The first fact we meet in the comparative psychology of the vertebrates, and which should be the empirical starting-point of all scientific human psychology, is the characteristic structure of the central nervous system. This central psychic organ has a particular position, shape, and texture in the vertebrate as it has in all the higher species. In every case we find a spinal medulla, a strong cylindrical nervous cord, which runs down the middle of the back, in the upper part of the vertebral column (or the cord which represents it). In every case a number of nerves branch off from this medulla in regular division, one pair to each segment or vertebra. In every case this medullary cord arises in the same way in the fœtus; a fine groove appears in the middle axis of the skin at the back; then the parallel borders of this medullary groove are lifted up a little, bend over towards each other, and form into a kind of tube.
The long dorsal cylindrical medullary tube which is thus formed is thoroughly characteristic of the vertebrates; it is always the same in the early embryonic sketch of the organism, and it is always the chief feature of the different kinds of psychic organ which evolve from it in time. Only one single group of invertebrates has a similar structure: the rare, marine tunicata, copelata, ascidia, and thalidiæ. These animals have other important peculiarities of structure (especially in the chorda and the gut) which show a striking divergence from the other invertebrates and resemblance to the vertebrates. The inference we draw is that both these groups, the vertebrates and the tunicates, have arisen from a common ancestral group of the vermalia, the prochordonia.[21] Still, there is a great difference between the two classes in the fact that the body of the tunicate does not articulate, or form members, and has a very simple organization (most of them subsequently attach themselves to the bottom of the sea and degenerate). The vertebrate, on the other hand, is characterized by an early development of internal members, and the formation of pro-vertebræ (vertebratio). This prepares the way for the much higher development of their organism, which finally attains perfection in man. This is easily seen in the finer structure of his spinal cord, and in the development of a number of segmental pairs of nerves, the spinal nerves, which proceed to the various parts of the body.
The long ancestral history of our “vertebrate soul” commences with the formation of the most rudimentary spinal cord in the earliest acrania; slowly and gradually, through a period of many millions of years, it conducts to that marvellous structure of the human brain which seems to entitle the highest primate form to quite an exceptional position in nature. Since a clear conception of this slow and steady progress of our phyletic psychogeny is indispensable for a true psychology, we must divide that vast period into a number of stages or sections: in each of them the perfecting of the structure of the nervous centre has been accompanied by a corresponding evolution of its function, the psyche. I distinguish eight of these periods in the phylogeny of the spinal cord, which are characterized by eight different groups of vertebrates: (1) the acrania; (2) the cyclostomata; (3) the fishes; (4) the amphibia; (5) the implacental mammals (monotremes and marsupials); (6) the earlier placental mammals, especially the prosimiæ; (7) the younger primates, the simiæ; and (8) the anthropoid apes and man.
I. First stage—the acrania: their only modern representative is the lancelot or amphioxus; the psychic organ remains a simple medullary tube, and contains a regularly segmented spinal cord, without brain.
II. Second stage—the cyclostomata: the oldest group of the craniota, now only represented by the petromyzontes and myxinoides: the fore-termination of the cord expands into a vesicle, which then subdivides into five successive parts—the great-brain, intermediate-brain, middle-brain, little-brain, and hind-brain: these five cerebral vesicles form the common type from which the brain of all craniota has evolved, from the lamprey to man.
III. Third stage—the primitive fishes (selachii): similar to the modern shark: in these oldest fishes, from which all the gnathostomata descend, the more pronounced division of the five cerebral vesicles sets in.
IV. Fourth stage—the amphibia. These earliest land animals, making their first appearance in the Carboniferous period, represent the commencement of the characteristic structure of the tetrapod and a corresponding development of the fish-brain: it advances still further in their Permian successors, the reptiles, the earliest representatives of which, the tocosauria, are the common ancestors of all the amniota (reptiles and birds on one side, mammals on the other).
V.-VIII. Fifth to the eighth stages—the mammals. I have exhaustively treated, and illustrated with a number of plates, in my Anthropogeny, the evolution of our nervous system and the correlative question of the development of the soul. I have now, therefore, merely to refer the reader to that work. It only remains for me to add a few remarks on the last and most interesting class of facts pertaining to this—to the evolution of the soul and its organs within the limits of the class mammalia. In doing so, I must remind the reader that the monophyletic origin of this class—that is, the descent of all the mammals from one common ancestral form (of the Triassic period)—is now fully established.
The most important consequence of the monophyletic origin of the mammals is the necessity of deriving the human soul from a long evolutionary series of other mammal souls. A deep anatomical and physiological gulf separated the brain structure and the dependent psychic activity of the higher mammals from those of the lower: this gulf, however, is completely bridged over by a long series of intermediate stages. The period of at least fourteen (more than a hundred, on other estimates) million years, which has elapsed since the commencement of the Triassic period, is amply sufficient to allow even the greatest psychological advance. The following is a summary of the results of investigation in this quarter, which has recently been very penetrating:
I. The brain of the mammal is differentiated from that of the other vertebrates by certain features, which are found in all branches of the class; especially by a preponderant development of the first and fourth vesicles, the cerebrum and cerebellum, while the third vesicle, the middle brain, disappears altogether.