Muscles, and Nerves—Typical Formation of the Nerve-Centre in the Various Groups of Animals—Psychic Organ of the Vertebrate: the Brain and the Spinal Cord—Phylogeny of the Mammal Soul
The theory of descent, combined with anthropological research, has convinced us of the descent of our human organism from a long series of animal ancestors by a slow and gradual transformation occupying many millions of years. Since, then, we cannot dissever man’s psychic life from the rest of his vital functions—we are rather forced to a conviction of the natural evolution of our whole body and mind—it becomes one of the main tasks of the modern monistic psychology to trace the stages of the historical development of the soul of man from the soul of the brute. Our “phylogeny of the soul” seeks to attain this object; it may also, as a branch of general psychology, be called phylogenetic psychology, or, in contradistinction to biontic (individual), phyletic psychogeny. And, although this new science has scarcely been taken up in earnest yet, and most of the “professional” psychologists deny its very right to existence, we must claim for it the utmost importance and the deepest interest. For, in our opinion, it is its special province to solve for us the great enigma of the nature and origin of the human soul.
The methods and paths which will lead us to the remote goal of a complete phylogenetic psychology—a goal that is still buried in the mists of the future, and almost imperceptible to many—do not differ from those of other branches of evolutionary research. Comparative anatomy, physiology, and ontogeny are of the first importance. Much support is given also by palæontology, for the order in which the fossil remains of the various classes of vertebrates succeed each other in the course of organic evolution reveals to us, to some extent, the gradual growth of their psychic power as well as their phyletic connection. We must admit that we are here, as we are in every branch of phylogenetic research, driven to the construction of a number of hypotheses in order to fill up the considerable lacunæ of empirical phylogeny. Yet these hypotheses cast so clear and significant a light on the chief stages of historical development that we are afforded a most gratifying insight into their entire course.
The comparative psychology of man and the higher animals enables us to learn from the highest group of the placentals, the primates, the long strides by which the human soul has advanced beyond the psyche of the anthropoid ape. The phylogeny of the mammals and of the lower vertebrates acquaints us with the long series of the earlier ancestors of the primates which have arisen within this stem since the Silurian age. All these vertebrates agree in the structure and development of their characteristic psychic organ—the spinal cord. We learn from the comparative anatomy of the vermalia that this spinal cord has been evolved from a dorsal acroganglion, or vertical brain, of an invertebrate ancestor. We learn, further, from comparative ontogeny that this simple psychic organ has been evolved from the stratum of cells in the outer germinal layer, the ectoderm, of the platodes. In these earliest flat-worms, which have no specialized nervous system, the outer skin-covering serves as a general sensitive and psychic organ. Finally, comparative embryology teaches us that these simple metazoa have arisen by gastrulation from blastæades, from hollow spheres, the wall of which is merely one simple layer of cells, the blastoderm; and the same science, with the aid of the biogenetic law, explains how these protozoic cœnobia originally sprang from the simplest unicellular organisms.
On a critical study of these different embryonic formations, the evolution of which from each other we can directly observe under the microscope, we arrive, by means of the great law of biogeny, at a series of most important conclusions as to the chief stages in the development of our psychic life. We may distinguish eight of these to begin with:
I. Unicellular protozoa with a simple cell-soul: the infusoria.