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.