piece of cell-machinery and the highest product of organic evolution. Millions of highly differentiated phronetal cells form the several stations of this telegraphic system, and thousands of millions of the finest nerve-fibrils represent the wires which connect the stations with one another and with the sense-centres on the one hand, and with the motor-centres on the other. Comparative anatomy, moreover, acquaints us with the long and gradual development which the phronema has undergone within the higher class of the vertebrates, from the amphibia and reptiles up to the birds and mammals, and, within the last class, from the monotremes and marsupials up to the apes and men. The human brain seems to us to-day to be the greatest marvel that plasm, or the "living substance," has produced in the course of millions of years.
The remarkable progress which has been made in the last few decades in the anatomic and histological investigation of the brain does not yet, it is true, enable us to make a clear delimitation of the region of the phronema and its relations to the neighboring sensory and motor spheres in the cortex. We must, in fact, assume that there is no sharp distinction in the lower stages of the vertebrate soul; in the older and phylogenetically more distant stages they were not yet differentiated. Even now there are still intermediaries between the æsthetal and phronetal cells. But we may expect with confidence that further progress in the comparative anatomy of the brain will, with the aid of embryology, throw more and more light on these complicated structures. In any case, the fundamental fact is now empirically established that the phronema (the real organ of the soul) forms a definite part of the cortex of the brain, and that without it there can be no reason, no mental life, no thought, and no knowledge.
Since we regard psychology as a branch of physiology, and examine the whole of the phenomena of mental life from the same monistic stand-point as all other vital functions, it follows that we can make no exception for knowledge and reason. In this we are diametrically opposed to the current systems of psychology, which regard psychology, not as a natural science, but as a mental science. In the next chapter we shall see that this position is wholly unjustified. Unfortunately, this dualistic attitude is shared by a number of distinguished modern physiologists, who otherwise adopt the monistic principles; they take the soul to be, in the Cartesian sense, a supernatural entity. Descartes—a pupil of the Jesuits—only applied his theory to man, and regarded animals as soulless automata. But the theory is quite absurd in modern physiologists, who know from innumerable observations and experiments that the brain, or psychic organ, in man behaves just as it does in the other mammals, and especially the primates. This paradoxical dualism of some of our modern physiologists may be partly explained by the perverse theory of knowledge which the great authority of Kant, Hegel, etc., has imposed on them; and partly by a concern for the current belief in immortality, and the dread of being decried as "materialists" if they abandon it. As I do not share this belief, I examine and appreciate the physiological work of the phroneta just as impartially as I deal with the organs of sense or the muscles. I find that the one is just as much subject as the other to the law of substance. Hence we must regard the chemical processes in the ganglionic cells of the cortex as the real factors of knowledge and all other psychic action. The chemistry of the neuroplasm determines the vital function of the phronema. The same must be said of its most perfect and enigmatic function, consciousness. Although this greatest wonder of life is only directly accessible by the introspective method, or by the mirroring of knowledge in knowledge, nevertheless the use of the comparative method in psychology leads us to believe confidently that the lofty self-consciousness of man differs only in degree, and not in kind, from that of the ape, dog, horse, and other higher mammals.
Our monistic conception of the nature and seat of the soul is strongly confirmed by psychiatry, or the science of mental disease. As an old medical maxim runs, Pathologia physiologiam illustrat—the science of disease throws light on the sound organism. This maxim is especially applicable to mental diseases, for they can all be traced to modifications of parts of the brain which discharge definite functions in the normal state. The localization of the disease in a definite part of the phronema diminishes or extinguishes the normal mental function which is discharged by this section. Thus disease of the speech-centre, in the third frontal convolution, destroys the power of speech; the destruction of the visual region (in the occipital convolutions) does away with the power of sight; the lesion of the temporal convolutions destroys hearing. Nature herself here conducts delicate experiments which the physiologist could only accomplish very imperfectly or not at all. And although we have in this way only succeeded as yet in showing the functional dependence of a certain part of the mental functions on the respective parts of the cerebrum, no unprejudiced physician doubts to-day that it is equally true of the other parts. Each special mental activity is determined by the normal constitution of the relevant part of the brain, a section of the phronema. Very striking examples of this are afforded in the case of idiots and microcephali, the unfortunate beings whose cerebrum is more or less stunted, and who have accordingly to remain throughout life at a low stage of mental capacity. These poor creatures would be in a very pitiable condition if they had a clear consciousness of it, but that is not the case. They are like vertebrates from which the cerebrum has been partly or wholly removed in the laboratory. These may live for a long time, be artificially fed, and execute automatic or reflex (and in part purposive) motions, without our perceiving a trace of consciousness, reason, or other mental function in them.
The embryology of the child-soul has been known in a general way for thousands of years, and has been an object of keen interest to all observant parents and teachers; but it was not until about twenty years ago that a strictly scientific study was made of this remarkable and important phenomenon. In 1884 Kussmaul published his Untersuchungen über das Seelenleben des neugeborenen Menschen, and in 1882 W. Preyer published his Mind of the Child [English translation; Dr. J. Sully has several works on the same subject]. From the careful manuals which these and other observers have published, it is clear that the new-born infant not only has no reason or consciousness, but is also deaf, and only gradually develops its sense and thought-centres. It is only by gradual contact with the outer world that these functions successively appear, such as speech, laughing, etc.; later still come the power of association, the forming of concepts and words, etc. Recent anatomic observations quite accord with these physiological facts. Taken together, they convince us that the phronema is undeveloped in the new-born infant; and so we can no more speak in this case of a "seat of the soul" than of a "human spirit" as a centre of thought, knowledge, and consciousness. Hence the destruction of abnormal new-born infants—as the Spartans practised it, for instance, in selecting the bravest—cannot rationally be classed as "murder," as is done in even modern legal works. We ought rather to look upon it as a practice of advantage both to the infants destroyed and to the community. As the whole course of embryology is, according to our biogenetic law, an abbreviated repetition of the history of the race, we must say the same of psychogenesis, or the development of the "soul" and its organ—the phronema.
Comparative psychology comes next in importance to embryology as a means of studying the ancestral history of the soul. Within the ranks of the vertebrates we find to-day a long series of evolutionary stages which reach up from the lowest acrania and cyclostoma to the fishes and dipneusta, from these to the amphibia, and from these again to the amniota. Among the latter, moreover, the various orders of reptiles and birds on the one hand, and of mammals on the other, show us how the higher psychic powers have been developed step by step from the lower. To this physiological scale corresponds exactly the morphological gradation revealed by the comparative anatomy of the brain. The most interesting and important part of this is that which relates to the highest developed class—the mammals; within this class we find the same ever-advancing gradation. At its summit are the primates (man, the apes, and the half-apes), then the carnivora, a part of the ungulates, and the other placentals. A wide interval seems to separate these intelligent mammals from the lower placentals, the marsupials and monotremes. We do not find in the latter the high quantitative and qualitative development of the phronema which we have in the former; yet we find every intermediate stage between the two. The gradual development of the cerebrum and its chief part—the phronema—took place during the Tertiary period, the duration of which is estimated by many recent geologists at from twelve to fifteen (at the least three to five) million years.
As I have gone somewhat fully, in chapters vi.-ix. of the Riddle, into the chief results of the modern study of the brain and