It is true that modern vegetal physiology has given a purely physical explanation of many of these stimulated movements, or tropisms, by special features of growth, variations of pressure, etc. Yet these mechanical causes are neither more nor less psychophysical than the similar “reflex movements” of the sponges, polyps, and other nerveless metazoa, even though their mechanism is entirely different. The character of the tissue-soul reveals itself in the same way in both cases—the cells of the tissue (the regular, orderly structure of cells) transmit the stimuli they have received in one part, and thus provoke movements of other parts, or of the whole organ. This transmission of stimuli has as much title to be called “psychic activity” as its more complete form in the higher animals with nerves; the anatomic explanation of it is that the social cells of the tissue, or cell-community, are not isolated from each other (as was formerly supposed), but are connected throughout by fine threads or bridges of protoplasm. When the sensitive mimosa closes its graceful leaves and droops its stalk at contact, or on being shaken; when the irritable fly-trap (the dionæa) swiftly clasps its leaves together at a touch, and captures a fly; the sensation seems to be keener, the transmission of the stimulus more rapid, and the movement more energetic than in the reflex action of the stimulated bath-sponge and many other sponges.
B. The soul of the nerveless metazoa.—Of very special interest for comparative psychology in general, and for the phylogeny of the animal soul in particular, is the psychic activity of those lower metazoa which have tissues, and sometimes differentiated organs, but no nerves or specific organs of sense. To this category belong four different groups of the earliest cœlenterates: (a) the gastræads, (b) the platodaria, (c) the sponges, and (d) the hydropolyps, the lowest form of cnidaria.
The gastraeads (or animals with a primitive gut) form a small group of the lowest cœlenterates, which is of great importance as the common ancestral group of all the metazoa. The body of these little swimming animals looks like a tiny (generally oval) vesicle, which has a simple cavity with one opening—the primitive gut and the primitive mouth. The wall of the digestive cavity is formed of two simple layers of cells, or epithelium, the inner of which—the gut-layer—is responsible for the vegetal activity of nourishment, while the outer, or skin-layer, discharges the animal functions of movement and sensation. The homogeneous sensitive cells of the skin-layer bear long, slender hairs or lashes (cilia), by the vibration of which the swimming motion is effected. The few surviving forms of gastræads, the gastræmaria (trichoplacidae) and cyemaria (orthonectidae), are extremely interesting, from the fact that they remain throughout life at a stage of structure which is passed by all the other metazoa (from the sponge to man) at the commencement of their embryonic development. As I have shown in my Theory of the Gastraea (1872), a very characteristic embryonic form, the gastrula, is immediately developed from the blastula in all the tissue animals. The germinal membrane (blastoderm), which represents the wall of the hollow vesicle, forms a depression at one side, and this soon sinks in so deep that the inner cavity of the vesicle disappears. The half of the membrane which bends in is thus laid on, and inside, the other half; the latter forms the skin-layer, or outer germinal layer (ectoderm or epiblast), and the former becomes the gut-layer, or inner germinal layer (endoderm or hypoblast). The new cavity of the cup-shaped body is the digestive stomach cavity (the progaste), and its opening is the primitive mouth (or prostoma).[19] The skin-layer, or ectoderm, is the primitive psychic organ in the metazoa; from it, in all the nerve animals, not only the external skin and the organs of sense, but also the nervous system, are developed. In the gastræads, which have no nerves, all the cells which compose the simple epithelium of the ectoderm are equally organs of sensation and of movement; we have here the tissue-soul in its simplest form.
The platodaria, the earliest and simplest form of the platodes, seem to be of the same primitive construction. Some of these cryptocœla—the convoluta, etc.—have no specific nervous system, while their nearest relatives, the turbellaria, have already differentiated one, and even developed a vertical brain.
The sponges form a peculiar group in the animal world, which differs widely in organization from all the other metazoa. The innumerable kinds of sponges grow, as a rule, at the bottom of the sea. The simplest form of sponge, the olynthus, is in reality nothing more than a gastraea, the body-wall of which is perforated like a sieve, with fine pores, in order to permit the entrance of the nourishing stream of water. In the majority of sponges—even in the most familiar one, the bath-sponge—the bulbous organism constructs a kind of stem or tree, which is made up of thousands of these gastræads, and permeated by a nutritive system of canals. Sensation and movement are only developed in the faintest degree in the sponges; they have no nerves, muscles, or organs of sense. It was therefore quite natural that such stationary, shapeless, insensitive animals should have been commonly taken to be plants in earlier years. Their psychic life—for which no special organs have been differentiated—is far inferior to that of the mimosa and other sensitive plants.
The soul of the cnidaria is of the utmost importance in comparative and phylogenetic psychology; for in this numerous group of the cœlenterates the historical evolution of the nerve-soul out of the tissue-soul is repeated before our eyes. To this group belong the innumerable classes of stationary polyps and corals, and of swimming medusæ and siphonophora. As the common ancestor of all the cnidaria we can safely assign a very simple polyp, which is substantially the same in structure as the common, still surviving, fresh-water polyp—the hydra. Yet the hydræ, and the stationary, closely related hydropolyps, have no nerves or higher sense-organs, although they are extremely sensitive. On the other hand, the free-swimming medusæ, which are developed from them—and are still connected with them by alternation of generations—have an independent nervous system and specific sense-organs. Here, also, we may directly observe the ontogenetic evolution of the nerve-soul (neuropsyche) out of the tissue-soul (histopsyche), and thus learn its phylogenetic origin. This is the more interesting as such phenomena are polyphyletic—that is, they have occurred several times—more than once, at least—quite independently. As I have shown elsewhere, the hydromedusæ have arisen from the hydropolyps in a different manner from that of the evolution of the scyphomedusæ from the scyphopolyps; the gemmation is terminal in the case of the latter, and lateral with the former. In addition, both groups have characteristic hereditary differences in the more minute structure of their psychic organs. The class of siphonophora is also very interesting to the psychologist. In these pretty, free-swimming organisms, which come from the hydromedusæ we can observe a double soul: the personal soul of the numerous individualities which compose them, and the common, harmoniously acting psyche of the entire colony.
IV. The nerve-soul (neuropsyche): fourth stage of phyletic psychogeny.—The psychic life of all the higher animals is conducted, as in man, by means of a more or less complicated “psychic apparatus.” This apparatus is always composed of three chief sections: the organs of sense are responsible for the various sensations; the muscles effect the movements; the nerves form the connection between the two by means of a special central organ, the brain or ganglion. The arrangement and action of this psychic mechanism have been frequently compared with those of a telegraphic system: the nerves are the wires, the brain the central, and the sense-organs subordinate stations. The motor nerves conduct the commands of the will centrifugally from the nerve-centre to the muscles, by the contraction of which they produce the movements: the sensitive nerves transmit the various sensations centripetally—that is, from the peripheral sense-organs to the brain, and thus render an account of the impressions they receive from the outer world. The ganglionic cells, or “psychic cells,” which compose the central nervous organ, are the most perfect of all organic elements; they not only conduct the commerce between the muscles and the organs of sense, but they also effect the highest performances of the animal soul, the formation of ideas and thoughts, and especially consciousness.
The great progress of anatomy, physiology, histology, and ontogeny has recently added a wealth of interesting discoveries to our knowledge of the mechanism of the soul. If speculative philosophy assimilated only the most important of these significant results of empirical biology, it would have a very different character from that it unfortunately presents. As I have not space for an exhaustive treatment of them here, I will confine myself to a relation of the chief facts.
Each of the higher animal species has a characteristic psychic organ; the central nervous system of each has certain peculiarities of shape, position, and composition. The medusæ, among the radiating cnidaria, have a ring of nervous matter at the border of the fringe, generally provided with four or eight ganglia. The mouth of the five-rayed cnidarion is girt with a nerve-ring, from which proceed five branches. The bi-symmetrical platodes and the vermalia have a vertical brain, or acroganglion, composed of two dorsal ganglia, lying above the mouth; from these “upper ganglia” two branch nerves proceed to the skin and the muscles. In some of the vermalia and in the mollusca a pair of ventral “lower ganglia” are added, which are connected with the former by a ring round the gullet. This ring is found also in the articulata; but in these it is continued on the belly side of the long body as a ventral medulla, a double fibre like a rope-ladder, which expands into a double ganglion in each member. The vertebrates have an entirely different formation of the psychic organ; they have always a spinal medulla developed at the back of the body; and from an expansion of its fore part there arises subsequently the characteristic vesicular brain.[20]
Although the psychic organs of the higher species of animals differ very materially in position, form, and composition, nevertheless comparative anatomy is in a position to prove a common origin for most of them—namely, from the vertical brain of the platodes and vermalia; they have all, moreover, had their origin in the outermost layer of the embryo, the ectoderm, or outer skin-layer. Hence we find the same typical structure in all varieties of the central nervous organ—a combination of ganglionic cells, or “psychic cells” (the real active elementary organs of the soul), and of nerve-fibres, which effect the connection and transmission of the action.