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 (Coenopsyche): 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 coenobia 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.

II. Multicellular protozoa with a communal soul: the catallacta.

III. The earliest metazoa with an epithelial soul: the platodes.

IV. Invertebrate ancestors with a simple vertical brain: the vermalia.

V. Vertebrates without skull or brain, with a simple spinal cord: the acrania.

VI. Animals with skull and brain (of five vesicles): the craniota.

VII. Mammals with predominant development of the cortex of the brain: the placentals.

VIII. The higher anthropoid apes and man, with organs of thought (in the cerebrum): the anthropomorpha.

Among these eight stages in the development of the human soul we may further distinguish more or less clearly a number of subordinate stages. Naturally, however, in reconstructing them we have to fall back on the same defective evidence of empirical psychology which the comparative anatomy and physiology of the actual fauna affords us. As the craniote animals of the sixth stage--and these are true fishes--are already found fossilized in the Silurian system, we are forced to assume that the five preceding series of ancestors (which were incapable of fossilization) were evolved in an earlier, pre-Silurian age.

I. _The cell-soul_ (_or cytopsyche_): first stage of phyletic psychogenesis.--The earliest ancestors of man and all other animals were unicellular protozoa. This fundamental hypothesis of rational phylogeny is based, in virtue of the phylogenetic law, on the familiar embryological fact that every man, like every other metazoon (_i.e._, every multicellular organism with tissues), begins his personal existence as a simple cell, the stem-cell (_cytula_), or the impregnated egg-cell (see p. 63). As this cell has a "soul" from the commencement, so had also the corresponding unicellular _ancestral forms_, which were represented in the oldest series of man's ancestors by a number of different protozoa.

We learn the character of the psychic activity of these unicellular organisms from the comparative physiology of the protists of to-day. Close observation and careful experiment have opened out to us in this respect, in the second half of the nineteenth century, a new world of the most interesting phenomena. The best description of them was given by Max Verworn in his thoughtful work, based on original research, _Psycho-physiological Studies of the Protists_. The work includes also the few earlier observations of the "psychic life of the protist." Verworn came to the firm conclusion that the psychic processes are unconscious in all the protists, that the phenomena of sensation and movement coincide with the molecular vital processes in their protoplasm, and that their ultimate causes are to be sought in the properties of the protoplasmic molecules (the _plastidules_). "Hence the psychic phenomena of the protists form a bridge that connects the chemical processes of the inorganic world with the psychic life of the highest animals; they represent the germ of the highest psychic phenomena of the metazoa and of man."

The careful observations and many experiments of Verworn, together with those of Wilhelm Engelmann, Wilhelm Preyer, Richard Hertwig, and other more recent students of the protists, afford conclusive evidence for my "theory of the cell-soul" (1866). On the strength of several years of study of different kinds of protists, especially rhizopods and infusoria, I published a theory thirty-three years ago to the effect that every living cell has psychic properties, and that the psychic life of the multicellular animals and plants is merely the sum total of the psychic functions of the cells which build up their structure. In the lower groups (in algæ and sponges, for instance) _all_ the cells of the body have an equal share in it (or with very slight differences); in the higher groups, in harmony with the law of the "division of labor," only a select portion of them are involved--the "soul-cells." The important consequences of this "cellular psychology" were partly treated in my work on _The Perigenesis of the Plastidule_ (1876), and partly in my speech at Munich, in 1877, on "Modern Evolution in Relation to the Whole of Science." A more popular presentation of them is to be found in my two Vienna papers (1878) on "The Origin and Development of the Sense-Organs" and on "Cell-Souls and Soul-Cells."

Moreover, the cell-soul, even within the limits of the protist world, presents a long series of stages of development, from the most simple and primitive to a comparatively elaborate activity. In the earliest and simplest protists the faculty of sensation and movement is equally distributed over the entire protoplasm of the homogeneous morsel; in the higher forms certain "cell-instruments," or _organella_, appear, as their physiological organs. Motor cell-parts of that character are found in the pseudopodia of the rhizopods, and the vibrating hairs, lashes, or cilia of the infusoria. The cell-nucleus, which is wanting in the earlier and lower protists, is considered to be an internal central organ of the cell-life. It is especially noteworthy, from a physiologico-chemical point of view, that the very earliest protists were plasmodomous, with plant-like nutrition--hence _protophyta_, or primitive plants; from these came as a secondary stage, by metasitism, the first plasmophagi, with animal nutrition--the _protozoa_, or primitive animals.[18] This metasitism, or circulation of nutritive matter, implies an important psychological advance; with it began the development of those characteristic properties of the animal soul which are wanting in the plant.

We find the highest development of the animal cell-soul in the class of ciliata, or ciliated infusoria. When we compare their activity with the corresponding psychic life of the higher, multicellular animals, we find scarcely any psychological difference; the sensitive and motor _organella_ of these protozoa seem to accomplish the same as the sense-organs, nerves, and muscles of the metazoa. Indeed, we have found in the great cell-nucleus (_meganucleus_) of the infusoria a central organ of psychic activity, which plays much the same part in their unicellular organism as the brain does in the psychic life of higher animals. However, it is very difficult to determine how far this comparison is justified; the views of experts diverge considerably over the matter. Some take all spontaneous bodily movement in them to be automatic, or impulsive, and all stimulated movement to be reflex; others are convinced that such movements are partly voluntary and intentional. The latter would attribute to the infusoria a certain degree of consciousness, and even self-consciousness; but this is rejected by the others. However that very difficult question may be settled, it does not alter the fact that these unicellular protozoa give proof of the possession of a highly developed "cell-soul," which is of great interest for a correct decision as to the _psyche_ of our earliest unicellular ancestors.

II. _The communal or cenobitic soul_ (_coenopsyche_): second stage of phyletic psychogenesis.--Individual development begins, in man and in all other multicellular animals, with the repeated segmentation of one simple cell. This _stem-cell_, the impregnated ovum, divides first into two daughter cells, by a process of ordinary indirect segmentation; as the process is repeated there arise (by equal division of the egg) successively four, eight, sixteen, thirty-two, sixty-four such new cells, or "blastomeres." Usually (that is, in the case of the majority of animals) an irregular enlargement sooner or later takes the place of this original regular division of cells. But the result is the same in all cases--the formation of a (generally spherical) cluster of heterogeneous (originally homogeneous) cells. This stage is called the _morula_ ("mulberry," which it somewhat resembles in shape). Then, as a rule, a fluid gathers in the interior of this aggregate of cells; it changes into a spherical vesicle; all the cells go to its surface, and arrange themselves in one simple layer--the _blastoderm_. The hollow sphere which is thus formed is the important stage of the "germinal vesicle," the _blastula_, or blastosphere.

The psychological phenomena which we directly observe in the formation of the blastula are partly sensations, partly movements, of this community of cells. The movements may be divided into two groups: (1) the inner movements, which are always repeated in substantially the same manner in the process of ordinary (indirect) segmentation of cells (formation of the axis of the nucleus, mitosis, karyokinesis, etc.); (2) the outer movements, which are seen in the regular change of position of the social cells and their grouping for the construction of the blastoderm. We assume that these movements are hereditary and unconscious, because they are always determined in the same fashion by heredity from the earlier protist ancestors. The sensations also fall into two groups: (1) the sensations of the individual cells, which reveal themselves in the assertion of their individual independence and their relation to neighboring cells (with which they are in contact, and partly in direct combination, by means of protoplasmic fibres); (2) the common sensation of the entire community of cells, which is seen in the individual formation of the _blastula_ as a hollow vesicle.

The causal interpretation of the formation of the blastula is given us by the biogenetic law, which explains the phenomena we directly observe to be the outcome of heredity, and relates them to corresponding historical processes which took place long ago in the origin of the earliest protist-coenobia, the blastæads. But we get a physiological and psychological insight into these important phenomena of the earliest cell-communities by observation and experiment on their modern representatives. Such permanent cell-communities or colonies are still found in great numbers both among the plasmodomous primitive plants (for instance, the paulotomacea, diatomacea, volvocinæ, etc.) and the plasmophagous primitive animals (the infusoria and rhizopods). In all these coenobia we can easily distinguish two different grades of psychic activity: (1) the cell-soul of the individual cells (the "elementary organisms") and (2) the communal soul of the entire colony.

III. _The tissue-soul_ (_histopsyche_): third stage of phyletic psychogenesis.--In all multicellular, tissue-forming plants (_metaphyta_) and in the lowest, nerveless classes of tissue-forming animals (_metazoa_) we have to distinguish two different forms of psychic activity--namely: (1) the _psyche_ of the individual cells which compose the tissue, and (2) the _psyche_ of the tissue itself, or of the "cell-state" which is made up of the tissues. This "tissue-soul" is the higher psychological function which gives physiological individuality to the compound multicellular organism as a true "cell-commonwealth." It controls all the separate "cell-souls" of the social cells--the mutually dependent "citizens" which constitute the community. This fundamental twofold character of the _psyche_ in the metaphyta and the lower, nerveless metazoa is very important. It may be verified by unprejudiced observation and suitable experiment. In the first place, each single cell has its own sensation and movement, and, in addition, each tissue and each organ, composed of a number of homogeneous cells, has its special irritability and psychic unity (_e.g._, the pollen and stamens).

A. _The plant-soul_ (_phytopsyche_) is, in our view, the summary of the entire psychic activity of the tissue-forming, multicellular plant (the _metaphyton_, as distinct from the unicellular _protophyton_); it is, however, the subject of the most diverse opinions even at the present day. It was once customary to draw an essential distinction between the plant and the animal, on the ground that the latter had a "soul" and the plant had none. However, an unprejudiced comparison of the irritability and movements of various higher plants and lower animals convinced many observers, even at the beginning of the century, that there must be a "soul" on both sides. At a later date Fechner, Leitgeb, and others strongly contended for the plant-soul. But a profounder knowledge of the subject was obtained when the similarity of the elementary structure of the plant and of the animal was proved by the cellular theory, and especially when the similarity of conduct of the active, living protoplasm in both was shown in the plasma theory of Max Schultze (1859). Modern comparative physiology has shown that the physiological attitude towards various stimuli (light, heat, electricity, gravity, friction, chemical action, etc.) of the "sensitive" portions of many plants and animals is exactly the same, and that the reflex movements which the stimuli elicit take place in precisely the same manner on both sides. Hence, if it was necessary to attribute this activity to a "soul" in the lower, nerveless metazoa (sponges, polyps, etc.), it was also necessary in the case of many (if not all) metaphyta, at least in the very sensitive _mimosa_, the "fly-traps" (_dionaea_ and _drosera_), and the numerous kinds of climbing plants.

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 coelenterates: (_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 coelenterates, 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 cryptocoela--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 coelenterates 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.

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 foetus; 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.

II. The brain development of the lowest and earliest mammals (the monotremes, marsupials, and prochoriates) is closely allied to that of their palæozoic ancestors, the Carboniferous amphibia (the _stegocephala_) and the Permian reptiles (the _tocosauria_).

III. During the Tertiary period commences the typical development of the cerebrum, which distinguishes the younger mammals so strikingly from the older.

IV. The special development (quantitatively and qualitatively) of the cerebrum which is so prominent a feature in man, and which is the root of his pre-eminent psychic achievements, is only found, outside humanity, in a small section of the most highly developed mammals of the earlier Tertiary epoch, especially in the anthropoid apes.

V. The differences of brain structure and psychic faculty which separate man from the anthropoid ape are slighter than the corresponding interval between the anthropoid apes and the lower primates (the earliest simiæ and prosimiæ).

VI. Consequently, the historical, gradual evolution of the human soul from a long chain of higher and lower mammal souls must, by application of the universally valid phyletic laws of the theory of descent, be regarded as a _fact_ which has been scientifically proved.