SENSATION

SENSATIONSensation and consciousness—Unconscious and conscious sensation—Sensibility and irritability—Reflex sensation and perception of stimuli—Sensation and living force—Reaction to stimuli—Resolution of stimuli—External and internal stimuli—Conveyance of stimuli—Sensation and striving—Sensation and feeling—Inorganic and organic sensation—Light sensation, phototaxis, sight—Sensation of warmth, thermotaxis—Sensation of matter, chemotaxis—Taste and smell—Erotic chemicotropism—Organic sensations—Sensation of pressure—Geotaxis—Sensation of sound—Electric sensation.Sensation is one of those general terms that have at all times been liable to the most varied interpretations. Like the cognate idea of the "soul," it is still extremely ambiguous. During the eighteenth century it was generally believed that the function of sensation was peculiar to animals, and was not present in plants. This opinion found its most important expression in the well-known principle in Linné'sSystema Naturæ: "Stones grow: plants grow and live: animals grow, live, and feel." Albrecht Haller, who gathered up all the knowledge of his time relating to organic life in hisElementa Physiologiæ(1766), distinguished as its two chief characters "sensibility" and "irritability." The one he ascribed exclusively to the nerves, and the other to the muscles. This erroneous idea was subsequently refuted, and in our own time irritability is conceived to be a general property of all living matter.The great advance made by the comparative anatomy and experimental physiology of animals and plants in the first half of the nineteenth century brought to light the fact that irritability or sensibility is a common quality of all organisms, and that it is one of the principal characteristics of vital force (cf.chapter ii.). The greatest merit in connection with its experimental study attaches to the famous Johannes Müller. In his classicalManual of Human Physiology(1840) he established his theory of the specific energy of the nerves and their dependence on the sense-organs on the one hand and the mental life on the other. He devoted the fifth chapter of his book to the former and the sixth to the latter, approaching particularly to Spinoza in his general psychological views; he treated psychology as a part of physiology, and thus put on a sound scientific basis that naturalistic conception of the place of psychology in the biological system which we now regard as the correct view. At the same time he proved that sensation is a function of the organism as much as movement or nutrition.The view of sensation that prevailed in the second half of the nineteenth century was very different. On the one hand the experimental and comparative physiology of the sense-organs and the nervous system immensely enriched our exact knowledge by the invention of ingenious methods of research and the use of the great advance made by physics and chemistry. The famous investigations of Helmholtz and Hertwig on the physics of the senses, of Matteucci and Dubois-Reymond on the electricity of the muscles and nerves, and the great progress made in vegetal physiology by Sachs and Pfeffer, and in physiological chemistry by Moleschott and Bunge, enabled us to realize that even the most mysterious of the wonders of life depend on physical and chemical processes. By the application of the differentstimuli—light, heat, electricity, and chemical action—to the various sensitive or irritable organs under definitely controlled conditions, scientists succeeded in subjecting with exactness a great part of the phenomena of stimulation to mathematical measurements and formulæ. The science of the stimuli and their effects acquired a strictly physical character.On the other hand, in most striking contradiction to the immense advance of experimental physiology, we see that the general conception of the various vital processes, and especially of the inner nerve-action that converts the functions of the senses into mental life, is most curiously neglected. Even the fundamental idea of sensation, which plays the chief part in it, is disregarded more and more. In many of the most valuable modern manuals of physiology, containing long chapters on stimuli and stimulation, there is little or no mention of sensation as such. This is chiefly due to the mischievous and unjustifiable gulf that has once more been artificially created between physiology and psychology. As the "exact" physiologists found the study of the inner psychic processes which take place in sense-action and sensation inconvenient and unprofitable, they gladly handed over this difficult and obscure field to the "psychologists proper"—in other words, to the metaphysicians, who had for the starting-point of their airy speculations the belief in an immortal soul and divine consciousness. The psychologists readily abandoned the inconvenient burden of experience anda posterioriknowledge, to which the modern anatomic physiology of the brain laid special claim.The greatest and most fatal error committed by modern physiology in this was the admission of the baseless dogma that all sensation must be accompanied by consciousness. As most physiologists share the view of Dubois-Reymond, that consciousness is not a naturalphenomenon, but a hyperphysical problem, they leave it and this inconvenient "sensation" outside the range of their researches. This decision is, naturally, very agreeable to the prevalent metaphysics; it has just as much interest in the transcendental character of sensation as in the liberty of the will, and thus the whole of psychology passes from the empirical province of natural science into the mystical province of mental science. For its foundation they then take the "critical theory of knowledge," which ignores the results of the real physiological organs—the senses, nerves, and brain—and draws its "superior wisdom" from the inner mirroring of self by the introspective analysis of presentations and their associations. It is extraordinary that even distinguished monistic physiologists suffer themselves to be taken in with this sort of metaphysical jugglery, and dismiss the whole of psychology from their province; their psychomonism readmits the soul as a supernatural entity, and delivers it, in contrast with the "world of bodies," from the yoke of the law of substance.Impartial reflection on our personal experience during sensation and consciousness will soon convince us that these are two different physiological functions, which are by no means necessarily associated; and the same may be said of the third principal function of the soul—the will. When we learn an art—for instance, painting or playing the piano—we need months of daily practice in order to become expert at it. In this we experience every day hundreds of thousands of sensations and movements which are learned and repeated with full consciousness. The longer we continue the practice and the more we adapt and accustom ourselves to the function, the easier and less conscious it becomes. And when we have practised the art for some years, we paint our picture or play our piano unconsciously; we thinkno longer of all the small, subtle shades of sensation and acts of will which were necessary in learning. The mere impulse of the will to paint the picture once more or play the piece again suffices to release the whole chain of complicated movements and accompanying sensations which had originally to be learned slowly, laboriously, and with full consciousness. An experienced pianist plays the most difficult piece—if he has learned it and repeated it thousands of times—"half in a dream." But it needs only a slight accident, such as a mistake or a sudden interruption, to bring back the wandering attention to the work. The piece is now played with clear consciousness. The same may be said of thousands of sensations and movements which we learned at first consciously in childhood, and then repeat daily afterwards without noticing—such as in walking, eating, speaking, and so on. These familiar facts prove of themselves that consciousness is a complicated function of the brain, by no means necessarily connected with sensation or will. To bind up the ideas of consciousness and sensation inseparably is the more absurd, as the mechanism or the real nature of consciousness seems very obscure to us, while the idea of it is perfectly clear: we know that we know, feel, and will.The word "irritability" is generally taken by modern physiology to mean that the living matter has the property of reacting on stimuli—that is to say, of responding by changes in itself to changes in its environment. The stimulus, or action of a foreign energy, must, however, be felt by the plasm before the corresponding stimulated movement (in the form of various manifestations of energy) will be produced. Hence the question whether this sensation is (in certain cases) associated with consciousness or (generally) remains unconscious is of a subordinate interest. The plant that is caused to open its floral calyx by thestimulus of light acts just as unconsciously in this as the coral that spreads out its crown of tentacles under the same influence; and when the sensitive carnivorous plant (dionæaordrosera) closes its leaves in order to catch and destroy the insect sitting on them, it acts in the same way as the sensitive actinia or coral when it draws in its crown of tentacles for the same object—in both cases without consciousness! We call these unconscious movements "reflex actions." I have dealt somewhat fully with these reflex movements in the seventh chapter of theRiddle, and must refer the reader thereto. This elementary psychic function always depends on a conjunction of sensation and movement (in the widest sense). The movement that the stimulus provokes is always preceded by a sensation of the influence exerted.Modern physiology makes desperate efforts to avoid the use of the word "sensation" and substitute for it "perception of stimulus." The chief blame for this misleading expression is due to the arbitrary and unjustified separation of psychology from physiology. The latter is supposed to occupy itself with the material phenomena and physical changes, leaving to psychology the privilege of dealing with the higher mental phenomena and metaphysical problems. As we reject this distinction altogether on monistic principles, we cannot consent to separate sensation from the perception of stimuli—whether this sensation be accompanied with consciousness or not. Moreover, modern physiology, in spite of its desire to keep clear of psychology, sees itself compelled in a thousand ways to use the words "sensation" and "sensitive," especially in the science of the organs of sense.What we call sensation or perception of stimuli may be regarded as a special form of the living force or actual energy (Ostwald). Sensitiveness or irritability, on the other hand, is a form of virtual or potentialenergy. The living substance at rest, which is sensitive or irritable, is in a state of equilibrium and indifference to its environment. But the active plasm, that receives and feels a stimulus, has its equilibrium disturbed, and corresponds to the change in its environment and its internal condition. This response of the organism to a stimulus is called "reaction"—a term that is also used (in the same sense) in chemistry to express the interaction of bodies on each other. At each stimulation the virtual energy of the plasm (sensitiveness) is converted into living or kinetic force (sensation). The share of the stimulus in this conversion is described as a "release" of energy.The term "reaction" stands in general for the change which any body experiences from the action of another body. Thus, for instance, to take the simplest case, the interaction of two substances in chemistry is called a reaction. In chemical analysis the word is used in a narrower sense to denote that action of one body on another which serves to reveal its nature. Even here we must assume that the two bodies feel their different characters; otherwise they could not act on each other. Hence every chemist speaks of a more or less "sensitive reaction." But this process is not different in principle from the reaction of the living organism to outer stimuli, whatever be their chemical or physical nature. And there is no more essential difference in psychological reaction, which is always bound up with corresponding changes in the psychoplasm, and so with a chemical conversion of energy. In this case, however, the process of reaction is much more complicated, and we can distinguish several parts or phases of it: 1, the outer excitation; 2, the reaction of the sense-organ; 3, the conducting of the modified impression to the central organ; 4, the internal sensation of the conducted impression; and, 5, consciousness of the impression.The important idea of a release of energy—the term we give to the effect of the stimulus—is also used in physics. If we put a piece of burning wood in a barrel of powder, the flame causes an explosion. In the case of dynamite a simple mechanical shock is enough to produce the most enormous expenditure of force in the explosive matter. When we discharge a bow the slight pressure of the finger on the tense cord suffices to send out the arrow or bolt on its deadly mission. So also a sound or a ray of light that strikes the ear or eye suffices to bring about a number of complex effects by means of the nervous system. In the fertilization of the ovum by the male sperm the chemical conjunction of the two formative principles is sufficient to cause the growth of a new human being out of the microscopic plasma-globule, the stem-cell (cytula). In these and thousands of other reactions a very slight shock suffices to provoke the largest effects in the stimulated substance. This shock, which we call a release of energy, is not the direct cause of the considerable result, but merely the occasion for bringing it about. In these cases we have always a vast accumulation of virtual energy converted into living force or work. The magnitude of the two forces has no relation at all to the smallness of the shock which led to the conversion. In this we have the difference between stimulated action and the simple mechanical action of two bodies on each other, in which the quantity of the energy expended is equal on both sides, and there is no stimulus.The immediate effect of a stimulus on living matter can best be followed in external physical or chemical stimuli, such as light, heat, pressure, sound, electricity, and chemical action. In these cases physical science is often able to reduce the life-process to the laws of inorganic nature. This is more difficult with the internal stimuli within the organism itself, which are only partlyexposed to physiological investigation. It is true that here also the task of science is to reduce all the biological phenomena to physical and chemical laws. But it can only discharge a part of this difficult task, as the phenomena are too complicated, and their conditions too little known in detail, to say nothing of the crudeness and imperfectness of our methods of research. Yet, in spite of all this, comparative and phylogenetic physiology convinces us that even the most complicated of our internal excitations, and particularly the mental activity of the brain, depend just as much as the outer stimulations on physical processes, and are equally subject to the law of substance. This is, in fact, true of reason and consciousness.In man and all the higher animals the stimuli are received by the organs of sense and conducted by their nerves to the central organ. In the brain they are either converted into specific sensations in the sense-centres, or conveyed to the motor region, where they provoke movements. The conduction of stimuli is simpler in the lower animals and the plants; the tissue-cells either directly affect each other or are connected by fine threads of plasm. In the unicellular protists the stimulus which strikes one particular spot of the surface may be immediately communicated to the other parts of the unified plasmic body.We shall see in the course of our inquiry that the simplest form of sensation (in the widest sense) is common to inorganic and organic bodies, and thus that sensitiveness is really a fundamental property of all matter, or, more correctly, all substance. We may, therefore, ascribe sensation to the constituent atoms of matter. This fundamental thought of hylozoism, expressed long ago by Empedocles, has lately been very definitely urged, especially by Fechner. However, the able founder of psychophysics (cf.theRiddle, p. 35)assumes that consciousness (or thought, in the Spinozistic sense) always accompanies this universal property of sensation. In my opinion, consciousness is a secondary psychic function, only found in man and the higher animals, and bound up with the centralization of the nervous system. Hence it is better to speak of the unconscious sensation of the atoms asfeeling(æsthesis), and their unconscious will asinclination(tropesis). It finds expression in the one-sided action of a stimulus as a "directed movement" or "stimulated movement" (tropismusortaxis).The familiar ideas of sensation and feeling are often confused, and employed in very different ways in both physiology and psychology. The metaphysical tendency which so completely separates the two sciences, and the physiological tendency which agrees with it, regard feeling as a purely psychic or spiritual function, whereas in the case of sensation they have to admit the connection with bodily functions, especially sense-action. In my opinion, the two ideas are purely physiological and cannot be sharply separated, or only in the sense that sensation relates more to the external (objective) part of the sensory nerve-process, and feeling to the internal (subjective) part. Hence we may define the difference in a general way by saying that sensation perceives the different qualities of the stimuli, and feeling only the quantity, the positive or negative action of the stimulus (pleasure or pain). In this last and widest sense we may ascribe the feeling of pleasure and pain (in the contact with qualitatively differing atoms) to all atoms, and so explain the elective affinity in chemistry (synthesis of loving atoms, inclination; analysis of hating atoms, disinclination).Our monistic system (whether it be taken as energism or materialism, or more correctly as hylozoism) regards all substance as having "soul"—that is to say, endowedwith energy. In the chemical analysis of organisms we do not find any elements that are not found in inorganic nature; we find that the movements in organisms obey the same laws of mechanics as the latter; we believe that the conversion of energy in the living matter occurs in the same way, and is provoked by the same stimuli, as in inorganic matter. We are forced to conclude from. these experiences that the perception of stimuli—sensation in the objective and feeling in the subjective sense—is also generally present in the two. All bodies are in a certain sense "sensitive." It is just in this dynamic conception of substance that monism differs essentially from the materialistic system, which regards one part of matter as "dead" and insensitive. In this we have the best means of joining consistent materialism or realism with consistent spiritualism or idealism. But, as a first condition of such a union, we must demand a recognition that organic life is subject to the same general laws as inorganic nature. In both cases the outer world acts alike as a stimulus on the inner world of the body. We can easily see this if we glance at the various kinds of sensation which correspond to the various kinds of stimuli. Light and heat, external and internal chemical stimuli, pressure and electricity, cause analogous sensations and modifications in their effect on organic and inorganic bodies.The effect which the light-stimulus has on living matter, the sensation of light that results, and the chemical changes of energy that follow, are of great physiological importance in all organisms. We might even say that sunlight is the first, oldest, and chief source of organic life; all other exertions of force depend in the long run on the radiant energy of sunlight. The oldest and most important function of plasm—one which is at the same time a cause of its formation—is carbon-assimilation; and this plasmodomism is directly dependenton sunlight. If it acts in a one-sided way, it causes the particular form of stimulation which we call phototaxis or heliotropism. This is of a positive character—that is to say, they turn towards the source of the light—in the great majority of organisms, both protists and histona. Everybody knows that flowers that are growing in the window of a room turn to the light. However, many organisms which have grown accustomed to living in the dark are heliotropically negative; they shun the light and seek darkness, such as the fungi, many lucifugous mosses and ferns, and many deep-sea animals.The principal organs of light-sensation in the higher animals are the eyes; they are wanting in many of the lower animals as well as the plants. The essential difference between the real eye and a part of the skin that is merely sensitive to light is that the eye can form a picture of objects in the outer world. This faculty of vision begins with the formation of a small convergent lens, a biconvex refracting body at a certain spot on the surface. Dark pigment-cells which surround it absorb the light-rays. From this first phylogenetic form of the organ of vision up to the elaborate human eye there is a long scale of evolutionary stages—not less extensive and remarkable than the historical succession of artificial optical instruments from the simple lens to the complicated modern telescope or microscope. This great "wonder of life"—the long scale of the evolution of the eye—has an interesting tearing on many important questions of general physiology and phylogeny. We can, in this case, see clearly how a very complicated and purposive apparatus can arise in a purely mechanical way, without any preconceived design or plan. In other words, we can see how an entirely new function—and one of its principal functions, vision—has arisen in the organism by mechanical means.The advanced vision of the higher animals is made up of a great number of different functions, with a corresponding complexity of detail in the anatomic structure of the eye. No other organ, after the brain, is so necessary as the eye for the multifarious vital activities of the higher animals, and especially for the mental life of civilized man and the progress of art and science. What would the human mind be if we could not read, write, and draw, and have a direct knowledge through the eye of the forms and colors of the outer world? Yet this invaluable structure is only the highest and most perfect stage in the long chain of evolutionary processes which has its starting-point in the general sensitiveness to light, or the photic irritability of plasm. However, we find a number of varieties and grades of this even among the unicellular protists, and, indeed, the very lowest and oldest of the protists, the monera. Various species of both the chromacea and the bacteria are heliotropic to different degrees, and have a fine sensitiveness to the strength of the light stimulus.The stimulating effect which light has on the homogeneous plasm of the monera is also found in a number of inorganic bodies. In these cases the photic stimulus produces partly chemical and partly mechanical changes. Every chemist speaks of substances that are more or less "sensitive" to light; the photographer speaks of his "sensitive plates," the painter of his "sensitive colors." Many chemical compounds are so sensitive to light that they are destroyed at once in sunlight, and so have to be kept in the dark. There is no other word but "sensation" to express the attitude of the atoms towards each other which becomes so conspicuous in these cases under the influence of sunlight. It seems to me that this phenomenon is a clear justification of our hylozoic monism when it affirms that all matter is psychic. In metaphysics sensation is held to be an essential property of the soul.In the same general way as light the heat-stimulus acts on organisms, and causes the sensations, sometimes pleasant and sometimes unpleasant, which we call the subjective feeling of heat, warmth, coolness, or cold. The sense-organ that receives these impressions of temperature is the surface of the unicellular plasmic body in the protists, and the skin (epidermis) that protects the surface from the outer world in the histona. In all living things the temperature of the surrounding medium (water or air) has a great influence in regulating the life-processes; in the stationary animals and plants it is the temperature of the ground to which they are attached. This temperature must always be between the freezing-point and boiling-point of water, as fluid water is indispensable for the imbibition of the living matter and the molecular movements within the plasm. At the same time, some of the lower protists (chromacea, bacteria) can endure very high and very low temperatures, but only for a short time. Some protists (monera and diatomes) can stand a temperature of 200° C. for several days, and others can be heated above boiling-point without being killed. Arctic and High-Alpine plants and animals may be in a frozen condition for several months, yet live again when they are thawed. However, the resistance to these extremes of cold lasts for only a limited time, and in the frozen state all vital functions are at a standstill.In the great majority of living things the vital activity is confined within narrow limits of temperature. Many plants and animals in the tropics which have been accustomed for thousands of years to the constancy of the hot equatorial climate can endure only very restricted variations of temperature. On the other hand, many of the inhabitants of Central Siberia, where the climate is very hot in the short summer and very cold in the long winter, can stand great variations. Thus the livingplasm has experienced considerable changes in its sense of warmth through adaptation to different environments; not only the maximum and the minimum, but the optimum (most agreeable point), is subject to very great variations. This can easily be observed and followed experimentally in the phenomena of thermotaxis or thermotropism—that is to say, the effect that follows from a one-sided action of the heat-stimulus. The organism that falls below the minimum of temperature is said to be stiff with cold, while the organism that rises above the maximum is stiff with heat.The heat-stimulus acts on inorganic as well as organic bodies, like the light-stimulus. The law holds good in both cases that higher temperatures increase sensation, while lower ones paralyze it. There is a minimum, an optimum, and a maximum, for many chemical and physical processes in the inorganic world. As far as the melting effect of water is concerned, freezing is the minimum of the heat stimulus and boiling the maximum. As the various chemical compounds meet in water at very different temperatures, we have an optimum for many substances—that is to say, a degree of warmth which is most favorable to the solution of a given quantity of a solid body in water. On the whole, the law holds for chemical processes that they are accelerated by high temperatures and retarded by low ones (like the human passions!); the former have a stimulating and the latter a benumbing effect. As the action of the various chemical compounds on each other is determined by the nature of the elements and their affinities, we must trace the variations in their conduct towards thermic stimuli to a sensation of temperature in the constituent atoms; increase of temperature stimulates it, while decrease lessens or paralyzes it. Here, again, the simple inorganic processes have a general resemblance to the complicated vital phenomena in the organic body.Since we regard the whole of organic life as, in the ultimate analysis, merely a very elaborate chemical process, we shall quite expect that chemical stimuli are the most important factors in sensation. And this is so in point of fact; from the simplest moneron up to the most highly differentiated cell and on to the flower in the plant and the mental life of man, the vital processes are dominated by chemical forces and conversions of energy, which are set in play by external or internal chemical stimuli. The excitation which they produce is called, in a general way, "sensation of matter" or chemæsthesis; the basis of it is the mutual relation of the chemical elements which we describe as chemical affinity. In this affinity we have the play of attractive forces which lie in the nature of the elements themselves, especially in the peculiar properties of their constituent atoms; and this cannot be explained unless we ascribe unconscious sensation (in the widest sense) to the atoms, an inherent feeling of pleasure and the reverse, which they experience in the contact of other atoms (the "loves and hatreds of the elements" of Empedocles).The numbers of different stimuli that act chemically on the plasm and excite its "sensation of matter" may be divided into two groups—external and internal stimuli. The latter lie within the organism itself, and cause the internal "organic sensations"; the former are in the outer world, and are felt as taste, smell, sex-impulse, etc. In the higher animals special chemical sense-organs have been developed for these chemical stimuli. As these are well known to us from our own human experience, and comparative physiology shows us the same structures in the higher animals, we will deal first with them. In general the same law holds for these external chemical stimuli as for optical and thermic stimuli; we can recognize a maximum limit oftheir action, a minimum below which they fail to stimulate, and an optimum or stage in which their influence is strongest.The important part played in human life by taste and the pleasure associated with it is well known. The careful choice and preparation of savory food—which has become an art in gastronomy and a branch of practical philosophy in gastrosophy—was just as important two thousand years ago with the Greeks and Romans as it is to-day in royal banquets or the Lucullic dinners of millionaires. The excitement that we see associated with this refined combination of rich foods and drinks, and that finds expression in so many speeches and toasts, has its philosophic root in the harmony of gustatory sensations and the varying play of stimuli that the delicate dishes and wines exercise on the organs of taste, the tongue and palate. The microscopic organs of these parts of the mouth are the gustatory papillæ—cup-shaped structures, covered with spindle-shaped "taste-cells," and having a narrow opening into the cavity of the mouth. When sapid matters, drinks and fluid or loose particles of food, touch the taste-cells, they excite the fine terminal branchlets of the gustatory nerve which enters the cells. As we find that there are similar structures in most of the higher animals, and that they also choose their food with some care, we may confidently assume that they have sensations of taste like man. However, no trace of this is found in many of the lower animals; in these cases it is impossible to lay down a line of demarcation between taste and smell.In man and the higher air-breathing vertebrates the seat of the sense of smell is in the nostrils; in man it is especially that part of the mucous lining of the nasal cavity which we call the "olfactory region" (the uppermost part of the nasal dividing wall, the superior and middle meatus). It is necessary for a sensation of smellthat the odorous matter, or olfactory stimuli, be brought in a finely divided condition over the moist olfactory membranes. When they touch the olfactory cells—slender, rod-shaped cells with very fine hairs at the free end—they excite the ends of the olfactory nerve which are connected with the cells.In many animals, especially mammals, the sense of smell has a much more important part in life than it has in man, in whom it is relatively feeble. It is well known that dogs and other carnivora, and even ungulates, have a much keener smell. In these cases the nasal cavity, which is the seat of the sense, is much larger, and the muscles in it are much stronger. The nostrils of the air-breathing vertebrates have been developed from a pair of open nasal depressions in the skin of the fish's head. But in these aquatic vertebrates the chemical action of the olfactory stimuli must be of a different character, like the sensation of taste. The odorous matter is, in these cases, brought into contact with the olfactory membrane in a liquid form (in which condition it is not perceptible to man). In fact, the division between the senses of smell and taste disappears altogether in the lower animals. These two "chemical senses" are closely related, and have a common feature in the direct chemical action of the stimulus on the sensitive part of the skin.A chemical sensation of matter that corresponds completely to the real taste-sensation in the higher animals is found in some of the higher carnivorous plants. The leaves of the sun-dew (drosera rotundifolia) are very sensitive insect-traps, and are armed at the edge with knob-like tentacles, sticky hairs that secrete an acid, flesh-digesting juice. When a solid body (but not a raindrop) touches the surface of the leaf the stimulus acts in such a way on the tentacle heads as to contract the leaf. But the acid fluid which serves fordigestion, and corresponds to the gastric juice in the animal, is only secreted by the corpuscles if the solid foreign body is nitrogenous (flesh or cheese). Hence the leaves of these insectivorous plants taste their meat diet, and distinguish it from other solids, to which they are indifferent. In the broader sense, in fact, we may describe the points of the roots of plants as organs of taste; they plunge into the richer parts of the earth which yield more nourishment, and avoid the poor parts. In unicellular plants and animals the action of chemical stimuli is especially conspicuous when it is one-sided, and provokes definite movements in one particular direction (chemotaxis).The movements of unicellular organisms that are provoked by chemical stimuli and are known as chemotropism (more recently as chemotaxis) are particularly interesting because they show the existence of a chemical sensitiveness, somewhat resembling taste or smell, in the lowest organisms, and even in the homogeneous plasm of the monera. Repeated experiments of Wilhelm Engelmann, Max Verworn, and others, have shown that many bacteria, diatomes, infusoria, rhizopods, and other protists, have a similar sense of taste; they move towards certain acids (for instance, a drop of malic acid) or a bubble of oxygen that lies on one side of the drop of water in which the protists are under the microscope. Many pathogenetic bacteria secrete poisonous substances which are very injurious to the human frame. The active white blood-cells, leucocytes, in the human blood have a special "taste" for these bacteria-poisons, and concentrate in large quantities, by means of their amœboid movements, at those parts of the body where they are secreted. If the leucocytes prove the stronger in their struggle with the bacteria, they destroy them, and in this way they act as sanitary officers in keeping poisonous infection out of our organism. But if thebacteria win the battle, they are transported into other parts of the body by the leucocytes; they distinguish their plasm by taste, and may cause a deadly infection.We have a particularly interesting and important species of chemical irritation in the mutual attraction of the two sex-cells, to which I gave the name of chemotropism thirty years ago, and which I described as the earliest phylogenetic source of sexual love (see theAnthropogeny, chapters vii. and xxix.). The remarkable phenomena of impregnation, the most important of all the processes of sexual generation, consist in the coalescence of the female ovum and the male sperm-cell. This could not take place if the two cells had not a sensation of their respective chemical constitution and disposition for union; they come together under this impulse. This sexual affinity is found at the lowest stages of plant life, in the protophyta and algæ. With these both cells—the smaller male microgameta and the larger female macrogameta—are often mobile, and swim about in order to effect a union. In the higher plants and animals only the small male cell is mobile as a rule, and swims towards the large immobile ovum in order to blend with it. The sensation that impels it is of a chemical nature, allied to taste and smell. This has been proved by the splendid experiments of Pfeffer, who showed that the male ciliated cells of ferns are attracted by malic acid, and those of the mosses by cane-sugar, just in the same way as by the exhalation from the female ovum. Conception depends on exactly the same erotic chemotropism in the fertilization of all the higher organisms.Erotic chemotropism must be regarded as a general sense-function of the sexual cells in all amphigonous organisms, but in the higher organisms special forms of the sex-sense, connected with specific organs, aredeveloped; as the source of sexual love they play a most important part in the life of many of the histona. In man and most of the higher animals these feelings of love are associated with the highest features of psychic life, and have led to the formation of some most remarkable customs, instincts, and passions. Wilhelm Bölsche has given us an admirable selection from this infinitely rich and attractive realm in his famousLife of Love in Nature(1903). It is well known that this sexual sense as we have it in man has been developed from the nearest related mammals, the apes. But while it offers a shameless and repulsive spectacle in many of the apes, it has been greatly ennobled and refined in man in the development of civilization. However, the sexual sense-organs and their specific energy have remained the same. In the vertebrates and the articulates and many other metazoa the copulative organs are equipped with special cell-forms (voluptuous particles), which are the seat of intensely pleasurable feelings (see theAnthropogeny, chapter xxix., plate 30). The pubic hairs which clothe themons Venerisare also delicate organs of the sex-sense, and so are the tactile hairs about the mouth. In these cases the correlation between the sensitive forms of energy in the copulative organs and the psychic functions of the central nervous system has been remarkably developed. Moreover, a large part of the rest of the skin may co-operate as a secondary organ of the sex-sense, as is seen in the effect of caressing, stroking, embracing, kissing, etc. Goethe, at once the greatest lyric poet and the subtlest and profoundest monistic philosopher of Germany, has given unrivalled expression to this sensual, yet supersensual, basis of sexual love. Ontogeny teaches unmistakably that its elementary organs, the epidermic cells, develop entirely from the ectoderm.By "organic sensations" modern physiology understands the perception of certain internal bodily states,which are mostly brought about by chemical stimuli (to a small extent by mechanical and other irritation) in the organs themselves. As subjective feelings of the organism itself these states are most aptly called "feelings"—the positive states, pleasure, comfort, delight; the negative, discomfort, pain, etc. These organic sensations (also called common sensations or feelings) are of great importance for the self-regulation of the complicated organism. To the positive organic sensations belong not only the bodily feeling of satiety, repose, or comfort, but also the psychic feelings of joy, good humor, mental rest, etc. Among negative common feelings we have not only hunger and thirst, bodily fatigue, bodily pain, sea-sickness, etc., but also mental strain, vertigo, bad humor, and so on. Between the two groups we have the third category of neutral organic sensations, which involve neither pleasure nor pain, but merely the perception of certain internal conditions, such as muscular strain (in lifting heavy objects), the disposal of the limbs (in crossing the legs), and so on.Chemical sensation is just as general and important in organic nature as in the life of organisms. In this case it is nothing less than the basis of chemical affinity. No chemical process can be thoroughly understood unless we attribute a mutual sensation to the atoms, and explain their combination as due to a feeling of pleasure and their separation to a feeling of displeasure. The great Empedocles (fifth centuryB.C.) explained the origin of all things long ago by the various combination of pure elements, the interaction of love (attraction) and hate (repulsion). This attraction or repulsion is, of course, unconscious, just as in the instincts of plants and animals. If one prefers to avoid the term "sensation," it may be called "feeling" (æsthesis), while the (involuntary) movement it provokes may be called "inclination" (tropesis), and the capacity for the latter "tropism"(more recentlytaxis,cf.chapter xii. of theRiddle). We may illustrate it from the simplest case of chemical combination. When we rub together sulphur and mercury, two totally different elements, the atoms of the finely divided matter combine and form a third and different chemical body, cinnabar. How would this simple synthesis be possible unless the two elementsfeeleach other, move towards each other, andthenunite?We find universally distributed in nature the sensation of the mechanical stimulus of gravitation, the most comprehensive statement of which is given in Newton's law of gravity. According to this fundamental and all-ruling law, any two particles of matter are attracted in direct proportion to their mass and inverse proportion to the square of their distance. This form of attraction, also, can be traced to a "sensation of matter" in the mutually attracting atoms. The local sensation that any body provokes by contact with the surface of an organism is felt as pressure (baros). A stimulus that causes this pressure alone brings about a counter-pressure as a reaction, and an effort to neutralize it, the pressure-movement (barotaxisorbarotropism). Sensitiveness to pressure or the contact of solid bodies is found throughout the organic world; it can be proved experimentally among the protists as well as the histona. Special sense-organs have been developed in the skin of the higher animals as the instruments of this pressure-sense (baræsthesis) in the form of tactile corpuscles; they are most numerous at the finger-tips and other particularly sensitive parts. In many of the higher animals there is a fine sense of touch in the feelers or tentacles, or (in the higher articulates) in the horns or antennæ. Moreover, these tactile and prehensile organs are also very widely found among the higher plants, especially the climbing plants (vines, bryony, etc.). Their slender creepers, which roll out spirally, have avery delicate feeling for the nature of the supports which they embrace; they distinguish between smooth and rough, thick and thin supports, and prefer the latter. Many of the higher plants, which are particularly sensitive to pressure, have, to an extent, special organs of touch (tentacles), and reveal this by the movements of their leaves (the sensitive plants,mimosa,dionæa,oxalis). But even among the unicellular protists we find that the contact of solid bodies has an irritating effect, the perception of which provokes corresponding movements (thigmotaxisorthigmotropismus). A peculiar form of pressure-sensation is produced in many organisms by the flow of liquids; in the mycetozoa, for instance, it provokes counter-movements (rheotaxis,rheotropismus), as Ernst Strahl showed by his experiments onæthelium septicum.We have an interesting analogy to the thigmotaxis of the viscous living plasm in the elasticity of solid inorganic bodies, such as an elastic steel-rod. In virtue of its springy nature, the elastic rod reacts on the pressure of force that has bent it, and endeavors to regain its former position. The spiral spring sets the works of the clock in motion in virtue of its elasticity.A very important part is played in botany by the action of gravitation on the growth of plants. The attraction towards the centre of the earth causes the positively geotropic roots to grow vertically into the earth, while the negatively geotropic stalk pushes out in the opposite direction. This applies also to a number of stationary animals which are attached to the ground by roots, such as polyps, corals, bryozoa, etc. And even the locomotion of free animals, the disposition of their bodies to the ground, the position and posture of their limbs, etc., is determined partly by the feeling of gravitation, and partly by adaptation to certain functions which resist this, as in running, swimming, and so on.All these geotropic sensations belong to the same group of barotactile phenomena, as the fall of a stone or any other effect of gravitation that depends on an inorganic feeling of attraction.As a result of these adaptations, we find a distinct sense of space developed in the higher, free-moving animals. The feeling of the three dimensions of space becomes an important means of orientation, and in the vertebrates, from the fishes up to man, the three spiral canals in the inner ear are developed as special organs of this. These three semicircular canals, which lie vertically to each other in the three dimensions of space, are the organs of the sensation that guides the movements of the head, and, in relation to this, for the normal posture of the body and the feeling of equilibrium. If the three spiral canals are destroyed, the equilibrium is lost; the body totters and falls. Hence, these organs are not of an acoustic, but a static or geotactic character; and the same may be said of the so-called "auditory vesicles" of many of the lower animals—round vesicles which contain a liquid and a solid body, the otolith. When this body changes its position with the change of posture of the whole frame, it presses on the fine auditory hairs, or delicate terminations of the auscultory nerve, which enters the vesicle. In fact, the sense of equilibrium is often combined with the sense of hearing.The perception of noises and tones, which we call hearing, is restricted to a section of the higher, free-moving animals; if, that is to say, the above-mentioned "auditory vesicles" in the lower animals do not have acoustic as well as static sensations. The specific sensation of hearing is due to vibration of the medium in which the animal lives (air or water), or to vibrations of solid bodies (such as tuning-forks) which are brought into touch with them. If the vibrations are irregular, they are felt as "noises"; if regular, they are heard as"tones" or notes; when a number of tones together (fundamental and over-tones) excite a complex sensation, we have "timbre." The vibrations of the sounding body are borne to the auditory cells, which represent the terminal extensions of the auscultory nerve. The specific sensation of hearing can, therefore, be traced originally to the sense of pressure, from which it has been evolved. As the organ of hearing is, like the eye, one of the principal instruments of the higher mental life, and as the refined musical hearing of civilized man is often taken to be a metaphysical power of the soul, it is important to note that here again the starting-point was purely physical—that is to say, it can be traced to the sense of pressure of matter, or gravitation.The great importance of electricity as an agency in nature, both organic and inorganic, has only lately been fully appreciated. Electric changes are connected with many (if not, as is now supposed, with all) chemical and optical processes. Man himself and most of the higher animals have no electric organs (apart from the eye), and no sense-organs that experience a specific electric sensation. It is probably otherwise with many of the lower animals, especially those that develop free electricity, such as the electric fishes. The larvæ of frogs and embryos of fishes, if put in a vessel of water through which a galvanic current is sent, place themselves when it is closed with their longitudinal axis in the direction of the current, with the head directed to the anode and the tail to the cathode (Hermann). Again, the luminous sea-animals which cause the beautiful phenomenon of the illumination of the sea, and the glow-worms and other luminous organisms, have probably an unconscious feeling of the flow of electric energy associated with these phenomena. Many plants show a direct reaction to electric stimuli; when, for instance, we send a constant galvanic current for some time through the pointsof their roots (very sensitive organs, compared by Darwin to the brain of the animal), they bend towards the cathode.Many of the protists are very sensitive to electric currents, as Max Verworn especially proved by a series of beautiful experiments. Most of the ciliated infusoria and many of the rhizopods (amœba) are cathodically sensitive or negatively galvanotactic. When we send a constant electric current through a drop of water in which thousands ofparamœciumare moving about, all the infusoria swim at once, with the anterior pole of the body foremost, towards the cathode or negative pole; they accumulate about it in great crowds. If the direction of the current is now changed, the whole swarm at once make in the opposite direction for the new cathode. Most of the flagellate infusoria do just the reverse; they are anodically sensitive or positively galvanotactic. In a drop of water, in which swarms ofpolytomaare moving about, all the cells swim at once towards the anode or positive pole, when an electric current is sent through. The opposite galvanotropic behavior of these two groups of infusoria in a drop of water, in which they are mixed together, is very interesting; as soon as a constant stream enters it, the ciliata fly to the cathode and the flagellata to the anode. When the current is reversed the two swarms rush at each other like hostile armies, cross in the middle of the drop, and gather at the opposite poles. These and other phenomena of galvanic sensation show clearly that the living plasm is subject to the same physical laws as the water that is decomposed into hydrogen and oxygen by an electric current. Both elementsfeelthe opposite electricities.SCALE OF SENSATION AND IRRITABILITY1stStageSensation of Atoms.Affinity of the elements in every chemical combination.2dStageSensation of Molecules(groups of atoms): in the attraction and repulsion of molecules (positive and negative electricity, etc.).3dStage:Sensation of Plastidules(micella, biogens, or plasma-molecules): in the simplest vital process of the monera (chromacea and bacteria).4thStage:Sensation of Cells: irritability of the unicellular protists (protophyta and protozoa): erotic chemotropism connected with the nucleus and trophic with the cell-body.5thStage:Sensation of Cœnobia(volvox, magosphæra). With the formation of cell-communities we have association of sensations (individual feeling on the part of the social cells together with common feeling on the part of the community).6thStage:Sensation of the Lower Plants.In the metaphyta or tissue-plants all the cells are still equally sensitive at the lower stages: there are no special sense-organs.7thStage:Sensation of the Higher Plants.In the higher metaphyta specially sensitive cells, or groups of cells, with a specific energy, are developed at certain points: sense-organs.8thStage:Sensation of the Lower Metazoa, without differentiated nerves or sense-organs. Lower cœlenteria: sponges, polyps, platodaria.9thStage:Sensation of the Higher Metazoa, with differentiated nerves and sense-organs, but still without consciousness(?). The higher cœlenteria and most of the cœlomaria.10thStage:Sensation with Dawning Consciousness, with independent formation of the phronema. The higher articulata (spiders and insects) and vertebrates (amphibia, lower reptiles, lower mammals).11thStage:Sensation with Consciousness and Thought: amniotes: higher reptiles, birds, and mammals: savages.12thStage:Sensation with Productive Mental Action in Art and Science: civilized men.XIVMENTAL LIFEMind and soul—Intelligence and reason—Pure reason—Kant's dualism—Anthropology—Anthropogeny—Embryology of the mind—Mind of the embryo—The canonical mind—Legal rights of the embryo—Phylogeny of the mind—Paleontology of the mind—Psyche and phronema—Mental energy—Diseases of the mind—Mental powers—Conscious and unconscious mental life—Monistic and dualistic theory—Mental life of the mammals, of savages, and of civilized and educated people.The greatest and most commanding of all the wonders of life is unquestionably the mind of man. That function of the human organism, to which we give the name of "mind," is not only the chief source of all the higher enjoyment of life for ourselves, but it is also the power that most effectually separates man from the brute according to conventional beliefs. Hence it is supremely important for our biological philosophy to devote a few careful pages to the study of its nature, its origin and development, and its relation to the body.At the very outset of our psychological inquiry we are met by the difficulty of giving a clear definition of "mind," and distinguishing it from "soul." Both ideas are extremely ambiguous: their content and connotation are described in the most various ways by the representatives of science. Generally speaking, we mean by mind that part of the life of the soul which is connected with consciousness and thought, and is, therefore, only found in the higher animals which have intelligence and reason.In a narrower sense reason is regarded as the proper function of mind, and as the essential prerogative of man in the animal world. In this sense Kant especially has done much to strengthen the prevailing conception of mental action, and has, by hisCritique of Pure Reason, converted philosophy into a mere "science of reason." In consequence of this conception, which still prevails widely in scientific circles, we will first study the mental life in the action of reason, and try to form a clear idea of this great wonder of life.Psychologists and metaphysicians are of very varied opinions as to the difference between intelligence and reason. Schopenhauer, for instance, considers causality to be the sole function of intelligence, and the formation of concepts to be the province of reason; in his opinion the latter power alone marks off man from the brute. However, the power of abstraction, which collects the common features in a number of different presentations, is also found in the higher animals. Intelligent dogs not only discriminate between individual men, cats, etc., according as they are sympathetic or the reverse, but they have a general idea of man or cat, and behave very differently towards the two. On the other hand, the power of forming concepts is still so slight in uncivilized races that it rises but little above the mind of dogs, horses, etc.; the mental interval between them and civilized man is extremely wide. However, a long scale of reason unites the various stages of association of presentations which lead up to the formation of concepts; it is quite impossible to lay down a strict line of demarcation between the lower and higher mental functions of animals, or between the latter and reason. Hence the distinction between the two cerebral functions is only relative; the intelligence comprises the narrower circle of concrete and more proximate associations, while reason deals with the wider sphere of abstract and morecomprehensive groups of association. In the scientific life of the mind, therefore, the intelligence is always occupied with empirical investigation, and reason with speculative knowledge. But the two faculties are equally functions of the phronema, and depend on the normal anatomic and chemical condition of this organ of thought.Since Kant won so great a prominence in modern philosophy for the idea of pure reason by his famousCritique(1781), it has been much discussed, especially in the modern metaphysical theory of knowledge. It has, however, like all other ideas, undergone considerable changes of meaning in the course of time. Kant himself at first understood by pure reason "reason independent of all experience." But impartial modern psychology based on the physiology of the brain and the phylogeny of its functions, has shown that there is no such thing as this purea prioriknowledge, independent of all experience. Those principles of reason which at present seem to be a priori in this sense have been attained in virtue of thousands of experiences. In so far as this is a question of real knowledge of the truth, Kant himself has frequently recognized the point. He says expressly in hisProlegomena to any future metaphysic that can be regarded as Science(1783, p. 204): "A knowledge of things by pure reason or pure intelligence is nothing but an empty appearance; only in experience is there truth." In subscribing to this empirical theory of knowledge of Kant I. and rejecting the transcendental theory of Kant II., we may on our side understand by pure reason "knowledge without prejudices," free from all dogma—all fictions of faith.The familiar cry of modern metaphysicians, "Return to Kant," has become so general in Germany that not only nearly all metaphysicians—the official representatives of "philosophy" at our universities—but alsomany distinguished scientists, regard Kant's dualistic theory of knowledge as a necessary condition for the attainment of truth. Kant dominated philosophy in the nineteenth century much as Aristotle did in the Middle Ages. His authority became especially powerful when the prevailing Christian faith believed that his "practical reason" fully supported its own three fundamental dogmas—the personality of God, the immortality of the soul, and the freedom of the will. It overlooked the fact that Kant had utterly failed to find proofs of these dogmas in hisCritique of Pure Reason. Even conservative governments found favorable features in this dualistic philosophy. We are, therefore, forced to return once more to this mischievous system; though Kant's antinomy of the two reasons has now been refuted so often and so thoroughly that we need not dwell any further on this point.Although the great Königsberg philosopher brought every side of human life within his comprehensive sphere of study, man remained to him—as he had been to Plato and Aristotle, Christ and Descartes—a dual being, made up of a physical body and a transcendental mind or spirit. Comparative anatomy and evolution, which have provided the solid morphological basis of monistic anthropology, did not come into existence until the beginning of the nineteenth century; they were quite unknown to Kant. He had, however, a presentiment of their importance, as Fritz Schultze has shown in his interesting work onKant and Darwin(1875). We find in various places expressions which may be described as anticipations of Darwinism. Kant also gave lectures on "Pragmatic Anthropology," and studied the psychology of races and peoples. It is remarkable that he did not arrive at a phylogenetic conception of the human mind, and a recognition of the possibility of its evolution from the mind of other vertebrates. It is clear thathe was held back from this by the profound mystic tendency of his theory of reason, and the dogma of the immortality of the soul, the freedom of the will, and the categorical imperative. Reason remained in Kant's view a transcendental phenomenon, and this dualistic error had a great influence on the whole structure of his philosophy. It must be remembered, of course, that our knowledge of the psychology of peoples was then very imperfect; but a critical study of the facts then known should have sufficed to convince him of the lower and animal condition of their minds. If Kant had had children, and followed patiently the development of the child's soul (as Preyer did a century later), he would hardly have persisted in his erroneous idea that reason, with its power of attaininga prioriknowledge, is a transcendental and supernatural wonder of life, or a unique gift to man from Heaven.The root of the error is that Kant had no idea of the natural evolution of the mind. He did not employ the comparative and genetic methods to which we owe the chief scientific achievements of the last half-century. Kant and his followers, who confined themselves almost exclusively to the introspective method or the self-observation of their own mind, regarded as the model of the human soul the highly developed and versatile mind of the philosopher, and disregarded altogether the lower stages of mental life which we find in the child and the savage.The immense advance made by the science of man in the second half of the nineteenth century cut the ground from under the older anthropology and the dualistic system of Kant. A number of newly founded branches of science co-operated in the work. Comparative anatomy showed that our whole complicated frame resembles that of the other mammals, and in particular differs only by slight stages of growth, and therefore inthe details of the organs, from that of the anthropoid apes. The comparative histology of the brain especially showed that this is also true of the brain, the real organ of mind. From comparative embryology we learned that man develops from a simple ovum just like the anthropoid ape; in fact, that it is almost impossible to distinguish between the ape and the human embryo even at a late stage of development. Comparative animal chemistry explained that the chemical compounds which build up our organs, and the conversions of energy which accompany its metabolism, resemble those in the other vertebrates. Comparative physiology taught us that all man's vital functions—nutrition and reproduction, movement and sensation—can be traced to the same physical laws in man as in all the other vertebrates. Above all, the comparative and experimental study of the sense-organs and the various parts of the brain showed that these organs of the mind work in the same way in man as in the other primates. Modern paleontology taught that man is, it is true, more than a hundred thousand years old, but only appeared on earth towards the close of the Tertiary Period. Prehistoric research and comparative ethnology have shown that civilized nations were preceded by older and lower races, and these by savages, which have a close bodily and mental affinity to the apes. Finally, the reformed theory of descent (1859) enabled us to unite the chief results of the various branches of anthropological study, and explain them phylogenetically by the development of man from other primates (anthropoid apes, cynocephali, lemures, etc.). By this means a new and monistic basis was provided for modern anthropology; the position assigned to man in nature by dualistic metaphysics was shown to be utterly untenable. I have attempted in the last edition of myAnthropogeny(of which an English edition is in preparation) to combine all theseresults of empirical investigation in a sketch of the natural evolution of man, paying special regard to embryology. I pointed out in chapters ii.-vi. of theRiddlehow important a part of our monistic philosophy this phylogenetic anthropology is.The monistic conception of the human body and mind, which the theory of descent has put on a zoological basis, was bound to meet with the sternest resistance in dualistic and metaphysical circles. It was, however, also regarded with great disapproval by many modern empirical anthropologists, especially those who take it to be their chief task to make as "exact" a study as possible of the human frame, and measure and describe its various parts. We might have expected these descriptive anthropologists and ethnologists to extend a friendly hand to the new anthropogeny, and avail themselves of its leading ideas, in order to bring unity and causal connection into the enormous mass of empirical material accumulated. However, this took place only to a limited extent, The majority of anthropologists regarded evolution, and especially the evolution of man, as an undemonstrated hypothesis. They confined themselves to accumulating huge masses of raw empirical material, without having any clear aim or any definite questions in view. This was chiefly the case in Germany, where the Society of Anthropology and Prehistoric Research was for thirty years under the lead of Rudolph Virchow. This famous scientist had won great honor in connection with the reform of medicine by his cellular pathology and a number of distinguished works on pathological anatomy and histology since the middle of the nineteenth century. But when he afterwards (subsequently to his removal to Berlin, 1856) devoted himself chiefly to political and social questions, he lost sight of the great advance made in other branches of biology. He completely failed to appreciate itsgreatest achievement—the establishment of the science of evolution by Darwin. To this we must add the psychological metamorphosis (similar to that of Wundt, Baer, Dubois-Reymond, and others), of which I have spoken in the sixth chapter of theRiddle. The extraordinary authority of Virchow, and the indefatigable zeal with which he struggled every year until his death (1903) against the descent of man from other vertebrates, caused a wide-spread opposition to the doctrine of evolution. This was supported especially by Johannes Ranke, of Munich, the secretary of the Anthropological Society. Happily, a change has recently set in. However, myAnthropogenyhas remained for thirty years the only work of its kind—namely, a comprehensive treatment of man's ancestral history, especially in the light of embryology.As I pointed out in the eighth and ninth chapters of theRiddle, the most solid foundation of our monistic psychology is the fact that the human mind grows. Like every other function of our organism, our mental activity exhibits the phenomenon of development in two directions, individually in each human being and phyletically in the whole race. The ontogeny of the mind—or the embryology of the human soul—brings before us in direct observation the various stages of development through which the mind of every man passes from the beginning to the close of life. The phylogeny of the mind—or the ancestral history of the human soul—does not afford us this direct observation; it can only be deduced by a comparison and synthesis of the historical indications which are supplied by history and prehistoric research on the one hand, and the critical study of the various stages of mental life in savages and the higher vertebrates on the other. In this the biogenetic law is used with great success (chapter xvi.).As everybody knows, the new-born child shows as yet no trace of mind or reason or consciousness; these functions are wanting in it as completely as in the embryo from which it has been developed during the nine months in the mother's womb. Even in the ninth month, when most of the organs of the human embryo are formed and arranged as they appear later, there is no more trace of mind in its psychic life than in the ovum and spermatozoon from which it was evolved. The moment in which these sexual cells unite marks precisely the real commencement of individual existence, and therefore of the soul also (as a potential function of the plasm). But the mind proper—or reason, the higher conscious function of the soul—only develops, slowly and gradually, long after birth. As Flechsig has shown anatomically, the cortex in the new-born child is not yet organized or capable of functioning. Rational consciousness is even impossible for the child when it begins to speak; it reveals itself for the first time (after the first year) at the moment when the child speaks of itself, not in the-third person, but as "I." With this self-consciousness comes also the antithesis of the individual to the outer world, or world-consciousness. This is the real beginning of mental life.In defining the appearance of the individual mind by the awakening of self-consciousness, we make it possible to distinguish, from the monistic physiological point of view, between "soul" (psyche) and "spirit" (pneuma). There is a soul even in the maternal ovum and the paternal spermatozoon (cf.chapter xi.); there is an individual soul in the stem-cell (cytula) which arises at conception by the blending of the parent cells. But the mind proper, the thinking reason, develops out of the animal intelligence (or earlier instincts) of the child only with the consciousness of its personality as opposed to the outer world. At the same time the child reaches thehigher stage of personality, which law has for a long time taken under its protection and made morally responsible to society by education. This shows how erroneous and untenable, from the physiological point of view, are the ideas still embodied in our code as to the psychic life and the mind of the embryo and the new-born infant. They came mostly from the canon law of the Catholic Church.The dualistic ideas of the soul of the human embryo which were taught by the Church in the Middle Ages are particularly interesting from the psychological point of view; and at the same time they are of great practical importance even in our own day, since many of their moral consequences form an important element in canon law, and have passed from this into civil law. This influential canon law was formed under ecclesiastical authority from the decisions of Church councils and the decretals of the popes. It is, like most of the dogmas and decrees which civilization owes to this powerful hierarchy, a curious tissue of old traditions and new fictions, political dogmas, and crass superstition. It is directed to the despotic ruling of the uneducated masses and the exclusive dominion of the Church—a Church that calls itself Christian while thus acting as the very reverse of pure Christianity. The canon law takes its name from the dogmatic rules (or canons) of the Church. They involuntarily suggest the metal tubes which are so often theultima ratio regisin the wars of Christian nations. The canonical regulations of the Church, as implements of a crude spiritual despotism, have no more to do with the ethical laws of pure reason than the cannons of secular authorities have as naked organs of physical force. We might write the motto,Ultima ratio ecclesiæ(the last argument of the Church), over the sacredCorpus Juris Canonici. A collection of later papal decretals which forms an appendix to the booksof canon law was very happily given the official title ofExtravagantes. Among the "extravagant" nonsense which the papacy included in canon law as a moral code for believers is its view of the psychic life of the embryo. The "immortal soul" is supposed to enter the soulless embryo only several weeks after conception. As theologians and metaphysicians are very much divided as to the period of this entrance of the soul, and know nothing about the structure of the embryo and its development, we will only recall the fact that the human fœtus cannot be distinguished from that of the anthropoid ape and other mammals even in the sixth week of its development. The outline of the five cerebral vesicles and the three higher sense-organs (nose, eye, and ear vesicle) is discernible in the head; the two pairs of limbs can be traced in the shape of four simple roundish unjointed plates; and the pointed tail sticks out at the lower part, the rudimentary legacy from our long-tailed ape-ancestors. Although the cortex is not yet developed at this stage, the embryo may be considered to have a "soul" (cf.chapters xiv. and xv. of myAnthropogeny, and plates 8-14).

Sensation and consciousness—Unconscious and conscious sensation—Sensibility and irritability—Reflex sensation and perception of stimuli—Sensation and living force—Reaction to stimuli—Resolution of stimuli—External and internal stimuli—Conveyance of stimuli—Sensation and striving—Sensation and feeling—Inorganic and organic sensation—Light sensation, phototaxis, sight—Sensation of warmth, thermotaxis—Sensation of matter, chemotaxis—Taste and smell—Erotic chemicotropism—Organic sensations—Sensation of pressure—Geotaxis—Sensation of sound—Electric sensation.

Sensation is one of those general terms that have at all times been liable to the most varied interpretations. Like the cognate idea of the "soul," it is still extremely ambiguous. During the eighteenth century it was generally believed that the function of sensation was peculiar to animals, and was not present in plants. This opinion found its most important expression in the well-known principle in Linné'sSystema Naturæ: "Stones grow: plants grow and live: animals grow, live, and feel." Albrecht Haller, who gathered up all the knowledge of his time relating to organic life in hisElementa Physiologiæ(1766), distinguished as its two chief characters "sensibility" and "irritability." The one he ascribed exclusively to the nerves, and the other to the muscles. This erroneous idea was subsequently refuted, and in our own time irritability is conceived to be a general property of all living matter.

The great advance made by the comparative anatomy and experimental physiology of animals and plants in the first half of the nineteenth century brought to light the fact that irritability or sensibility is a common quality of all organisms, and that it is one of the principal characteristics of vital force (cf.chapter ii.). The greatest merit in connection with its experimental study attaches to the famous Johannes Müller. In his classicalManual of Human Physiology(1840) he established his theory of the specific energy of the nerves and their dependence on the sense-organs on the one hand and the mental life on the other. He devoted the fifth chapter of his book to the former and the sixth to the latter, approaching particularly to Spinoza in his general psychological views; he treated psychology as a part of physiology, and thus put on a sound scientific basis that naturalistic conception of the place of psychology in the biological system which we now regard as the correct view. At the same time he proved that sensation is a function of the organism as much as movement or nutrition.

The view of sensation that prevailed in the second half of the nineteenth century was very different. On the one hand the experimental and comparative physiology of the sense-organs and the nervous system immensely enriched our exact knowledge by the invention of ingenious methods of research and the use of the great advance made by physics and chemistry. The famous investigations of Helmholtz and Hertwig on the physics of the senses, of Matteucci and Dubois-Reymond on the electricity of the muscles and nerves, and the great progress made in vegetal physiology by Sachs and Pfeffer, and in physiological chemistry by Moleschott and Bunge, enabled us to realize that even the most mysterious of the wonders of life depend on physical and chemical processes. By the application of the differentstimuli—light, heat, electricity, and chemical action—to the various sensitive or irritable organs under definitely controlled conditions, scientists succeeded in subjecting with exactness a great part of the phenomena of stimulation to mathematical measurements and formulæ. The science of the stimuli and their effects acquired a strictly physical character.

On the other hand, in most striking contradiction to the immense advance of experimental physiology, we see that the general conception of the various vital processes, and especially of the inner nerve-action that converts the functions of the senses into mental life, is most curiously neglected. Even the fundamental idea of sensation, which plays the chief part in it, is disregarded more and more. In many of the most valuable modern manuals of physiology, containing long chapters on stimuli and stimulation, there is little or no mention of sensation as such. This is chiefly due to the mischievous and unjustifiable gulf that has once more been artificially created between physiology and psychology. As the "exact" physiologists found the study of the inner psychic processes which take place in sense-action and sensation inconvenient and unprofitable, they gladly handed over this difficult and obscure field to the "psychologists proper"—in other words, to the metaphysicians, who had for the starting-point of their airy speculations the belief in an immortal soul and divine consciousness. The psychologists readily abandoned the inconvenient burden of experience anda posterioriknowledge, to which the modern anatomic physiology of the brain laid special claim.

The greatest and most fatal error committed by modern physiology in this was the admission of the baseless dogma that all sensation must be accompanied by consciousness. As most physiologists share the view of Dubois-Reymond, that consciousness is not a naturalphenomenon, but a hyperphysical problem, they leave it and this inconvenient "sensation" outside the range of their researches. This decision is, naturally, very agreeable to the prevalent metaphysics; it has just as much interest in the transcendental character of sensation as in the liberty of the will, and thus the whole of psychology passes from the empirical province of natural science into the mystical province of mental science. For its foundation they then take the "critical theory of knowledge," which ignores the results of the real physiological organs—the senses, nerves, and brain—and draws its "superior wisdom" from the inner mirroring of self by the introspective analysis of presentations and their associations. It is extraordinary that even distinguished monistic physiologists suffer themselves to be taken in with this sort of metaphysical jugglery, and dismiss the whole of psychology from their province; their psychomonism readmits the soul as a supernatural entity, and delivers it, in contrast with the "world of bodies," from the yoke of the law of substance.

Impartial reflection on our personal experience during sensation and consciousness will soon convince us that these are two different physiological functions, which are by no means necessarily associated; and the same may be said of the third principal function of the soul—the will. When we learn an art—for instance, painting or playing the piano—we need months of daily practice in order to become expert at it. In this we experience every day hundreds of thousands of sensations and movements which are learned and repeated with full consciousness. The longer we continue the practice and the more we adapt and accustom ourselves to the function, the easier and less conscious it becomes. And when we have practised the art for some years, we paint our picture or play our piano unconsciously; we thinkno longer of all the small, subtle shades of sensation and acts of will which were necessary in learning. The mere impulse of the will to paint the picture once more or play the piece again suffices to release the whole chain of complicated movements and accompanying sensations which had originally to be learned slowly, laboriously, and with full consciousness. An experienced pianist plays the most difficult piece—if he has learned it and repeated it thousands of times—"half in a dream." But it needs only a slight accident, such as a mistake or a sudden interruption, to bring back the wandering attention to the work. The piece is now played with clear consciousness. The same may be said of thousands of sensations and movements which we learned at first consciously in childhood, and then repeat daily afterwards without noticing—such as in walking, eating, speaking, and so on. These familiar facts prove of themselves that consciousness is a complicated function of the brain, by no means necessarily connected with sensation or will. To bind up the ideas of consciousness and sensation inseparably is the more absurd, as the mechanism or the real nature of consciousness seems very obscure to us, while the idea of it is perfectly clear: we know that we know, feel, and will.

The word "irritability" is generally taken by modern physiology to mean that the living matter has the property of reacting on stimuli—that is to say, of responding by changes in itself to changes in its environment. The stimulus, or action of a foreign energy, must, however, be felt by the plasm before the corresponding stimulated movement (in the form of various manifestations of energy) will be produced. Hence the question whether this sensation is (in certain cases) associated with consciousness or (generally) remains unconscious is of a subordinate interest. The plant that is caused to open its floral calyx by thestimulus of light acts just as unconsciously in this as the coral that spreads out its crown of tentacles under the same influence; and when the sensitive carnivorous plant (dionæaordrosera) closes its leaves in order to catch and destroy the insect sitting on them, it acts in the same way as the sensitive actinia or coral when it draws in its crown of tentacles for the same object—in both cases without consciousness! We call these unconscious movements "reflex actions." I have dealt somewhat fully with these reflex movements in the seventh chapter of theRiddle, and must refer the reader thereto. This elementary psychic function always depends on a conjunction of sensation and movement (in the widest sense). The movement that the stimulus provokes is always preceded by a sensation of the influence exerted.

Modern physiology makes desperate efforts to avoid the use of the word "sensation" and substitute for it "perception of stimulus." The chief blame for this misleading expression is due to the arbitrary and unjustified separation of psychology from physiology. The latter is supposed to occupy itself with the material phenomena and physical changes, leaving to psychology the privilege of dealing with the higher mental phenomena and metaphysical problems. As we reject this distinction altogether on monistic principles, we cannot consent to separate sensation from the perception of stimuli—whether this sensation be accompanied with consciousness or not. Moreover, modern physiology, in spite of its desire to keep clear of psychology, sees itself compelled in a thousand ways to use the words "sensation" and "sensitive," especially in the science of the organs of sense.

What we call sensation or perception of stimuli may be regarded as a special form of the living force or actual energy (Ostwald). Sensitiveness or irritability, on the other hand, is a form of virtual or potentialenergy. The living substance at rest, which is sensitive or irritable, is in a state of equilibrium and indifference to its environment. But the active plasm, that receives and feels a stimulus, has its equilibrium disturbed, and corresponds to the change in its environment and its internal condition. This response of the organism to a stimulus is called "reaction"—a term that is also used (in the same sense) in chemistry to express the interaction of bodies on each other. At each stimulation the virtual energy of the plasm (sensitiveness) is converted into living or kinetic force (sensation). The share of the stimulus in this conversion is described as a "release" of energy.

The term "reaction" stands in general for the change which any body experiences from the action of another body. Thus, for instance, to take the simplest case, the interaction of two substances in chemistry is called a reaction. In chemical analysis the word is used in a narrower sense to denote that action of one body on another which serves to reveal its nature. Even here we must assume that the two bodies feel their different characters; otherwise they could not act on each other. Hence every chemist speaks of a more or less "sensitive reaction." But this process is not different in principle from the reaction of the living organism to outer stimuli, whatever be their chemical or physical nature. And there is no more essential difference in psychological reaction, which is always bound up with corresponding changes in the psychoplasm, and so with a chemical conversion of energy. In this case, however, the process of reaction is much more complicated, and we can distinguish several parts or phases of it: 1, the outer excitation; 2, the reaction of the sense-organ; 3, the conducting of the modified impression to the central organ; 4, the internal sensation of the conducted impression; and, 5, consciousness of the impression.

The important idea of a release of energy—the term we give to the effect of the stimulus—is also used in physics. If we put a piece of burning wood in a barrel of powder, the flame causes an explosion. In the case of dynamite a simple mechanical shock is enough to produce the most enormous expenditure of force in the explosive matter. When we discharge a bow the slight pressure of the finger on the tense cord suffices to send out the arrow or bolt on its deadly mission. So also a sound or a ray of light that strikes the ear or eye suffices to bring about a number of complex effects by means of the nervous system. In the fertilization of the ovum by the male sperm the chemical conjunction of the two formative principles is sufficient to cause the growth of a new human being out of the microscopic plasma-globule, the stem-cell (cytula). In these and thousands of other reactions a very slight shock suffices to provoke the largest effects in the stimulated substance. This shock, which we call a release of energy, is not the direct cause of the considerable result, but merely the occasion for bringing it about. In these cases we have always a vast accumulation of virtual energy converted into living force or work. The magnitude of the two forces has no relation at all to the smallness of the shock which led to the conversion. In this we have the difference between stimulated action and the simple mechanical action of two bodies on each other, in which the quantity of the energy expended is equal on both sides, and there is no stimulus.

The immediate effect of a stimulus on living matter can best be followed in external physical or chemical stimuli, such as light, heat, pressure, sound, electricity, and chemical action. In these cases physical science is often able to reduce the life-process to the laws of inorganic nature. This is more difficult with the internal stimuli within the organism itself, which are only partlyexposed to physiological investigation. It is true that here also the task of science is to reduce all the biological phenomena to physical and chemical laws. But it can only discharge a part of this difficult task, as the phenomena are too complicated, and their conditions too little known in detail, to say nothing of the crudeness and imperfectness of our methods of research. Yet, in spite of all this, comparative and phylogenetic physiology convinces us that even the most complicated of our internal excitations, and particularly the mental activity of the brain, depend just as much as the outer stimulations on physical processes, and are equally subject to the law of substance. This is, in fact, true of reason and consciousness.

In man and all the higher animals the stimuli are received by the organs of sense and conducted by their nerves to the central organ. In the brain they are either converted into specific sensations in the sense-centres, or conveyed to the motor region, where they provoke movements. The conduction of stimuli is simpler in the lower animals and the plants; the tissue-cells either directly affect each other or are connected by fine threads of plasm. In the unicellular protists the stimulus which strikes one particular spot of the surface may be immediately communicated to the other parts of the unified plasmic body.

We shall see in the course of our inquiry that the simplest form of sensation (in the widest sense) is common to inorganic and organic bodies, and thus that sensitiveness is really a fundamental property of all matter, or, more correctly, all substance. We may, therefore, ascribe sensation to the constituent atoms of matter. This fundamental thought of hylozoism, expressed long ago by Empedocles, has lately been very definitely urged, especially by Fechner. However, the able founder of psychophysics (cf.theRiddle, p. 35)assumes that consciousness (or thought, in the Spinozistic sense) always accompanies this universal property of sensation. In my opinion, consciousness is a secondary psychic function, only found in man and the higher animals, and bound up with the centralization of the nervous system. Hence it is better to speak of the unconscious sensation of the atoms asfeeling(æsthesis), and their unconscious will asinclination(tropesis). It finds expression in the one-sided action of a stimulus as a "directed movement" or "stimulated movement" (tropismusortaxis).

The familiar ideas of sensation and feeling are often confused, and employed in very different ways in both physiology and psychology. The metaphysical tendency which so completely separates the two sciences, and the physiological tendency which agrees with it, regard feeling as a purely psychic or spiritual function, whereas in the case of sensation they have to admit the connection with bodily functions, especially sense-action. In my opinion, the two ideas are purely physiological and cannot be sharply separated, or only in the sense that sensation relates more to the external (objective) part of the sensory nerve-process, and feeling to the internal (subjective) part. Hence we may define the difference in a general way by saying that sensation perceives the different qualities of the stimuli, and feeling only the quantity, the positive or negative action of the stimulus (pleasure or pain). In this last and widest sense we may ascribe the feeling of pleasure and pain (in the contact with qualitatively differing atoms) to all atoms, and so explain the elective affinity in chemistry (synthesis of loving atoms, inclination; analysis of hating atoms, disinclination).

Our monistic system (whether it be taken as energism or materialism, or more correctly as hylozoism) regards all substance as having "soul"—that is to say, endowedwith energy. In the chemical analysis of organisms we do not find any elements that are not found in inorganic nature; we find that the movements in organisms obey the same laws of mechanics as the latter; we believe that the conversion of energy in the living matter occurs in the same way, and is provoked by the same stimuli, as in inorganic matter. We are forced to conclude from. these experiences that the perception of stimuli—sensation in the objective and feeling in the subjective sense—is also generally present in the two. All bodies are in a certain sense "sensitive." It is just in this dynamic conception of substance that monism differs essentially from the materialistic system, which regards one part of matter as "dead" and insensitive. In this we have the best means of joining consistent materialism or realism with consistent spiritualism or idealism. But, as a first condition of such a union, we must demand a recognition that organic life is subject to the same general laws as inorganic nature. In both cases the outer world acts alike as a stimulus on the inner world of the body. We can easily see this if we glance at the various kinds of sensation which correspond to the various kinds of stimuli. Light and heat, external and internal chemical stimuli, pressure and electricity, cause analogous sensations and modifications in their effect on organic and inorganic bodies.

The effect which the light-stimulus has on living matter, the sensation of light that results, and the chemical changes of energy that follow, are of great physiological importance in all organisms. We might even say that sunlight is the first, oldest, and chief source of organic life; all other exertions of force depend in the long run on the radiant energy of sunlight. The oldest and most important function of plasm—one which is at the same time a cause of its formation—is carbon-assimilation; and this plasmodomism is directly dependenton sunlight. If it acts in a one-sided way, it causes the particular form of stimulation which we call phototaxis or heliotropism. This is of a positive character—that is to say, they turn towards the source of the light—in the great majority of organisms, both protists and histona. Everybody knows that flowers that are growing in the window of a room turn to the light. However, many organisms which have grown accustomed to living in the dark are heliotropically negative; they shun the light and seek darkness, such as the fungi, many lucifugous mosses and ferns, and many deep-sea animals.

The principal organs of light-sensation in the higher animals are the eyes; they are wanting in many of the lower animals as well as the plants. The essential difference between the real eye and a part of the skin that is merely sensitive to light is that the eye can form a picture of objects in the outer world. This faculty of vision begins with the formation of a small convergent lens, a biconvex refracting body at a certain spot on the surface. Dark pigment-cells which surround it absorb the light-rays. From this first phylogenetic form of the organ of vision up to the elaborate human eye there is a long scale of evolutionary stages—not less extensive and remarkable than the historical succession of artificial optical instruments from the simple lens to the complicated modern telescope or microscope. This great "wonder of life"—the long scale of the evolution of the eye—has an interesting tearing on many important questions of general physiology and phylogeny. We can, in this case, see clearly how a very complicated and purposive apparatus can arise in a purely mechanical way, without any preconceived design or plan. In other words, we can see how an entirely new function—and one of its principal functions, vision—has arisen in the organism by mechanical means.

The advanced vision of the higher animals is made up of a great number of different functions, with a corresponding complexity of detail in the anatomic structure of the eye. No other organ, after the brain, is so necessary as the eye for the multifarious vital activities of the higher animals, and especially for the mental life of civilized man and the progress of art and science. What would the human mind be if we could not read, write, and draw, and have a direct knowledge through the eye of the forms and colors of the outer world? Yet this invaluable structure is only the highest and most perfect stage in the long chain of evolutionary processes which has its starting-point in the general sensitiveness to light, or the photic irritability of plasm. However, we find a number of varieties and grades of this even among the unicellular protists, and, indeed, the very lowest and oldest of the protists, the monera. Various species of both the chromacea and the bacteria are heliotropic to different degrees, and have a fine sensitiveness to the strength of the light stimulus.

The stimulating effect which light has on the homogeneous plasm of the monera is also found in a number of inorganic bodies. In these cases the photic stimulus produces partly chemical and partly mechanical changes. Every chemist speaks of substances that are more or less "sensitive" to light; the photographer speaks of his "sensitive plates," the painter of his "sensitive colors." Many chemical compounds are so sensitive to light that they are destroyed at once in sunlight, and so have to be kept in the dark. There is no other word but "sensation" to express the attitude of the atoms towards each other which becomes so conspicuous in these cases under the influence of sunlight. It seems to me that this phenomenon is a clear justification of our hylozoic monism when it affirms that all matter is psychic. In metaphysics sensation is held to be an essential property of the soul.

In the same general way as light the heat-stimulus acts on organisms, and causes the sensations, sometimes pleasant and sometimes unpleasant, which we call the subjective feeling of heat, warmth, coolness, or cold. The sense-organ that receives these impressions of temperature is the surface of the unicellular plasmic body in the protists, and the skin (epidermis) that protects the surface from the outer world in the histona. In all living things the temperature of the surrounding medium (water or air) has a great influence in regulating the life-processes; in the stationary animals and plants it is the temperature of the ground to which they are attached. This temperature must always be between the freezing-point and boiling-point of water, as fluid water is indispensable for the imbibition of the living matter and the molecular movements within the plasm. At the same time, some of the lower protists (chromacea, bacteria) can endure very high and very low temperatures, but only for a short time. Some protists (monera and diatomes) can stand a temperature of 200° C. for several days, and others can be heated above boiling-point without being killed. Arctic and High-Alpine plants and animals may be in a frozen condition for several months, yet live again when they are thawed. However, the resistance to these extremes of cold lasts for only a limited time, and in the frozen state all vital functions are at a standstill.

In the great majority of living things the vital activity is confined within narrow limits of temperature. Many plants and animals in the tropics which have been accustomed for thousands of years to the constancy of the hot equatorial climate can endure only very restricted variations of temperature. On the other hand, many of the inhabitants of Central Siberia, where the climate is very hot in the short summer and very cold in the long winter, can stand great variations. Thus the livingplasm has experienced considerable changes in its sense of warmth through adaptation to different environments; not only the maximum and the minimum, but the optimum (most agreeable point), is subject to very great variations. This can easily be observed and followed experimentally in the phenomena of thermotaxis or thermotropism—that is to say, the effect that follows from a one-sided action of the heat-stimulus. The organism that falls below the minimum of temperature is said to be stiff with cold, while the organism that rises above the maximum is stiff with heat.

The heat-stimulus acts on inorganic as well as organic bodies, like the light-stimulus. The law holds good in both cases that higher temperatures increase sensation, while lower ones paralyze it. There is a minimum, an optimum, and a maximum, for many chemical and physical processes in the inorganic world. As far as the melting effect of water is concerned, freezing is the minimum of the heat stimulus and boiling the maximum. As the various chemical compounds meet in water at very different temperatures, we have an optimum for many substances—that is to say, a degree of warmth which is most favorable to the solution of a given quantity of a solid body in water. On the whole, the law holds for chemical processes that they are accelerated by high temperatures and retarded by low ones (like the human passions!); the former have a stimulating and the latter a benumbing effect. As the action of the various chemical compounds on each other is determined by the nature of the elements and their affinities, we must trace the variations in their conduct towards thermic stimuli to a sensation of temperature in the constituent atoms; increase of temperature stimulates it, while decrease lessens or paralyzes it. Here, again, the simple inorganic processes have a general resemblance to the complicated vital phenomena in the organic body.

Since we regard the whole of organic life as, in the ultimate analysis, merely a very elaborate chemical process, we shall quite expect that chemical stimuli are the most important factors in sensation. And this is so in point of fact; from the simplest moneron up to the most highly differentiated cell and on to the flower in the plant and the mental life of man, the vital processes are dominated by chemical forces and conversions of energy, which are set in play by external or internal chemical stimuli. The excitation which they produce is called, in a general way, "sensation of matter" or chemæsthesis; the basis of it is the mutual relation of the chemical elements which we describe as chemical affinity. In this affinity we have the play of attractive forces which lie in the nature of the elements themselves, especially in the peculiar properties of their constituent atoms; and this cannot be explained unless we ascribe unconscious sensation (in the widest sense) to the atoms, an inherent feeling of pleasure and the reverse, which they experience in the contact of other atoms (the "loves and hatreds of the elements" of Empedocles).

The numbers of different stimuli that act chemically on the plasm and excite its "sensation of matter" may be divided into two groups—external and internal stimuli. The latter lie within the organism itself, and cause the internal "organic sensations"; the former are in the outer world, and are felt as taste, smell, sex-impulse, etc. In the higher animals special chemical sense-organs have been developed for these chemical stimuli. As these are well known to us from our own human experience, and comparative physiology shows us the same structures in the higher animals, we will deal first with them. In general the same law holds for these external chemical stimuli as for optical and thermic stimuli; we can recognize a maximum limit oftheir action, a minimum below which they fail to stimulate, and an optimum or stage in which their influence is strongest.

The important part played in human life by taste and the pleasure associated with it is well known. The careful choice and preparation of savory food—which has become an art in gastronomy and a branch of practical philosophy in gastrosophy—was just as important two thousand years ago with the Greeks and Romans as it is to-day in royal banquets or the Lucullic dinners of millionaires. The excitement that we see associated with this refined combination of rich foods and drinks, and that finds expression in so many speeches and toasts, has its philosophic root in the harmony of gustatory sensations and the varying play of stimuli that the delicate dishes and wines exercise on the organs of taste, the tongue and palate. The microscopic organs of these parts of the mouth are the gustatory papillæ—cup-shaped structures, covered with spindle-shaped "taste-cells," and having a narrow opening into the cavity of the mouth. When sapid matters, drinks and fluid or loose particles of food, touch the taste-cells, they excite the fine terminal branchlets of the gustatory nerve which enters the cells. As we find that there are similar structures in most of the higher animals, and that they also choose their food with some care, we may confidently assume that they have sensations of taste like man. However, no trace of this is found in many of the lower animals; in these cases it is impossible to lay down a line of demarcation between taste and smell.

In man and the higher air-breathing vertebrates the seat of the sense of smell is in the nostrils; in man it is especially that part of the mucous lining of the nasal cavity which we call the "olfactory region" (the uppermost part of the nasal dividing wall, the superior and middle meatus). It is necessary for a sensation of smellthat the odorous matter, or olfactory stimuli, be brought in a finely divided condition over the moist olfactory membranes. When they touch the olfactory cells—slender, rod-shaped cells with very fine hairs at the free end—they excite the ends of the olfactory nerve which are connected with the cells.

In many animals, especially mammals, the sense of smell has a much more important part in life than it has in man, in whom it is relatively feeble. It is well known that dogs and other carnivora, and even ungulates, have a much keener smell. In these cases the nasal cavity, which is the seat of the sense, is much larger, and the muscles in it are much stronger. The nostrils of the air-breathing vertebrates have been developed from a pair of open nasal depressions in the skin of the fish's head. But in these aquatic vertebrates the chemical action of the olfactory stimuli must be of a different character, like the sensation of taste. The odorous matter is, in these cases, brought into contact with the olfactory membrane in a liquid form (in which condition it is not perceptible to man). In fact, the division between the senses of smell and taste disappears altogether in the lower animals. These two "chemical senses" are closely related, and have a common feature in the direct chemical action of the stimulus on the sensitive part of the skin.

A chemical sensation of matter that corresponds completely to the real taste-sensation in the higher animals is found in some of the higher carnivorous plants. The leaves of the sun-dew (drosera rotundifolia) are very sensitive insect-traps, and are armed at the edge with knob-like tentacles, sticky hairs that secrete an acid, flesh-digesting juice. When a solid body (but not a raindrop) touches the surface of the leaf the stimulus acts in such a way on the tentacle heads as to contract the leaf. But the acid fluid which serves fordigestion, and corresponds to the gastric juice in the animal, is only secreted by the corpuscles if the solid foreign body is nitrogenous (flesh or cheese). Hence the leaves of these insectivorous plants taste their meat diet, and distinguish it from other solids, to which they are indifferent. In the broader sense, in fact, we may describe the points of the roots of plants as organs of taste; they plunge into the richer parts of the earth which yield more nourishment, and avoid the poor parts. In unicellular plants and animals the action of chemical stimuli is especially conspicuous when it is one-sided, and provokes definite movements in one particular direction (chemotaxis).

The movements of unicellular organisms that are provoked by chemical stimuli and are known as chemotropism (more recently as chemotaxis) are particularly interesting because they show the existence of a chemical sensitiveness, somewhat resembling taste or smell, in the lowest organisms, and even in the homogeneous plasm of the monera. Repeated experiments of Wilhelm Engelmann, Max Verworn, and others, have shown that many bacteria, diatomes, infusoria, rhizopods, and other protists, have a similar sense of taste; they move towards certain acids (for instance, a drop of malic acid) or a bubble of oxygen that lies on one side of the drop of water in which the protists are under the microscope. Many pathogenetic bacteria secrete poisonous substances which are very injurious to the human frame. The active white blood-cells, leucocytes, in the human blood have a special "taste" for these bacteria-poisons, and concentrate in large quantities, by means of their amœboid movements, at those parts of the body where they are secreted. If the leucocytes prove the stronger in their struggle with the bacteria, they destroy them, and in this way they act as sanitary officers in keeping poisonous infection out of our organism. But if thebacteria win the battle, they are transported into other parts of the body by the leucocytes; they distinguish their plasm by taste, and may cause a deadly infection.

We have a particularly interesting and important species of chemical irritation in the mutual attraction of the two sex-cells, to which I gave the name of chemotropism thirty years ago, and which I described as the earliest phylogenetic source of sexual love (see theAnthropogeny, chapters vii. and xxix.). The remarkable phenomena of impregnation, the most important of all the processes of sexual generation, consist in the coalescence of the female ovum and the male sperm-cell. This could not take place if the two cells had not a sensation of their respective chemical constitution and disposition for union; they come together under this impulse. This sexual affinity is found at the lowest stages of plant life, in the protophyta and algæ. With these both cells—the smaller male microgameta and the larger female macrogameta—are often mobile, and swim about in order to effect a union. In the higher plants and animals only the small male cell is mobile as a rule, and swims towards the large immobile ovum in order to blend with it. The sensation that impels it is of a chemical nature, allied to taste and smell. This has been proved by the splendid experiments of Pfeffer, who showed that the male ciliated cells of ferns are attracted by malic acid, and those of the mosses by cane-sugar, just in the same way as by the exhalation from the female ovum. Conception depends on exactly the same erotic chemotropism in the fertilization of all the higher organisms.

Erotic chemotropism must be regarded as a general sense-function of the sexual cells in all amphigonous organisms, but in the higher organisms special forms of the sex-sense, connected with specific organs, aredeveloped; as the source of sexual love they play a most important part in the life of many of the histona. In man and most of the higher animals these feelings of love are associated with the highest features of psychic life, and have led to the formation of some most remarkable customs, instincts, and passions. Wilhelm Bölsche has given us an admirable selection from this infinitely rich and attractive realm in his famousLife of Love in Nature(1903). It is well known that this sexual sense as we have it in man has been developed from the nearest related mammals, the apes. But while it offers a shameless and repulsive spectacle in many of the apes, it has been greatly ennobled and refined in man in the development of civilization. However, the sexual sense-organs and their specific energy have remained the same. In the vertebrates and the articulates and many other metazoa the copulative organs are equipped with special cell-forms (voluptuous particles), which are the seat of intensely pleasurable feelings (see theAnthropogeny, chapter xxix., plate 30). The pubic hairs which clothe themons Venerisare also delicate organs of the sex-sense, and so are the tactile hairs about the mouth. In these cases the correlation between the sensitive forms of energy in the copulative organs and the psychic functions of the central nervous system has been remarkably developed. Moreover, a large part of the rest of the skin may co-operate as a secondary organ of the sex-sense, as is seen in the effect of caressing, stroking, embracing, kissing, etc. Goethe, at once the greatest lyric poet and the subtlest and profoundest monistic philosopher of Germany, has given unrivalled expression to this sensual, yet supersensual, basis of sexual love. Ontogeny teaches unmistakably that its elementary organs, the epidermic cells, develop entirely from the ectoderm.

By "organic sensations" modern physiology understands the perception of certain internal bodily states,which are mostly brought about by chemical stimuli (to a small extent by mechanical and other irritation) in the organs themselves. As subjective feelings of the organism itself these states are most aptly called "feelings"—the positive states, pleasure, comfort, delight; the negative, discomfort, pain, etc. These organic sensations (also called common sensations or feelings) are of great importance for the self-regulation of the complicated organism. To the positive organic sensations belong not only the bodily feeling of satiety, repose, or comfort, but also the psychic feelings of joy, good humor, mental rest, etc. Among negative common feelings we have not only hunger and thirst, bodily fatigue, bodily pain, sea-sickness, etc., but also mental strain, vertigo, bad humor, and so on. Between the two groups we have the third category of neutral organic sensations, which involve neither pleasure nor pain, but merely the perception of certain internal conditions, such as muscular strain (in lifting heavy objects), the disposal of the limbs (in crossing the legs), and so on.

Chemical sensation is just as general and important in organic nature as in the life of organisms. In this case it is nothing less than the basis of chemical affinity. No chemical process can be thoroughly understood unless we attribute a mutual sensation to the atoms, and explain their combination as due to a feeling of pleasure and their separation to a feeling of displeasure. The great Empedocles (fifth centuryB.C.) explained the origin of all things long ago by the various combination of pure elements, the interaction of love (attraction) and hate (repulsion). This attraction or repulsion is, of course, unconscious, just as in the instincts of plants and animals. If one prefers to avoid the term "sensation," it may be called "feeling" (æsthesis), while the (involuntary) movement it provokes may be called "inclination" (tropesis), and the capacity for the latter "tropism"(more recentlytaxis,cf.chapter xii. of theRiddle). We may illustrate it from the simplest case of chemical combination. When we rub together sulphur and mercury, two totally different elements, the atoms of the finely divided matter combine and form a third and different chemical body, cinnabar. How would this simple synthesis be possible unless the two elementsfeeleach other, move towards each other, andthenunite?

We find universally distributed in nature the sensation of the mechanical stimulus of gravitation, the most comprehensive statement of which is given in Newton's law of gravity. According to this fundamental and all-ruling law, any two particles of matter are attracted in direct proportion to their mass and inverse proportion to the square of their distance. This form of attraction, also, can be traced to a "sensation of matter" in the mutually attracting atoms. The local sensation that any body provokes by contact with the surface of an organism is felt as pressure (baros). A stimulus that causes this pressure alone brings about a counter-pressure as a reaction, and an effort to neutralize it, the pressure-movement (barotaxisorbarotropism). Sensitiveness to pressure or the contact of solid bodies is found throughout the organic world; it can be proved experimentally among the protists as well as the histona. Special sense-organs have been developed in the skin of the higher animals as the instruments of this pressure-sense (baræsthesis) in the form of tactile corpuscles; they are most numerous at the finger-tips and other particularly sensitive parts. In many of the higher animals there is a fine sense of touch in the feelers or tentacles, or (in the higher articulates) in the horns or antennæ. Moreover, these tactile and prehensile organs are also very widely found among the higher plants, especially the climbing plants (vines, bryony, etc.). Their slender creepers, which roll out spirally, have avery delicate feeling for the nature of the supports which they embrace; they distinguish between smooth and rough, thick and thin supports, and prefer the latter. Many of the higher plants, which are particularly sensitive to pressure, have, to an extent, special organs of touch (tentacles), and reveal this by the movements of their leaves (the sensitive plants,mimosa,dionæa,oxalis). But even among the unicellular protists we find that the contact of solid bodies has an irritating effect, the perception of which provokes corresponding movements (thigmotaxisorthigmotropismus). A peculiar form of pressure-sensation is produced in many organisms by the flow of liquids; in the mycetozoa, for instance, it provokes counter-movements (rheotaxis,rheotropismus), as Ernst Strahl showed by his experiments onæthelium septicum.

We have an interesting analogy to the thigmotaxis of the viscous living plasm in the elasticity of solid inorganic bodies, such as an elastic steel-rod. In virtue of its springy nature, the elastic rod reacts on the pressure of force that has bent it, and endeavors to regain its former position. The spiral spring sets the works of the clock in motion in virtue of its elasticity.

A very important part is played in botany by the action of gravitation on the growth of plants. The attraction towards the centre of the earth causes the positively geotropic roots to grow vertically into the earth, while the negatively geotropic stalk pushes out in the opposite direction. This applies also to a number of stationary animals which are attached to the ground by roots, such as polyps, corals, bryozoa, etc. And even the locomotion of free animals, the disposition of their bodies to the ground, the position and posture of their limbs, etc., is determined partly by the feeling of gravitation, and partly by adaptation to certain functions which resist this, as in running, swimming, and so on.All these geotropic sensations belong to the same group of barotactile phenomena, as the fall of a stone or any other effect of gravitation that depends on an inorganic feeling of attraction.

As a result of these adaptations, we find a distinct sense of space developed in the higher, free-moving animals. The feeling of the three dimensions of space becomes an important means of orientation, and in the vertebrates, from the fishes up to man, the three spiral canals in the inner ear are developed as special organs of this. These three semicircular canals, which lie vertically to each other in the three dimensions of space, are the organs of the sensation that guides the movements of the head, and, in relation to this, for the normal posture of the body and the feeling of equilibrium. If the three spiral canals are destroyed, the equilibrium is lost; the body totters and falls. Hence, these organs are not of an acoustic, but a static or geotactic character; and the same may be said of the so-called "auditory vesicles" of many of the lower animals—round vesicles which contain a liquid and a solid body, the otolith. When this body changes its position with the change of posture of the whole frame, it presses on the fine auditory hairs, or delicate terminations of the auscultory nerve, which enters the vesicle. In fact, the sense of equilibrium is often combined with the sense of hearing.

The perception of noises and tones, which we call hearing, is restricted to a section of the higher, free-moving animals; if, that is to say, the above-mentioned "auditory vesicles" in the lower animals do not have acoustic as well as static sensations. The specific sensation of hearing is due to vibration of the medium in which the animal lives (air or water), or to vibrations of solid bodies (such as tuning-forks) which are brought into touch with them. If the vibrations are irregular, they are felt as "noises"; if regular, they are heard as"tones" or notes; when a number of tones together (fundamental and over-tones) excite a complex sensation, we have "timbre." The vibrations of the sounding body are borne to the auditory cells, which represent the terminal extensions of the auscultory nerve. The specific sensation of hearing can, therefore, be traced originally to the sense of pressure, from which it has been evolved. As the organ of hearing is, like the eye, one of the principal instruments of the higher mental life, and as the refined musical hearing of civilized man is often taken to be a metaphysical power of the soul, it is important to note that here again the starting-point was purely physical—that is to say, it can be traced to the sense of pressure of matter, or gravitation.

The great importance of electricity as an agency in nature, both organic and inorganic, has only lately been fully appreciated. Electric changes are connected with many (if not, as is now supposed, with all) chemical and optical processes. Man himself and most of the higher animals have no electric organs (apart from the eye), and no sense-organs that experience a specific electric sensation. It is probably otherwise with many of the lower animals, especially those that develop free electricity, such as the electric fishes. The larvæ of frogs and embryos of fishes, if put in a vessel of water through which a galvanic current is sent, place themselves when it is closed with their longitudinal axis in the direction of the current, with the head directed to the anode and the tail to the cathode (Hermann). Again, the luminous sea-animals which cause the beautiful phenomenon of the illumination of the sea, and the glow-worms and other luminous organisms, have probably an unconscious feeling of the flow of electric energy associated with these phenomena. Many plants show a direct reaction to electric stimuli; when, for instance, we send a constant galvanic current for some time through the pointsof their roots (very sensitive organs, compared by Darwin to the brain of the animal), they bend towards the cathode.

Many of the protists are very sensitive to electric currents, as Max Verworn especially proved by a series of beautiful experiments. Most of the ciliated infusoria and many of the rhizopods (amœba) are cathodically sensitive or negatively galvanotactic. When we send a constant electric current through a drop of water in which thousands ofparamœciumare moving about, all the infusoria swim at once, with the anterior pole of the body foremost, towards the cathode or negative pole; they accumulate about it in great crowds. If the direction of the current is now changed, the whole swarm at once make in the opposite direction for the new cathode. Most of the flagellate infusoria do just the reverse; they are anodically sensitive or positively galvanotactic. In a drop of water, in which swarms ofpolytomaare moving about, all the cells swim at once towards the anode or positive pole, when an electric current is sent through. The opposite galvanotropic behavior of these two groups of infusoria in a drop of water, in which they are mixed together, is very interesting; as soon as a constant stream enters it, the ciliata fly to the cathode and the flagellata to the anode. When the current is reversed the two swarms rush at each other like hostile armies, cross in the middle of the drop, and gather at the opposite poles. These and other phenomena of galvanic sensation show clearly that the living plasm is subject to the same physical laws as the water that is decomposed into hydrogen and oxygen by an electric current. Both elementsfeelthe opposite electricities.

SCALE OF SENSATION AND IRRITABILITY

XIV

Mind and soul—Intelligence and reason—Pure reason—Kant's dualism—Anthropology—Anthropogeny—Embryology of the mind—Mind of the embryo—The canonical mind—Legal rights of the embryo—Phylogeny of the mind—Paleontology of the mind—Psyche and phronema—Mental energy—Diseases of the mind—Mental powers—Conscious and unconscious mental life—Monistic and dualistic theory—Mental life of the mammals, of savages, and of civilized and educated people.

The greatest and most commanding of all the wonders of life is unquestionably the mind of man. That function of the human organism, to which we give the name of "mind," is not only the chief source of all the higher enjoyment of life for ourselves, but it is also the power that most effectually separates man from the brute according to conventional beliefs. Hence it is supremely important for our biological philosophy to devote a few careful pages to the study of its nature, its origin and development, and its relation to the body.

At the very outset of our psychological inquiry we are met by the difficulty of giving a clear definition of "mind," and distinguishing it from "soul." Both ideas are extremely ambiguous: their content and connotation are described in the most various ways by the representatives of science. Generally speaking, we mean by mind that part of the life of the soul which is connected with consciousness and thought, and is, therefore, only found in the higher animals which have intelligence and reason.In a narrower sense reason is regarded as the proper function of mind, and as the essential prerogative of man in the animal world. In this sense Kant especially has done much to strengthen the prevailing conception of mental action, and has, by hisCritique of Pure Reason, converted philosophy into a mere "science of reason." In consequence of this conception, which still prevails widely in scientific circles, we will first study the mental life in the action of reason, and try to form a clear idea of this great wonder of life.

Psychologists and metaphysicians are of very varied opinions as to the difference between intelligence and reason. Schopenhauer, for instance, considers causality to be the sole function of intelligence, and the formation of concepts to be the province of reason; in his opinion the latter power alone marks off man from the brute. However, the power of abstraction, which collects the common features in a number of different presentations, is also found in the higher animals. Intelligent dogs not only discriminate between individual men, cats, etc., according as they are sympathetic or the reverse, but they have a general idea of man or cat, and behave very differently towards the two. On the other hand, the power of forming concepts is still so slight in uncivilized races that it rises but little above the mind of dogs, horses, etc.; the mental interval between them and civilized man is extremely wide. However, a long scale of reason unites the various stages of association of presentations which lead up to the formation of concepts; it is quite impossible to lay down a strict line of demarcation between the lower and higher mental functions of animals, or between the latter and reason. Hence the distinction between the two cerebral functions is only relative; the intelligence comprises the narrower circle of concrete and more proximate associations, while reason deals with the wider sphere of abstract and morecomprehensive groups of association. In the scientific life of the mind, therefore, the intelligence is always occupied with empirical investigation, and reason with speculative knowledge. But the two faculties are equally functions of the phronema, and depend on the normal anatomic and chemical condition of this organ of thought.

Since Kant won so great a prominence in modern philosophy for the idea of pure reason by his famousCritique(1781), it has been much discussed, especially in the modern metaphysical theory of knowledge. It has, however, like all other ideas, undergone considerable changes of meaning in the course of time. Kant himself at first understood by pure reason "reason independent of all experience." But impartial modern psychology based on the physiology of the brain and the phylogeny of its functions, has shown that there is no such thing as this purea prioriknowledge, independent of all experience. Those principles of reason which at present seem to be a priori in this sense have been attained in virtue of thousands of experiences. In so far as this is a question of real knowledge of the truth, Kant himself has frequently recognized the point. He says expressly in hisProlegomena to any future metaphysic that can be regarded as Science(1783, p. 204): "A knowledge of things by pure reason or pure intelligence is nothing but an empty appearance; only in experience is there truth." In subscribing to this empirical theory of knowledge of Kant I. and rejecting the transcendental theory of Kant II., we may on our side understand by pure reason "knowledge without prejudices," free from all dogma—all fictions of faith.

The familiar cry of modern metaphysicians, "Return to Kant," has become so general in Germany that not only nearly all metaphysicians—the official representatives of "philosophy" at our universities—but alsomany distinguished scientists, regard Kant's dualistic theory of knowledge as a necessary condition for the attainment of truth. Kant dominated philosophy in the nineteenth century much as Aristotle did in the Middle Ages. His authority became especially powerful when the prevailing Christian faith believed that his "practical reason" fully supported its own three fundamental dogmas—the personality of God, the immortality of the soul, and the freedom of the will. It overlooked the fact that Kant had utterly failed to find proofs of these dogmas in hisCritique of Pure Reason. Even conservative governments found favorable features in this dualistic philosophy. We are, therefore, forced to return once more to this mischievous system; though Kant's antinomy of the two reasons has now been refuted so often and so thoroughly that we need not dwell any further on this point.

Although the great Königsberg philosopher brought every side of human life within his comprehensive sphere of study, man remained to him—as he had been to Plato and Aristotle, Christ and Descartes—a dual being, made up of a physical body and a transcendental mind or spirit. Comparative anatomy and evolution, which have provided the solid morphological basis of monistic anthropology, did not come into existence until the beginning of the nineteenth century; they were quite unknown to Kant. He had, however, a presentiment of their importance, as Fritz Schultze has shown in his interesting work onKant and Darwin(1875). We find in various places expressions which may be described as anticipations of Darwinism. Kant also gave lectures on "Pragmatic Anthropology," and studied the psychology of races and peoples. It is remarkable that he did not arrive at a phylogenetic conception of the human mind, and a recognition of the possibility of its evolution from the mind of other vertebrates. It is clear thathe was held back from this by the profound mystic tendency of his theory of reason, and the dogma of the immortality of the soul, the freedom of the will, and the categorical imperative. Reason remained in Kant's view a transcendental phenomenon, and this dualistic error had a great influence on the whole structure of his philosophy. It must be remembered, of course, that our knowledge of the psychology of peoples was then very imperfect; but a critical study of the facts then known should have sufficed to convince him of the lower and animal condition of their minds. If Kant had had children, and followed patiently the development of the child's soul (as Preyer did a century later), he would hardly have persisted in his erroneous idea that reason, with its power of attaininga prioriknowledge, is a transcendental and supernatural wonder of life, or a unique gift to man from Heaven.

The root of the error is that Kant had no idea of the natural evolution of the mind. He did not employ the comparative and genetic methods to which we owe the chief scientific achievements of the last half-century. Kant and his followers, who confined themselves almost exclusively to the introspective method or the self-observation of their own mind, regarded as the model of the human soul the highly developed and versatile mind of the philosopher, and disregarded altogether the lower stages of mental life which we find in the child and the savage.

The immense advance made by the science of man in the second half of the nineteenth century cut the ground from under the older anthropology and the dualistic system of Kant. A number of newly founded branches of science co-operated in the work. Comparative anatomy showed that our whole complicated frame resembles that of the other mammals, and in particular differs only by slight stages of growth, and therefore inthe details of the organs, from that of the anthropoid apes. The comparative histology of the brain especially showed that this is also true of the brain, the real organ of mind. From comparative embryology we learned that man develops from a simple ovum just like the anthropoid ape; in fact, that it is almost impossible to distinguish between the ape and the human embryo even at a late stage of development. Comparative animal chemistry explained that the chemical compounds which build up our organs, and the conversions of energy which accompany its metabolism, resemble those in the other vertebrates. Comparative physiology taught us that all man's vital functions—nutrition and reproduction, movement and sensation—can be traced to the same physical laws in man as in all the other vertebrates. Above all, the comparative and experimental study of the sense-organs and the various parts of the brain showed that these organs of the mind work in the same way in man as in the other primates. Modern paleontology taught that man is, it is true, more than a hundred thousand years old, but only appeared on earth towards the close of the Tertiary Period. Prehistoric research and comparative ethnology have shown that civilized nations were preceded by older and lower races, and these by savages, which have a close bodily and mental affinity to the apes. Finally, the reformed theory of descent (1859) enabled us to unite the chief results of the various branches of anthropological study, and explain them phylogenetically by the development of man from other primates (anthropoid apes, cynocephali, lemures, etc.). By this means a new and monistic basis was provided for modern anthropology; the position assigned to man in nature by dualistic metaphysics was shown to be utterly untenable. I have attempted in the last edition of myAnthropogeny(of which an English edition is in preparation) to combine all theseresults of empirical investigation in a sketch of the natural evolution of man, paying special regard to embryology. I pointed out in chapters ii.-vi. of theRiddlehow important a part of our monistic philosophy this phylogenetic anthropology is.

The monistic conception of the human body and mind, which the theory of descent has put on a zoological basis, was bound to meet with the sternest resistance in dualistic and metaphysical circles. It was, however, also regarded with great disapproval by many modern empirical anthropologists, especially those who take it to be their chief task to make as "exact" a study as possible of the human frame, and measure and describe its various parts. We might have expected these descriptive anthropologists and ethnologists to extend a friendly hand to the new anthropogeny, and avail themselves of its leading ideas, in order to bring unity and causal connection into the enormous mass of empirical material accumulated. However, this took place only to a limited extent, The majority of anthropologists regarded evolution, and especially the evolution of man, as an undemonstrated hypothesis. They confined themselves to accumulating huge masses of raw empirical material, without having any clear aim or any definite questions in view. This was chiefly the case in Germany, where the Society of Anthropology and Prehistoric Research was for thirty years under the lead of Rudolph Virchow. This famous scientist had won great honor in connection with the reform of medicine by his cellular pathology and a number of distinguished works on pathological anatomy and histology since the middle of the nineteenth century. But when he afterwards (subsequently to his removal to Berlin, 1856) devoted himself chiefly to political and social questions, he lost sight of the great advance made in other branches of biology. He completely failed to appreciate itsgreatest achievement—the establishment of the science of evolution by Darwin. To this we must add the psychological metamorphosis (similar to that of Wundt, Baer, Dubois-Reymond, and others), of which I have spoken in the sixth chapter of theRiddle. The extraordinary authority of Virchow, and the indefatigable zeal with which he struggled every year until his death (1903) against the descent of man from other vertebrates, caused a wide-spread opposition to the doctrine of evolution. This was supported especially by Johannes Ranke, of Munich, the secretary of the Anthropological Society. Happily, a change has recently set in. However, myAnthropogenyhas remained for thirty years the only work of its kind—namely, a comprehensive treatment of man's ancestral history, especially in the light of embryology.

As I pointed out in the eighth and ninth chapters of theRiddle, the most solid foundation of our monistic psychology is the fact that the human mind grows. Like every other function of our organism, our mental activity exhibits the phenomenon of development in two directions, individually in each human being and phyletically in the whole race. The ontogeny of the mind—or the embryology of the human soul—brings before us in direct observation the various stages of development through which the mind of every man passes from the beginning to the close of life. The phylogeny of the mind—or the ancestral history of the human soul—does not afford us this direct observation; it can only be deduced by a comparison and synthesis of the historical indications which are supplied by history and prehistoric research on the one hand, and the critical study of the various stages of mental life in savages and the higher vertebrates on the other. In this the biogenetic law is used with great success (chapter xvi.).

As everybody knows, the new-born child shows as yet no trace of mind or reason or consciousness; these functions are wanting in it as completely as in the embryo from which it has been developed during the nine months in the mother's womb. Even in the ninth month, when most of the organs of the human embryo are formed and arranged as they appear later, there is no more trace of mind in its psychic life than in the ovum and spermatozoon from which it was evolved. The moment in which these sexual cells unite marks precisely the real commencement of individual existence, and therefore of the soul also (as a potential function of the plasm). But the mind proper—or reason, the higher conscious function of the soul—only develops, slowly and gradually, long after birth. As Flechsig has shown anatomically, the cortex in the new-born child is not yet organized or capable of functioning. Rational consciousness is even impossible for the child when it begins to speak; it reveals itself for the first time (after the first year) at the moment when the child speaks of itself, not in the-third person, but as "I." With this self-consciousness comes also the antithesis of the individual to the outer world, or world-consciousness. This is the real beginning of mental life.

In defining the appearance of the individual mind by the awakening of self-consciousness, we make it possible to distinguish, from the monistic physiological point of view, between "soul" (psyche) and "spirit" (pneuma). There is a soul even in the maternal ovum and the paternal spermatozoon (cf.chapter xi.); there is an individual soul in the stem-cell (cytula) which arises at conception by the blending of the parent cells. But the mind proper, the thinking reason, develops out of the animal intelligence (or earlier instincts) of the child only with the consciousness of its personality as opposed to the outer world. At the same time the child reaches thehigher stage of personality, which law has for a long time taken under its protection and made morally responsible to society by education. This shows how erroneous and untenable, from the physiological point of view, are the ideas still embodied in our code as to the psychic life and the mind of the embryo and the new-born infant. They came mostly from the canon law of the Catholic Church.

The dualistic ideas of the soul of the human embryo which were taught by the Church in the Middle Ages are particularly interesting from the psychological point of view; and at the same time they are of great practical importance even in our own day, since many of their moral consequences form an important element in canon law, and have passed from this into civil law. This influential canon law was formed under ecclesiastical authority from the decisions of Church councils and the decretals of the popes. It is, like most of the dogmas and decrees which civilization owes to this powerful hierarchy, a curious tissue of old traditions and new fictions, political dogmas, and crass superstition. It is directed to the despotic ruling of the uneducated masses and the exclusive dominion of the Church—a Church that calls itself Christian while thus acting as the very reverse of pure Christianity. The canon law takes its name from the dogmatic rules (or canons) of the Church. They involuntarily suggest the metal tubes which are so often theultima ratio regisin the wars of Christian nations. The canonical regulations of the Church, as implements of a crude spiritual despotism, have no more to do with the ethical laws of pure reason than the cannons of secular authorities have as naked organs of physical force. We might write the motto,Ultima ratio ecclesiæ(the last argument of the Church), over the sacredCorpus Juris Canonici. A collection of later papal decretals which forms an appendix to the booksof canon law was very happily given the official title ofExtravagantes. Among the "extravagant" nonsense which the papacy included in canon law as a moral code for believers is its view of the psychic life of the embryo. The "immortal soul" is supposed to enter the soulless embryo only several weeks after conception. As theologians and metaphysicians are very much divided as to the period of this entrance of the soul, and know nothing about the structure of the embryo and its development, we will only recall the fact that the human fœtus cannot be distinguished from that of the anthropoid ape and other mammals even in the sixth week of its development. The outline of the five cerebral vesicles and the three higher sense-organs (nose, eye, and ear vesicle) is discernible in the head; the two pairs of limbs can be traced in the shape of four simple roundish unjointed plates; and the pointed tail sticks out at the lower part, the rudimentary legacy from our long-tailed ape-ancestors. Although the cortex is not yet developed at this stage, the embryo may be considered to have a "soul" (cf.chapters xiv. and xv. of myAnthropogeny, and plates 8-14).

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