Those are the main points of my “gastræa theory”; I have since enlarged the first sketch of it (given in 1872), and have endeavored to substantiate it in a series of “Studies on the gastræa theory” (1873-84). Although it was almost universally rejected at first, and fiercely combated for ten years by many authorities, it is now (and has been for the last fifteen years) accepted by nearly all my colleagues. Let us now see what far-reaching consequences follow from it, and from the evolution of the germ, especially with regard to our great question, “the place of man in nature.”
The human ovum, like that of all other animals, is a single cell, and this tiny globular egg cell (about the 120th of an inch in diameter) has just the same characteristic appearance as that of all other viviparous organisms. The little ball of protoplasm is surrounded by a thick, transparent, finely reticulated membrane, called the zona pellucida; even the little, globular, germinal vesicle (the cell-nucleus), which is enclosed in the protoplasm (the cell-body), is of the same size and the same qualities as in the rest of the mammals. The same applies to the active spermatozoa of the male, the minute, threadlike, ciliated cells of which millions are found in every drop of the seminal fluid; on account of their lifelike movements they were previously taken to be forms of life, as the name indicates (spermatozoa—sperm animals). Moreover, the origin of both these important sexual cells in their respective organs is the same in man as in the other mammals; both the ova in the ovary of the female and the spermatozoa in the spermarium of the male arise in the same fashion—they always come from cells, which are originally derived from the cœlous epithelium, the layer of cells which clothes the cavity of the body.
The most important moment in the life of every man, as in that of all other complex animals, is the moment in which he begins his individual existence; it is the moment when the sexual cells of both parents meet and coalesce for the formation of a single simple cell. This new cell, the impregnated egg cell, is the individual stem cell (the cytula), the continued segmentation of which produces the cells of the germinal layers and the gastrula. With the formation of this cytula, hence in the process of conception itself, the existence of the personality, the independent individual, commences. This ontogenic fact is supremely important, for the most far-reaching conclusions may be drawn from it. In the first place, we have a clear perception that man, like all the other complex animals, inherits all his personal characteristics, bodily and mental, from his parents; and, further, we come to the momentous conclusion that the new personality which arises thus can lay no claim to “immortality.”
Hence the minute processes of conception and sexual generation are of the first importance. We are, however, only familiar with their details since 1875, when Oscar Hertwig, my pupil and fellow-traveller at that time, began his researches into the impregnation of the egg of the sea-urchin at Ajaccio, in Corsica. The beautiful capital of the island in which Napoleon the Great was born, in 1769, was also the spot in which the mysteries of animal conception were carefully studied for the first time in their most important aspects. Hertwig found that the one essential element in conception is the coalescence of the two sexual cells and their nuclei. Only one out of the millions of male ciliated cells which press round the ovum penetrates to its nucleus. The nuclei of both cells, of the spermatozoon and of the ovum, drawn together by a mysterious force, which we take to be a chemical sense-activity, related to smell, approach each other and melt into one. Thus, by the sensitive perception of the sexual nuclei, following upon a kind of “erotic chemicotropism,” a new cell is formed, which unites in itself the inherited qualities of both parents; the nucleus of the spermatozoon conveys the paternal features, the nucleus of the ovum those of the mother, to the stem cell, from which the child is to be developed. That applies both to the bodily and to the mental characteristics.
The formation of the germinal layers by the repeated division of the stem cell, the growth of the gastrula and of the later germ structures which succeed it, take place in man in just the same manner as in the other higher mammals, under the peculiar conditions which differentiate this group from the lower vertebrates. In the earlier stages of development these special characters of the placentalia are not to be detected. The significant embryonic or larval form of the chordula, which succeeds the gastrula, has substantially the same structure in all vertebrates; a simple straight rod, the dorsal cord, lies lengthways along the main axis of the shield-shaped body—the “embryonic shield”; above the cord the spinal marrow develops out of the outer germinal layer, while the gut makes its appearance underneath. Then, on both sides, to the right and left of the axial rod, appear the segments of the “pro-vertebræ” and the outlines of the muscular plates, with which the formation of the members of the vertebrate body begins. The gill-clefts appear on either side of the fore-gut; they are the openings of the gullet, through which, in our primitive fish-ancestors, the water which had entered at the mouth for breathing purposes made its exit at the sides of the head. By a tenacious heredity these gill-clefts, which have no meaning except for our fish-like aquatic ancestors, are still preserved in the embryo of man and all the other vertebrates. They disappear after a time. Even after the five vesicles of the embryonic brain appear in the head, and the rudiments of the eyes and ears at the sides, and after the legs sprout out at the base of the fish-like embryo, in the form of two roundish, flat buds, the fœtus is still so like that of other vertebrates that it is indistinguishable from them.
The substantial similarity in outer form and inner structure which characterizes the embryo of man and other vertebrates in this early stage of development is an embryological fact of the first importance; from it, by the fundamental law of biogeny, we may draw the most momentous conclusions. There is but one explanation of it—heredity from a common parent form. When we see that, at a certain stage, the embryos of man and the ape, the dog and the rabbit, the pig and the sheep, although recognizable as higher vertebrates, cannot be distinguished from each other, the fact can only be elucidated by assuming a common parentage. And this explanation is strengthened when we follow the subsequent divergence of these embryonic forms. The nearer two animals are in their bodily structure, and, therefore, in the scheme of nature, so much the longer do we find their embryos to retain this resemblance, and so much the closer do they approach each other in the ancestral tree of their respective group, so much the closer is their genetic relationship. Hence it is that the embryos of man and the anthropoid ape retain the resemblance much later, at an advanced stage of development, when their distinction from the embryos of other mammals can be seen at a glance. I have illustrated this significant fact by a juxtaposition of corresponding stages in the development of a number of different vertebrates in my Natural History of Creation and in my Anthropogeny.
The great phylogenetic significance of the resemblance we have described is seen, not only in the comparison of the embryos of vertebrates, but also in the comparison of their protective membranes. All vertebrates of the three higher classes—reptiles, birds, and mammals—are distinguished from the lower classes by the possession of certain special fœig;tal membranes, the amnion and the serolemma. The embryo is enclosed in these membranes, or bags, which are full of water, and is thus protected from pressure or shock. This provident arrangement probably arose during the Permian period, when the oldest reptiles, the proreptilia, the common ancestors of all the amniotes (animals with an amnion), completely adapted themselves to a life on land. Their direct ancestors, the amphibia, and the fishes are devoid of these fœtal membranes; they would have been superfluous to these inhabitants of the water. With the inheritance of these protective coverings are closely connected two other changes in the amniotes: firstly, the entire disappearance of the gills (while the gill arches and clefts continue to be inherited as “rudimentary organs”); secondly, the construction of the allantois. This vesicular bag, filled with water, grows out of the hind-gut in the embryo of all the amniotes, and is nothing else than an enlargement of the bladder of their amphibious ancestors. From its innermost and inferior section is formed subsequently the permanent bladder of the amniotes, while the larger outer part shrivels up. Usually this has an important part to play for a long time as the respiratory organ of the embryo, a number of large blood-vessels spreading out over its inner surface. The formation of the membranes, the amnion and the serolemma, and of the allantois, is just the same, and is effected by the same complicated process of growth, in man as in all the other amniotes; man is a true amniote.
The nourishment of the fœtus in the maternal womb is effected, as is well known, by a peculiar organ, richly supplied with blood at its surface, called the placenta. This important nutritive organ is a spongy, round disk, from six to eight inches in diameter, about an inch thick, and one or two pounds in weight; it is separated after the birth of the child, and issues as the “after-birth.” The placenta consists of two very different parts, the fœtal and the maternal part. The latter contains highly developed sinuses, which retain the blood conveyed to them by the arteries of the mother. On the other hand, the fœtal placenta is formed by innumerable branching tufts or villi, which grow out of the outer surface of the allantois, and derive their blood from the umbilical vessels. The hollow, blood-filled villi of the fœtal placenta protrude into the sinuses of the maternal placenta, and the slender membrane between the two is so attenuated that it offers no impediment to the direct interchange of material through the nutritive blood-stream (by osmosis).
In the older and lower groups of the placentals the entire surface of the chorion is covered with a number of short villi; these “chorion-villi” take the form of pit-like depressions of the mucous membrane of the mother, and are easily detached at birth. That happens in most of the ungulata (the sow, camel, mare, etc.), the cetacea, and the prosimiæ; these “mallo-placentalia” (with a diffuse placenta) have been denominated the indeciduata. The same formation is present in man and the other placentals in the beginning. It is soon modified, however, as the villi on one part of the chorion are withdrawn; while on the other part they grow proportionately stronger, and unite intimately with the mucous membrane of the womb. It is in consequence of this intimate blending that a portion of the uterus is detached at birth, and carried away with loss of blood. This detachable membrane—the decidua—is a characteristic of the higher placentalia, which have, consequently, been grouped under the title of deciduata; to that category belong the carnassia, rodentia, simiæ, and man. In the carnassia and some of the ungulata (the elephant, for instance) the placenta takes the form of a girdle, hence they are known as the zonoplacentalia; in the rodentia, the insectivora (the mole and the hedge-hog), the apes, and man, it takes the form of a disk.
Even ten years ago the majority of embryologists thought that man was distinguished by certain peculiarities in the form of the placenta—namely, by the possession of what is called the decidua reflexa, and by a special formation of the umbilical chord which unites the decidua to the fœtus. It was supposed that the rest of the placentals, including the apes, were without these special embryonic structures. The funiculus umbilicalis is a smooth, cylindrical cord, from sixteen to twenty-three inches long, and as thick as the little finger. It forms the connecting link between the fœtus and the maternal placenta, since it conducts the nutritive vessels from the body of the fœtus to the placenta; it comprises, besides, the pedicle of the allantois and the yelk-sac. The yelk-sac in the human case forms the greater portion of the germinal vesicle during the third week of gestation; but it shrivels up afterwards so that it was formerly entirely missed in the mature fœtus. Yet it remains all the time in a rudimentary condition, and may be detected even after birth as the little umbilical vesicle. Moreover, even the vesicular structure of the allantois disappears at an early stage in the human case; with a deflection of the amnion, it gives rise to the pedicle. We cannot enter here into a discussion of the complicated anatomical and embryological relations of these structures. I have described and illustrated them in my Anthropogeny (twenty-third chapter).
The opponents of evolution still appealed to these “special features” of human embryology, which were supposed to distinguish man from all the other mammals, even so late as ten years ago. But in 1890 Emil Selenka proved that the same features are found in the anthropoid apes, especially in the orang (satyrus), while the lower apes are without them. Thus Huxley’s pithecometra thesis was substantiated once more: “The differences between man and the great apes are not so great as are those between the manlike apes and the lower monkeys.” The supposed “evidences against the near blood-relationship of man and the apes” proved, on a closer examination of the real circumstances, to be strong reasons in favor of it.
Every scientist who penetrates with open eyes into this dark but profoundly interesting labyrinth of our embryonic development, and who is competent to compare it critically with that of the rest of the mammals, will find in it a most important aid towards the elucidation of the descent of our species. For the various stages of our embryonic development, in the character of palingenetic phenomena of heredity, cast a brilliant light on the corresponding stages of our ancestral tree, in accordance with the great law of biogeny. But even the cenogenetic phenomena of adaptation, the formation of the temporary fœtal organs—the characteristic fœtal membranes, and especially the placenta—gives us sufficiently definite indications of our close genetic relationship with the primates.
Origin of Man—Mythical History of Creation—Moses and Linné—The Creation of Permanent Species—The Catastrophic Theory: Cuvier—Transformism: Goethe—Theory of Descent: Lamarck—Theory of Selection: Darwin—Evolution (Phylogeny)—Ancestral Trees—General Morphology—Natural History of Creation—Systematic Phylogeny—Fundamental Law of Biogeny—Anthropogeny—Descent of Man from the Ape—Pithecoid Theory—The Fossil Pithecanthropus of Dubois
The youngest of the great branches of the living tree of biology is the science we call biological evolution, or phylogeny. It came into existence much later, and under much more difficult circumstances, than its natural sister, embryonic evolution or ontogeny. The object of the latter was to attain a knowledge of the mysterious processes by which the individual organism, plant or animal, developed from the egg. Phylogeny has to answer the much more obscure and difficult question: “What is the origin of the different organic species of plants and animals?”
Ontogeny (embryology and metamorphism) could follow the empirical method of direct observation in the solution of its not remote problem; it needed but to follow, day by day and hour by hour, the visible changes which the fœtus experiences during a brief period in the course of its development from the ovum. Much more difficult was the remote problem of phylogeny; for the slow processes of gradual construction, which effect the rise of new species of animals and plants, go on imperceptibly during thousands and even millions of years. Their direct observation is possible only within very narrow limits; the vast majority of these historical processes can only be known by direct inference—by critical reflection, and by a comparative use of empirical sciences which belong to very different fields of thought, palæontology, ontogeny, and morphology. To this we must add the immense opposition which was everywhere made to biological evolution on account of the close connection between questions of organic creation and supernatural myths and religious dogmas. For these reasons it can easily be understood how it is that the scientific existence of a true theory of origins was only secured, amid fierce controversy, in the course of the last forty years.
Every serious attempt that was made before the beginning of the nineteenth century to solve the problem of the origin of species lost its way in the mythological labyrinth of the supernatural stories of creation. The efforts of a few distinguished thinkers to emancipate themselves from this tyranny and attain to a naturalistic interpretation proved unavailing. A great variety of creation myths arose in connection with their religion in all the ancient civilized nations. During the Middle Ages triumphant Christendom naturally arrogated to itself the sole right of pronouncing on the question; and, the Bible being the basis of the structure of the Christian religion, the whole story of creation was taken from the book of Genesis. Even Carl Linné, the famous Swedish scientist, started from that basis when, in 1735, in his classical Systema Naturae, he made the first attempt at a systematic arrangement, nomenclature, and classification of the innumerable objects in nature. As the best practical aid in that attempt he introduced the well-known double or binary nomenclature; to each kind of animals and plants he gave a particular specific name, and added to it the wider-reaching name of the genus. A genus served to unite the nearest related species; thus, for instance, Linné grouped under the genus “dog” (canis), as different species, the house-dog (canis familiaris), the jackal (canis aureus), the wolf (canis lupus) the fox (canis vulpes), etc. This binary nomenclature immediately proved of such great practical assistance that it was universally accepted, and is still always followed in zoological and botanical classification.
But the theoretical dogma which Linné himself connected with his practical idea of species was fraught with the gravest peril to science. The first question which forced itself on the mind of the thoughtful scientist was the question as to the nature of the concept of species, its contents, and its range. And the creator of the idea answered this fundamental question by a naïve appeal to the dominant Mosaic legend of creation: “Species tot sunt diversae, quot diversas formas ab initio creavit infinitum ens”—(There are just so many distinct species as there were distinct types created in the beginning by the Infinite). This theosophic dogma cut short all attempt at a natural explanation of the origin of species. Linné was acquainted only with the plant and animal worlds that exist to-day; he had no suspicion of the much more numerous extinct species which had peopled the earth with their varying forms in the earlier period of its development.
It was not until the beginning of the nineteenth century that we were introduced to these fossil animals by Cuvier. In his famous work on the fossil bones of the four-footed vertebrates he gave (1812) the first correct description and true interpretation of many of these fossil remains. He showed, too, that a series of very different animal populations have succeeded each other in the various stages of the earth’s history. Since Cuvier held firmly to Linné’s idea of the absolute permanency of species, he thought their origin could only be explained by the supposition that a series of great cataclysms and new creations had marked the history of the globe; he imagined that all living creatures were destroyed at the commencement of each of these terrestrial revolutions, and an entirely new population was created at its close. Although this “catastrophic theory” of Cuvier’s led to the most absurd consequences, and was nothing more than a bald faith in miracles, it obtained almost universal recognition, and reigned triumphant until the coming of Darwin.
It is easy to understand that these prevalent ideas of the absolute unchangeability and supernatural creation of organic species could not satisfy the more penetrating thinkers. We find several eminent minds already, in the second half of the last century, busy with the attempt to find a natural explanation of the “problem of creation.” Pre-eminent among them was the great German poet and philosopher, Wolfgang Goethe, who, by his long and assiduous study of morphology, obtained, more than a hundred years ago, a clear insight into the intimate connection of all organic forms, and a firm conviction of a common natural origin. In his famed Metamorphosis of Plants (1790) he derived all the different species of plants from one primitive type, and all their different organs from one primitive organ—the leaf. In his vertebral theory of the skull he endeavored to prove that the skulls of the vertebrates—including man—were all alike made up of certain groups of bones, arranged in a definite structure, and that these bones are nothing else than transformed vertebræ. It was his penetrating study of comparative osteology that led Goethe to a firm conviction of the unity of the animal organization; he had recognized that the human skeleton is framed on the same fundamental type as that of all other vertebrates—“built on a primitive plan that only deviates more or less to one side or other in its very constant features, and still develops and refashions itself daily.” This remodelling, or transformation, is brought about, according to Goethe, by the constant interaction of two powerful constructive forces—a centripetal force within the organism, the “tendency to specification,” and a centrifugal force without, the tendency to variation, or the “idea of metamorphosis”; the former corresponds to what we now call heredity, the latter to the modern idea of adaptation. How deeply Goethe had penetrated into their character by these philosophic studies of the “construction and reconstruction of organic natures,” and how far, therefore, he must be considered the most important precursor of Darwin and Lamarck,[12] may be gathered from the interesting passages from his works which I have collected in the fourth chapter of my Natural History of Creation. These evolutionary ideas of Goethe, however, like analogous ideas of Kant, Owen, Treviranus, and other philosophers of the commencement of the century (which we have quoted in the above work), did not amount to more than certain general conclusions. They had not that great lever which the “natural history of creation” needed for its firm foundation on a criticism of the dogma of fixed species; this lever was first supplied by Lamarck.
The first thorough attempt at a scientific establishment of transformism was made at the beginning of the nineteenth century by the great French scientist Jean Lamarck, the chief opponent of his colleague, Cuvier, at Paris. He had already, in 1802, in his Observations on Living Organisms, expressed the new ideas as to the mutability and formation of species, which he thoroughly established in 1809 in the two volumes of his profound work, Philosophie Zoologique. In this work he first gave expression to the correct idea, in opposition to the prevalent dogma of fixed species, that the organic “species” is an artificial abstraction, a concept of only relative value, like the wider-ranging concepts of genus, family, order, and class. He went on to affirm that all species are changeable, and have arisen from older species in the course of very long periods of time. The common parent forms from which they have descended were originally very simple and lowly organisms. The first and oldest of them arose by abiogenesis. While the type is preserved by heredity in the succession of generations, adaptation, on the other hand, effects a constant modification of the species by change of habits and the exercise of the various organs. Even our human organism has arisen in the same natural manner, by gradual transformation, from a group of pithecoid mammals. For all these phenomena—indeed, for all phenomena both in nature and in the mind—Lamarck takes exclusively mechanical, physical, and chemical activities to be the true efficient causes. His magnificent Philosophie Zoologique contains all the elements of a purely monistic system of nature on the basis of evolution. I have fully treated these achievements of Lamarck in the fourth chapter of my Anthropogeny, and in the fourth chapter of the Natural History of Creation.
Science had now to wait until this great effort to give a scientific foundation to the theory of evolution should shatter the dominant myth of a “specific creation, and open out the path of natural” development. In this respect Lamarck was not more successful in resisting the conservative authority of his great opponent, Cuvier, than was his colleague and sympathizer, Geoffrey St. Hilaire, twenty years later. The famous controversies which he had with Cuvier in the Parisian Academy in 1830 ended with the complete triumph of the latter. I have elsewhere fully described these conflicts, in which Goethe took so lively an interest. The great expansion which the study of biology experienced at that time, the abundance of interesting discoveries in comparative anatomy and physiology, the establishment of the cellular theory, and the progress of ontogeny, gave zoologists and botanists so overwhelming a flood of welcome material to deal with that the difficult and obscure question of the origin of species was easily forgotten for a time. People rested content with the old dogma of creation. Even when Charles Lyell refuted Cuvier’s extraordinary “catastrophic theory” in his Principles of Geology, in 1830, and vindicated a natural, continuous evolution for the inorganic structure of our planet, his simple principle of continuity found no one to apply it to the inorganic world. The rudiments of a natural phylogeny which were buried in Lamarck’s works were as completely forgotten as the germ of a natural ontogeny which Caspar Friedrich Wolff had given fifty years earlier in his Theory of Generation. In both cases a full half-century elapsed before the great idea of a natural development won a fitting recognition. Only when Darwin (in 1859) approached the solution of the problem from a different side altogether, and made a happy use of the rich treasures of empirical knowledge which had accumulated in the mean time, did men begin to think once more of Lamarck as his great precursor.
The unparalleled success of Charles Darwin is well known. It shows him to-day, at the close of the century, to have been, if not the greatest, at least the most effective of its distinguished scientists. No other of the many great thinkers of our time has achieved so magnificent, so thorough, and so far-reaching a success with a single classical work as Darwin did in 1859 with his famous Origin of Species. It is true that the reform of comparative anatomy and physiology by Johannes Müller had inaugurated a new and fertile epoch for the whole of biology, that the establishment of the cellular theory by Schleiden and Schwann, the reform of ontogeny by Baer, and the formulation of the law of substance by Robert Mayer and Helmholtz were scientific facts of the first importance; but no one of them has had so profound an influence on the whole structure of human knowledge as Darwin’s theory of the natural origin of species. For it at once gave us the solution of the mystic “problem of creation,” the great “question of all questions”—the problem of the true character and origin of man himself.
If we compare the two great founders of transformism, we find in Lamarck a preponderant inclination to deduction, and to forming a completely monistic scheme of nature; in Darwin we have a predominant application of induction, and a prudent concern to establish the different parts of the theory of selection as firmly as possible on a basis of observation and experiment. While the French scientist far outran the then limits of empirical knowledge, and rather sketched the programme of future investigation, the English empiricist was mainly preoccupied about securing a unifying principle of interpretation for a mass of empirical knowledge which had hitherto accumulated without being understood. We can thus understand how it was that the success of Darwin was just as overwhelming as that of Lamarck was evanescent. Darwin, however, had not only the signal merit of bringing all the results of the various biological sciences to a common focus in the principle of descent, and thus giving them a harmonious interpretation, but he also discovered, in the principle of selection, that direct cause of transformation which Lamarck had missed. In applying, as a practical breeder, the experience of artificial selection to organisms in a state of nature, and in recognizing in the “struggle for life” the selective principle of natural selection, Darwin created his momentous “theory of selection,” which is what we properly call Darwinism.
One of the most pressing of the many important tasks which Darwin proposed to modern biology was the reform of the zoological and botanical system. Since the innumerable species of animals and plants were not created by a supernatural miracle, but evolved by natural processes, their ancestral tree is their “natural system.” The first attempt to frame a system in this sense was made by myself in 1866, in my General Morphology of Organisms. The first volume of this work (“General Anatomy”) dealt with the “mechanical science of the developed forms”; the second volume (“General Evolution”) was occupied with the science of the “developing forms.” The systematic introduction to the latter formed a “genealogical survey of the natural system of organisms.” Until that time the term “evolution” had been taken to mean exclusively, both in zoology and botany, the development of individual organisms—embryology, or metamorphic science. I established the opposite view, that this history of the embryo (ontogeny) must be completed by a second, equally valuable, and closely connected branch of thought—the history of the race (phylogeny). Both these branches of evolutionary science are, in my opinion, in the closest causal connection; this arises from the reciprocal action of the laws of heredity and adaptation; it has a precise and comprehensive expression in my “fundamental law of biogeny.”
As the new views I had put forward in my General Morphology met with very little notice, and still less acceptance, from my scientific colleagues, in spite of their severely scientific setting, I thought I would make the most important of them accessible to a wider circle of informed readers by a smaller work, written in a more popular style. This was done in 1868, in The Natural History of Creation (a series of popular scientific lectures on evolution in general, and the systems of Darwin, Goethe, and Lamarck in particular). If the success of my General Morphology was far below my reasonable anticipation, that of The Natural History of Creation went far beyond it. In a period of thirty years nine editions and twelve different translations of it have appeared. In spite of its great defects, the book has contributed much to the popularization of the main ideas of modern evolution. Still, I could only give the barest outlines in it of my chief object, the phylogenetic construction of a natural system. I have, therefore, given the complete proof, which is wanting in the earlier work, of the phylogenetic system in a subsequent larger work, my Systematic Phylogeny (outlines of a natural system of organisms on the basis of their specific development). The first volume of it deals with the protists and plants (1894), the second with the invertebrate animals (1896), the third with the vertebrates (1895). The ancestral tree of both the smaller and the larger groups is carried on in this work as far as my knowledge of the three great “ancestral documents”—palæontology, ontogeny, and morphology—qualified me to extend it.
I had already, in my General Morphology (at the end of the fifth book), described the close causative connection which exists, in my opinion, between the two branches of organic evolution as one of the most important ideas of transformism, and I had framed a precise formula for it in a number of “theses on the causal nexus of biontic and phyletic development”: “Ontogenesis is a brief and rapid recapitulation of phylogenesis, determined by the physiological functions of heredity (generation) and adaptation (maintenance).” Darwin himself had emphasized the great significance of his theory for the elucidation of embryology in 1859, and Fritz Müller had endeavored to prove it as regards the Crustacea in the able little work, Facts and Arguments for Darwin (1864). My own task has been to prove the universal application and the fundamental importance of the biogenetic law in a series of works, especially in the Biology of the Calcispongiae (1872), and in Studies on the Gastraea Theory (1873-1884). The theory of the homology of the germinal layers and of the relations of palingenesis to cenogenesis which I have exposed in them has been confirmed subsequently by a number of works of other zoologists. That theory makes it possible to follow nature’s law of unity in the innumerable variations of animal embryology; it gives us for their ancestral history a common derivation from a simple primitive stem form.
The far-seeing founder of the theory of descent, Lamarck, clearly recognized in 1809 that it was of universal application; that even man himself, the most highly developed of the mammals, is derived from the same stem as all the other mammals; and that this in its turn belongs to the same older branch of the ancestral tree as the rest of the vertebrates. He had even indicated the agencies by which it might be possible to explain man’s descent from the apes as the nearest related mammals. Darwin, who was, naturally, of the same conviction, purposely avoided this least acceptable consequence of his theory in his chief work in 1859, and put it forward for the first time in his Descent of Man in 1871. In the mean time (1863) Huxley had very ably discussed this most important consequence of evolution in his famous Place of Man in Nature. With the aid of comparative anatomy and ontogeny, and the support of the facts of palæontology, Huxley proved that the “descent of man from the ape” is a necessary consequence of Darwinism, and that no other scientific explanation of the origin of the human race is possible. Of the same opinion was Karl Gegenbaur, the most distinguished representative of comparative anatomy, who lifted his science to a higher level by a consistent and ingenious application of the theory of descent.
As a further consequence of the “pithecoid theory” (the theory of the descent of man from the ape) there now arose the difficult task of investigating, not only the nearest related mammal ancestors of man in the Tertiary epoch, but also the long series of the older animal ancestors which had lived in earlier periods of the earth’s history and been developed in the course of countless millions of years. I had made a start with the hypothetical solution of this great historic problem in my General Morphology; a further development of it appeared in 1874 in my Anthropogeny (first section, Origin of the Individual; second section, Origin of the Race). The fourth, enlarged, edition of this work (1891) contains that theory of the development of man which approaches nearest, in my own opinion, to the still remote truth, in the light of our present knowledge of the documentary evidence. I was especially preoccupied in its composition to use the three empirical “documents”—palæontology, ontogeny, and morphology (or comparative anatomy)—as evenly and harmoniously as possible. It is true that my hypotheses were in many cases supplemented and corrected in detail by later phylogenetic research; yet I am convinced that the ancestral tree of human origin which I have sketched therein is substantially correct. For the historical succession of vertebrate fossils corresponds completely with the morphological evolutionary scale which is revealed to us by comparative anatomy and ontogeny. After the Silurian fishes come the dipnoi of the Devonian period—the Carboniferous amphibia, the Permian reptilia, and the Mesozoic mammals. Of these, again, the lowest forms, the monotremes, appear first in the Triassic period, the marsupials in the Jurassic, and then the oldest placentals in the Cretaceous. Of the placentals, in turn, the first to appear in the oldest Tertiary period (the Eocene) are the lowest primates, the prosimiæ, which are followed by the simiæ in the Miocene. Of the catarrhinæ, the cynopitheci precede the anthropomorpha; from one branch of the latter, during the Pliocene period, arises the ape-man without speech (the pithecanthropus alalus); and from him descends, finally, speaking man.
The chain of our earlier invertebrate ancestors is much more difficult to investigate and much less safe than this tree of our vertebrate predecessors; we have no fossilized relics of their soft, boneless structures, so palæontology can give us no assistance in this case. The evidence of comparative anatomy and ontogeny, therefore, becomes all the more important. Since the human embryo passes through the same chordula-stage as the germs of all other vertebrates, since it evolves, similarly, out of two germinal layers of a gastrula, we infer, in virtue of the biogenetic law, the early existence of corresponding ancestral forms—vermalia, gastræada, etc. Most important of all is the fact that the human embryo, like that of all other animals, arises originally from a single cell; for this “stem-cell” (cytula)—the impregnated egg cell—points indubitably to a corresponding unicellular ancestor, a primitive, Laurentian protozoon.
For the purpose of our monistic philosophy, however, it is a matter of comparative indifference how the succession of our animal predecessors may be confirmed in detail. Sufficient for us, as an incontestable historical fact, is the important thesis that man descends immediately from the ape, and secondarily from a long series of lower vertebrates. I have laid stress on the logical proof of this “pithecometra-thesis” in the seventh book of the General Morphology: “The thesis that man has been evolved from lower vertebrates, and immediately from the simiae, is a special inference which results with absolute necessity from the general inductive law of the theory of descent.”
For the definitive proof and establishment of this fundamental pithecometra-thesis the palæontological discoveries of the last thirty years are of the greatest importance; in particular, the astonishing discoveries of a number of extinct mammals of the Tertiary period have enabled us to draw up clearly in its main outlines the evolutionary history of this most important class of animals, from the lowest oviparous monotremes up to man. The four chief groups of the placentals, the heterogeneous legions of the carnassia, the rodentia, the ungulata, and the primates, seem to be separated by profound gulfs, when we confine our attention to their representatives of to-day. But these gulfs are completely bridged, and the sharp distinctions of the four legions are entirely lost, when we compare their extinct predecessors of the Tertiary period, and when we go back into the Eocene twilight of history, in the oldest part of the Tertiary period—at least three million years ago. There we find the great sub-class of the placentals, which to-day comprises more than two thousand five hundred species, represented by only a small number of little, insignificant “proplacentals”; and in these prochoriata the characters of the four divergent legions are so intermingled and toned down that we cannot in reason do other than consider them as the precursors of those features. The oldest carnassia (the ictopsales), the oldest rodentia (the esthonychales), the oldest ungulata (the condylarthrales) and the oldest primates (the lemuravales), all have the same fundamental skeletal structure, and the same typical dentition of the primitive placentals, consisting of forty-four teeth (three incisors, one canine, four premolars, and three molars in each half of the jaw); all are characterized by the small size and the imperfect structure of the brain (especially of its chief part, the cortex, which does not become a true “organ of thought” until later on in the Miocene and Pliocene representatives); they have all short legs and five-toed, flat-soled feet (plantigrada). In many cases among these oldest placentals of the Eocene period it was very difficult to say at first whether they should be classed with the carnassia, rodentia, ungulata, or primates; so very closely, even to confusion, do these four groups of the placentals, which diverge so widely afterwards, approach each other at that time. Their common origin from a single ancestral group follows incontestably. These prochoriata lived in the preceding Cretaceous period (more than three million years ago), and were probably developed in the Jurassic period from a group of insectivorous marsupials (amphitheria) by the formation of a primitive placenta diffusa, a placenta of the simplest type.
But the most important of all the recent palaeontological discoveries which have served to elucidate the origin of the placentals relate to our own stem, the legion of primates. Formerly fossil remains of the primates were very scarce. Even Cuvier, the great founder of palaeontology, maintained until his last day (1832) that there were no fossilized primates; he had himself, it is true, described the skull of an Eocene prosimiæ (adapis), but he had wrongly classed it with the ungulata. However, during the last twenty years a fair number of well-preserved fossilized skeletons of prosimiæ and simiæ have been discovered; in them we find all the chief intermediate members which complete the connecting chain of ancestors from the oldest prosimiæ to man.
The most famous and most interesting of these discoveries is the fossil ape-man of Java, the much-talked-of pithecanthropus erectus, found by a Dutch military doctor, Eugen Dubois, in 1894. It is in truth the much-sought “missing link,” supposed to be wanting in the chain of primates, which stretches unbroken from the lowest catarrhinæ to the highest-developed man. I have dealt exhaustively with the significance of this discovery in the paper which I read on August 26, 1898, at the Fourth International Zoological Congress at Cambridge.[13] The palæontologist, who knows the conditions of the formation and preservation of fossils, will think the discovery of the pithecanthropus an unusually lucky accident. The apes, being arboreal, seldom came into the circumstances (unless they happened to fall into the water) which would secure the preservation and petrifaction of their skeleton. Thus, by the discovery of this fossil man-monkey of Java the descent of man from the ape has become just as clear and certain from the palæontological side as it was previously from the evidence of comparative anatomy and ontogeny. We now have all the principal documents which tell the history of our race.
Fundamental Importance of Psychology—Its Definition and Methods—Divergence of Views Thereon—Dualistic and Monistic Psychology—Relation to the Law of Substance—Confusion of Ideas—Psychological Metamorphoses: Kant, Virchow, Du Bois-Reymond—Methods of Research of Psychic Science—Introspective Method (Self-Observation)—Exact Method (Psycho-Physics)—Comparative Method (Animal Psychology)—Psychological Change of Principles: Wundt—Folk-Psychology and Ethnography: Bastian—Ontogenetic Psychology: Preyer—Phylogenetic Psychology: Darwin, Romanes
The phenomena which are comprised under the title of the “life of the soul,” or the psychic activity, are, on the one hand, the most important and interesting, on the other the most intricate and problematical, of all the phenomena we are acquainted with. As the knowledge of nature, the object of the present philosophic study, is itself a part of the life of the soul, and as anthropology, and even cosmology, presuppose a correct knowledge of the “psyche,” we may regard psychology, the scientific study of the soul, both as the foundation and the postulate of all other sciences. From another point of view it is itself a part of philosophy, or physiology, or anthropology.
The great difficulty of establishing it on a naturalistic basis arises from the fact that psychology, in turn, presupposes a correct acquaintance with the human organism, especially the brain, the chief organ of psychic activity. The great majority of “psychologists” have little or no acquaintance with these anatomical foundations of the soul, and thus it happens that in no other science do we find such contradictions and untenable notions as to its proper meaning and its essential object as are current in psychology. This confusion has become more and more palpable during the last thirty years, in proportion as the immense progress of anatomy and physiology has increased our knowledge of the structure and the functions of the chief psychic organ.
What we call the soul is, in my opinion, a natural phenomenon; I therefore consider psychology to be a branch of natural science—a section of physiology. Consequently, I must emphatically assert from the commencement that we have no different methods of research for that science than for any of the others; we have in the first place observation and experiment, in the second place the theory of evolution, and in the third place metaphysical speculation, which seek to penetrate as far as possible into the cryptic nature of the phenomena by inductive and deductive reasoning. However, with a view to a thorough appreciation of the question, we must first of all put clearly before the reader the antithesis of the dualistic and the monistic theories.
The prevailing conception of the psychic activity, which we contest, considers soul and body to be two distinct entities. These two entities can exist independently of each other; there is no intrinsic necessity for their union. The organized body is a mortal, material nature, chemically composed of living protoplasm and its compounds (plasma-products). The soul, on the other hand, is an immortal, immaterial being, a spiritual agent, whose mysterious activity is entirely incomprehensible to us. This trivial conception is, as such, spiritualistic, and its contradictory is, in a certain sense, materialistic. It is, at the same time, supernatural and transcendental, since it affirms the existence of forces which can exist and operate without a material basis; it rests on the assumption that outside of and beyond nature there is a “spiritual,” immaterial world, of which we have no experience, and of which we can learn nothing by natural means.
This hypothetical “spirit world,” which is supposed to be entirely independent of the material universe, and on the assumption of which the whole artificial structure of the dualistic system is based, is purely a product of poetic imagination; the same must be said of the parallel belief in the “immortality of the soul,” the scientific impossibility of which we must prove more fully later on (chap. xi.). If the beliefs which prevail in these credulous circles had a sound foundation, the phenomena they relate to could not be subject to the “law of substance”; moreover, this single exception to the highest law of the cosmos must have appeared very late in the history of the organic world, since it only concerns the “soul” of man and of the higher animals. The dogma of “free will,” another essential element of the dualistic psychology, is similarly irreconcilable with the universal law of substance.
Our own naturalistic conception of the psychic activity sees in it a group of vital phenomena, which are dependent on a definite material substratum, like all other phenomena. We shall give to this material basis of all psychic activity, without which it is inconceivable, the provisional name of “psychoplasm”; and for this good reason—that chemical analysis proves it to be a body of the group we call protoplasmic bodies the albuminoid carbon-combinations which are at the root of all vital processes. In the higher animals, which have a nervous system and sense-organs, “neuroplasm,” the nerve-material, has been differentiated out of psychoplasm. Our conception is, in this sense, materialistic. It is at the same time empirical and naturalistic, for our scientific experience has never yet taught us the existence of forces that can dispense with a material substratum, or of a spiritual world over and above the realm of nature.
Like all other natural phenomena, the psychic processes are subject to the supreme, all-ruling law of substance; not even in this province is there a single exception to this highest cosmological law (compare chap. xii.). The phenomena of the lowly psychic life of the unicellular protist and the plant, and of the lowest animal forms—their irritability, their reflex movements, their sensitiveness and instinct of self-preservation—are directly determined by physiological action in the protoplasm of their cells—that is, by physical and chemical changes which are partly due to heredity and partly to adaptation. And we must say just the same of the higher psychic activity of the higher animals and man, of the formation of ideas and concepts, of the marvellous phenomena of reason and consciousness; for the latter have been phylogenetically evolved from the former, and it is merely a higher degree of integration or centralization, of association or combination of functions which were formerly isolated, that has elevated them in this manner.
The first task of every science is the clear definition of the object it has to investigate. In no science, however, is this preliminary task so difficult as in psychology; and this circumstance is the more remarkable since logic, the science of defining, is itself a part of psychology. When we compare all that has been said by the most distinguished philosophers and scientists of all ages on the fundamental idea of psychology, we find ourselves in a perfect chaos of contradictory notions. What, really, is the “soul”? What is its relation to the “mind”? What is the inner meaning of “consciousness”? What is the difference between “sensation” and “sentiment”? What is “instinct”? What is the meaning of “free will”? What is “presentation”? What is the difference between “intellect” and “reason”? What is the true nature of “emotion”? What is the relation between all these “psychic phenomena” and the “body”? The answers to these and many other cognate questions are infinitely varied; not only are the views of the most eminent thinkers on these questions widely divergent, but even the same scientific authority has often completely changed his views in the course of his psychological development. Indeed, this “psychological metamorphosis” of so many thinkers has contributed not a little to the colossal confusion of ideas which prevails in psychology more than in any other branch of knowledge.
The most interesting example of such an entire change of objective and subjective psychological opinions is found in the case of the most influential leader of German philosophy, Immanuel Kant. The young, severely critical Kant came to the conclusion that the three great buttresses of mysticism—“God, freedom, and immortality”—were untenable in the light of “pure reason”; the older, dogmatic Kant found that these three great hallucinations were postulates of “practical reason,” and were, as such, indispensable. The more the distinguished modern school of “Neokantians” urges a “return to Kant” as the only possible salvation from the frightful jumble of modern metaphysics, the more clearly do we perceive the undeniable and fatal contradiction between the fundamental opinions of the young and the older Kant. We shall return to this point later on.
Other interesting examples of this change of views are found in two of the most famous living scientists, R. Virchow and E. du Bois-Reymond; the metamorphoses of their fundamental views on psychology cannot be overlooked, as both these Berlin biologists have played a most important part at Germany’s greatest university for more than forty years, and have, therefore, directly and indirectly, had a most profound influence on the modern mind. Rudolph Virchow, the eminent founder of cellular pathology, was a pure monist in the best days of his scientific activity, about the middle of the century; he passed at that time as one of the most distinguished representatives of the newly awakened materialism, which appeared in 1855, especially through two famous works, almost contemporaneous in appearance—Ludwig Büchner’s Matter and Force and Carl Vogt’s Superstition and Science. Virchow published his general biological views on the vital processes in man—which he takes to be purely mechanical natural phenomena—in a series of distinguished papers in the first volumes of the Archiv für pathologische Anatomie, which he founded. The most important of these articles, and the one in which he most clearly expresses his monistic views of that period, is that on “The Tendencies Towards Unity in Scientific Medicine” (1849). It was certainly not without careful thought, and a conviction of its philosophic value, that Virchow put this “medical confession of faith” at the head of his Collected Essays on Scientific Medicine in 1856. He defended in it, clearly and definitely, the fundamental principles of monism, which I am presenting here with a view to the solution of the world-problem; he vindicated the exclusive title of empirical science, of which the only reliable sources are sense and brain activity; he vigorously attacked anthropological dualism, the alleged “revelation,” and the transcendental philosophy, with their two methods—“faith and anthropomorphism.” Above all, he emphasized the monistic character of anthropology, the inseparable connection of spirit and body, of force and matter. “I am convinced,” he exclaims, at the end of his preface, “that I shall never find myself compelled to deny the thesis of the unity of human nature.” Unhappily, this “conviction” proved to be a grave error. Twenty-eight years afterwards Virchow represented the diametrically opposite view; it is to be found in the famous speech on “The Liberty of Science in Modern States,” which he delivered at the Scientific Congress at Munich in 1877, and which contains attacks that I have repelled in my Free Science and Free Teaching (1878).
In Emil du Bois-Reymond we find similar contradictions with regard to the most important and fundamental theses of philosophy. The more completely the distinguished orator of the Berlin Academy had defended the main principles of the monistic philosophy, the more he had contributed to the refutation of vitalism and the transcendental view of life, so much the louder was the triumphant cry of our opponents when in 1872, in his famous Ignorabimus-Speech, he spoke of consciousness as an insoluble problem, and opposed it to the other functions of the brain as a supernatural phenomenon. I return to the point in the tenth chapter.
The peculiar character of many of the psychic phenomena, especially of consciousness, necessitates certain modifications of our ordinary scientific methods. We have, for instance, to associate with the customary objective, external observation, the introspective method, the subjective, internal observation which scrutinizes our own personality in the mirror of consciousness. The majority of psychologists have started from this “certainty of the ego”: “Cogito ergo sum,” as Descartes said—I think, therefore I am. Let us first cast a glance at this way of inquiry, and then deal with the second, complementary, method.
By far the greater part of the theories of the soul which have been put forward during the last two thousand years or more are based on introspective inquiry—that is, on “self-observation,” and on the conclusions which we draw from the association and criticism of these subjective experiences. Introspection is the only possible method of inquiry for an important section of psychology, especially for the study of consciousness. Hence this cerebral function occupies a special position, and has been a more prolific source of philosophic error than any of the others (cf. chap. x.). It is, however, most unsatisfactory, and it leads to entirely false or incomplete notions, to take this self-observation of the mind to be the chief, or, especially, to be the only source of mental science, as has happened in the case of many and distinguished philosophers. A great number of the principal psychic phenomena, particularly the activity of the senses and speech, can only be studied in the same way as every other vital function of the organism—that is, firstly, by a thorough anatomical study of their organs, and, secondly, by an exact physiological analysis of the functions which depend on them. In order, however, to complete this external study of the mental life, and to supplement the results of internal observation, one needs a thorough knowledge of human anatomy, histology, ontogeny, and physiology. Most of our so-called “psychologists” have little or no knowledge of these indispensable foundations of anthropology; they are, therefore, incompetent to pronounce on the character even of their own “soul.” It must be remembered, too, that the distinguished personality of one of these psychologists usually offers a specimen of an educated mind of the highest civilized races; it is the last link of a long ancestral chain, and the innumerable older and inferior links are indispensable for its proper understanding. Hence it is that most of the psychological literature of the day is so much waste paper. The introspective method is certainly extremely valuable and indispensable; still it needs the constant co-operation and assistance of the other methods.
In proportion as the various branches of the human tree of knowledge have developed during the century, and the methods of the different sciences have been perfected, the desire has grown to make them exact; that is, to make the study of phenomena as purely empirical as possible, and to formulate the laws that result as clearly as the circumstances permit—if possible, mathematically. The latter is, however, only feasible in a small province of human knowledge, especially in those sciences in which there is question of measurable quantities; in mathematics, in the first place, and to a greater or less extent in astronomy, mechanics, and a great part of physics and chemistry. Hence these studies are called “exact sciences” in the narrower sense. It is, however, productive only of error to call all the physical sciences exact, and oppose them to the historical, mental, and moral sciences. The greater part of physical science can no more be treated as an exact science than history can; this is especially true of biology and of its subsidiary branch, psychology. As psychology is a part of physiology, it must, as a general rule, follow the chief methods of that science. It must establish the facts of psychic activity by empirical methods as much as possible, by observation and experiment, and it must then gather the laws of the mind by inductive and deductive inferences from its observations, and formulate them with the utmost distinctness. But, for obvious reasons, it is rarely possible to formulate them mathematically. Such a procedure is only profitable in one section of the physiology of the senses; it is not practicable in the greater part of cerebral physiology.
One small section of physiology, which seems amenable to the “exact” method of investigation, has been carefully studied for the last twenty years and raised to the position of a separate science under the title of psycho-physics. Its founders, the physiologists Theodor Fechner and Ernst Heinrich Weber, first of all closely investigated the dependence of sensations on the external stimuli that act on the organs of sense, and particularly the quantitative relation between the strength of the stimulus and the intensity of the sensation. They found that a certain minimum strength of stimulus is requisite for the excitement of a sensation, and that a given stimulus must be varied to a definite amount before there is any perceptible change in the sensation. For the highest sensations (of sight, hearing, and pressure) the law holds good that their variations are proportionate to the changes in the strength of the stimulus. From this empirical “law of Weber” Fechner inferred, by mathematical operations, his “fundamental law of psycho-physics,” according to which the intensity of a sensation increases in arithmetical progression, the strength of the stimulus in geometrical progression. However, Fechner’s law and other psycho-physical laws are frequently contested, and their “exactness” is called into question. In any case modern psycho-physics has fallen far short of the great hopes with which it was greeted twenty years ago; the field of its applicability is extremely limited. One important result of its work is that it has proved the application of physical laws in one, if only a small, branch of the life of the “soul”—an application which was long ago postulated on principle by the materialist psychology for the whole province of mental life. In this, as in many other branches of physiology, the “exact” method has proved inadequate and of little service. It is the ideal to aim at everywhere, but it is unattainable in most cases. Much more profitable are the comparative and genetic methods.
The striking resemblance of man’s psychic activity to that of the higher animals—especially our nearest relatives among the mammals—is a familiar fact. Most uncivilized races still make no material distinction between the two sets of mental processes, as the well-known animal fables, the old legends, and the idea of the transmigration of souls prove. Even most of the philosophers of classical antiquity shared the same conviction, and discovered no essential qualitative difference, but merely a quantitative one, between the soul of man and that of the brute. Plato himself, who was the first to draw a fundamental distinction between soul and body, made one and the same soul (or “idea”) pass through a number of animal and human bodies in his theory of metempsychosis. It was Christianity, intimately connecting faith in immortality with faith in God, that emphasized the essential difference of the immortal soul of man from the mortal soul of the brute. In the dualistic philosophy the idea prevailed principally through the influence of Descartes (1643); he contended that man alone had a true “soul,” and, consequently, sensation and free will, and that the animals were mere automata, or machines, without will or sensibility. Ever since the majority of psychologists—including even Kant—have entirely neglected the mental life of the brute, and restricted psychological research to man: human psychology, mainly introspective, dispensed with the fruitful comparative method, and so remained at that lower point of view which human morphology took before Cuvier raised it to the position of a “philosophic science” by the foundation of comparative anatomy.
Scientific interest in the psychic activity of the brute was revived in the second half of the last century, in connection with the advance of systematic zoology and physiology. A strong impulse was given to it by the work of Reimarus: “General observations on the instincts of animals” (Hamburg, 1760). At the same time a deeper scientific investigation had been facilitated by the thorough reform of physiology by Johannes Müller. This distinguished biologist, having a comprehensive knowledge of the whole field of organic nature, of morphology, and of physiology, introduced the “exact methods” of observation and experiment into the whole province of physiology, and, with consummate skill, combined them with the comparative methods. He applied them, not only to mental life in the broader sense (to speech, senses, and brain-action), but to all the other phenomena of life. The sixth book of his Manual of Human Physiology treats specially of the life of the soul, and contains eighty pages of important psychological observations.
During the last forty years a great number of works on comparative animal psychology have appeared, principally occasioned by the great impulse which Darwin gave in 1859 by his work on The Origin of Species, and by the application of the idea of evolution to the province of psychology. The more important of these works we owe to Romanes and Sir J. Lubbock, in England; to W. Wundt, L. Büchner, G. Schneider, Fritz Schultze, and Karl Groos, in Germany; to Alfred Espinas and E. Jourdan, in France; and to Tito Vignoli, in Italy.
In Germany, Wilhelm Wundt, of Leipzig, is considered to be the ablest living psychologist; he has the inestimable advantage over most other philosophers of a thorough zoological, anatomical, and physiological education. Formerly assistant and pupil of Helmholtz, Wundt had early accustomed himself to follow the application of the laws of physics and chemistry through the whole field of physiology, and, consequently, in the sense of Johannes Müller, in psychology, as a subsection of the latter. Starting from this point of view, Wundt published his valuable “Lectures on human and animal psychology” in 1863. He proved, as he himself tells us in the preface, that the theatre of the most important psychic processes is in the “unconscious soul,” and he affords us “a view of the mechanism which, in the unconscious background of the soul, manipulates the impressions which arise from the external stimuli.” What seems to me, however, of special importance and value in Wundt’s work is that he “extends the law of the persistence of force for the first time to the psychic world, and makes use of a series of facts of electro-physiology by way of demonstration.”
Thirty years afterwards (1892) Wundt published a second, much abridged and entirely modified, edition of his work. The important principles of the first edition are entirely abandoned in the second, and the monistic is exchanged for a purely dualistic stand-point. Wundt himself says in the preface to the second edition that he has emancipated himself from the fundamental errors of the first, and that he “learned many years ago to consider the work a sin of his youth”; it “weighed on him as a kind of crime, from which he longed to free himself as soon as possible.” In fact, the most important systems of psychology are completely opposed to each other in the two editions of Wundt’s famous Observations. In the first edition he is purely monistic and materialistic, in the second edition purely dualistic and spiritualistic. In the one psychology is treated as a physical science, on the same laws as the whole of physiology, of which it is only a part; thirty years afterwards he finds psychology to be a spiritual science, with principles and objects entirely different from those of physical science. This conversion is most clearly expressed in his principle of psycho-physical parallelism, according to which “every psychic event has a corresponding physical change”; but the two are completely independent, and are not in any natural causal connection. This complete dualism of body and soul, of nature and mind, naturally gave the liveliest satisfaction to the prevailing school-philosophy, and was acclaimed by it as an important advance, especially seeing that it came from a distinguished scientist who had previously adhered to the opposite system of monism. As I myself continue, after more than forty years’ study, in this “narrow” position, and have not been able to free myself from it in spite of all my efforts, I must naturally consider the “youthful sin” of the young physiologist Wundt to be a correct knowledge of nature, and energetically defend it against the antagonistic view of the old philosopher Wundt.
This entire change of philosophical principles, which we find in Wundt, as we found it in Kant, Virchow, Du Bois-Reymond, Karl Ernst Baer, and others, is very interesting. In their youth these able and talented scientists embrace the whole field of biological research in a broad survey, and make strenuous efforts to find a unifying, natural basis for their knowledge; in their later years they have found that this is not completely attainable, and so they entirely abandon the idea. In extenuation of these psychological metamorphoses they can, naturally, plead that in their youth they overlooked the difficulties of the great task, and misconceived the true goal; with the maturer judgment of age and the accumulation of experience they were convinced of their errors, and discovered the true path to the source of truth. On the other hand, it is possible to think that great scientists approach their task with less prejudice and more energy in their earlier years—that their vision is clearer and their judgment purer; the experiences of later years sometimes have the effect, not of enriching, but of disturbing, the mind, and with old age there comes a gradual decay of the brain, just as happens in all other organs. In any case, this change of views is in itself an instructive psychological fact; because, like many other forms of change of opinion, it shows that the highest psychic functions are subject to profound individual changes in the course of life, like all the other vital processes.
For the profitable construction of comparative psychology it is extremely important not to confine the critical comparison to man and the brute in general, but to put side by side the innumerable gradations of their mental activity. Only thus can we attain a clear knowledge of the long scale of psychic development which runs unbroken from the lowest, unicellular forms of life up to the mammals, and to man at their head. But even within the limits of our own race such gradations are very noticeable, and the ramifications of the “psychic ancestral tree” are very numerous. The psychic difference between the crudest savage of the lowest grade and the most perfect specimen of the highest civilization is colossal—much greater than is commonly supposed. By the due appreciation of this fact, especially in the latter half of the century, the “Anthropology of the uncivilized races” (Waitz) has received a strong support, and comparative ethnography has come to be considered extremely important for psychological purposes. Unfortunately, the enormous quantity of raw material of this science has not yet been treated in a satisfactory critical manner. What confused and mystic ideas still prevail in this department may be seen, for instance, in the Völkergedanke of the famous traveller, Adolf Bastian, who, though a prolific writer, merely turns out a hopeless mass of uncritical compilation and confused speculation.