Transcriber's Note:

Every effort has been made to replicate this text as faithfully as possible, including inconsistencies in hyphenation. It seems that the italic typeface used in this book did not have an ae ligature. Names of genera and higher taxonomic groups are not capitalized in the printed book: they have bee left unchanged. Some changes have been made. They are listed at the end of the text.

THE RIDDLE OF THE UNIVERSE

ERNST HAECKEL

THE RIDDLE
OF THE UNIVERSE
AT THE CLOSE OF
THE NINETEENTH CENTURY

BY

ERNST HAECKEL
(Ph.D., M.D., LL.D., Sc.D., and Professor at the University of Jena)

AUTHOR OF “THE HISTORY OF CREATION”
“THE EVOLUTION OF MAN” ETC.

TRANSLATED BY

JOSEPH McCABE

HARPER & BROTHERS PUBLISHERS
NEW YORK AND LONDON
1905

Copyright, 1900, by Harper & Brothers.

All rights reserved.

[CONTENTS]

[AUTHOR’S PREFACE]

The present study of the monistic philosophy is intended for thoughtful readers of every condition who are united in an honest search for the truth. An intensification of this effort of man to attain a knowledge of the truth is one of the most salient features of the nineteenth century. That is easily explained, in the first place, by the immense progress of science, especially in its most important branch, the history of humanity; it is due, in the second place, to the open contradiction that has developed during the century between science and the traditional “Revelation”; and, finally, it arises from the inevitable extension and deepening of the rational demand for an elucidation of the innumerable facts that have been recently brought to light, and for a fuller knowledge of their causes.

Unfortunately, this vast progress of empirical knowledge in our “Century of Science” has not been accompanied by a corresponding advancement of its theoretical interpretation—that higher knowledge of the causal nexus of individual phenomena which we call philosophy. We find, on the contrary, that the abstract and almost wholly metaphysical science which has been taught in our universities for the last hundred years under the name of “philosophy” is far from assimilating our hard-earned treasures of experimental research. On the other hand, we have to admit, with equal regret, that most of the representatives of what is called “exact science” are content with the special care of their own narrow branches of observation and experiment, and deem superfluous the deeper study of the universal connection of the phenomena they observe—that is, philosophy. While these pure empiricists “do not see the wood for the trees,” the metaphysicians, on the other hand, are satisfied with the mere picture of the wood, and trouble not about its individual trees. The idea of a “philosophy of nature,” to which both those methods of research, the empirical and the speculative, naturally converge, is even yet contemptuously rejected by large numbers of representatives of both tendencies.

This unnatural and fatal opposition between science and philosophy, between the results of experience and of thought, is undoubtedly becoming more and more onerous and painful to thoughtful people. That is easily proved by the increasing spread of the immense popular literature of “natural philosophy” which has sprung up in the course of the last half-century. It is seen, too, in the welcome fact that, in spite of the mutual aversion of the scientific observer and the speculative philosopher, nevertheless eminent thinkers from both camps league themselves in a united effort to attain the solution of that highest object of inquiry which we briefly denominate the “world-riddles.” The studies of these “world-riddles” which I offer in the present work cannot reasonably claim to give a perfect solution of them; they merely offer to a wide circle of readers a critical inquiry into the problem, and seek to answer the question as to how nearly we have approached that solution at the present day. What stage in the attainment of truth have we actually arrived at in this closing year of the nineteenth century? What progress have we really made during its course towards that immeasurably distant goal?

The answer which I give to these great questions must, naturally, be merely subjective and only partly correct; for my knowledge of nature and my ability to interpret its objective reality are limited, as are those of every man. The one point that I can claim for it, and which, indeed, I must ask of my strongest opponents, is that my Monistic Philosophy is sincere from beginning to end—it is the complete expression of the conviction that has come to me, after many years of ardent research into Nature and unceasing reflection, as to the true basis of its phenomena. For fully half a century has my mind’s work proceeded, and I now, in my sixty-sixth year, may venture to claim that it is mature; I am fully convinced that this “ripe fruit” of the tree of knowledge will receive no important addition and suffer no substantial modification during the brief spell of life that remains to me.

I presented all the essential and distinctive elements of my monistic and genetic philosophy thirty-three years ago, in my General Morphology of Organisms, a large and laborious work, which has had but a limited circulation. It was the first attempt to apply in detail the newly established theory of evolution to the whole science of organic forms. In order to secure the acceptance of at least one part of the new thought which it contained, and to kindle a wider interest in the greatest advancement of knowledge that our century has witnessed, I published my Natural History of Creation two years afterwards. As this less complicated work, in spite of its great defects, ran into nine large editions and twelve different translations, it has contributed not a little to the spread of monistic views. The same may be said of the less known Anthropogeny[1] (1874), in which I set myself the difficult task of rendering the most important facts of the theory of man’s descent accessible and intelligible to the general reader; the fourth, enlarged, edition of that work appeared in 1891. In the paper which I read at the fourth International Congress of Zoology at Cambridge, in 1898, on “Our Present Knowledge of the Descent of Man”[2] (a seventh edition of which appeared in 1899), I treated certain significant and particularly valuable advances which this important branch of anthropology has recently made. Other isolated questions of our modern natural philosophy, which are peculiarly interesting, have been dealt with in my Collected Popular Lectures on the Subject of Evolution (1878). Finally, I have briefly presented the broad principles of my monistic philosophy and its relation to the dominant faith in my Confession of Faith of a Man of Science: Monism as a Connecting Link between Religion and Science[3] (1892, eighth edition, 1899).

The present work on The Riddle of the Universe is the continuation, confirmation, and integration of the views which I have urged for a generation in the aforesaid volumes. It marks the close of my studies on the monistic conception of the universe. The earlier plan, which I projected many years ago, of constructing a complete “System of Monistic Philosophy” on the basis of evolution will never be carried into effect now. My strength is no longer equal to the task, and many warnings of approaching age urge me to desist. Indeed, I am wholly a child of the nineteenth century, and with its close I draw the line under my life’s work.

The vast extension of human knowledge which has taken place during the present century, owing to a happy division of labor, makes it impossible to-day to range over all its branches with equal thoroughness, and to show their essential unity and connection. Even a genius of the highest type, having an equal command of every branch of science, and largely endowed with the artistic faculty of comprehensive presentation, would be incapable of setting forth a complete view of the cosmos in the space of a moderate volume. My own command of the various branches of science is uneven and defective, so that I can attempt no more than to sketch the general plan of such a world-picture, and point out the pervading unity of its parts, however imperfect be the execution. Thus it is that this work on the world-enigma has something of the character of a sketch-book, in which studies of unequal value are associated. As the material of the book was partly written many years ago, and partly produced for the first time during the last few years, the composition is, unfortunately, uneven at times; repetitions, too, have proved unavoidable. I trust those defects will be overlooked.

In taking leave of my readers, I venture the hope that, through my sincere and conscientious work—in spite of its faults, of which I am not unconscious—I have contributed a little towards the solution of the great enigma. Amid the clash of theories, I trust that I have indicated to many a reader who is absorbed in the zealous pursuit of purely rational knowledge that path which, it is my firm conviction, alone leads to the truth—the path of empirical investigation and of the Monistic Philosophy which is based upon it.

Ernst Haeckel.

Jena, Germany.

[PREFACE]

The hour is close upon us when we shall commence our retrospect of one of the most wonderful sections of time that was ever measured by the sweep of the earth. Already the expert is at work, dissecting out and studying his particular phase of that vast world of thought and action we call the nineteenth century. Art, literature, commerce, industry, politics, ethics—all have their high interpreters among us; but in the chance of life it has fallen out that there is none to read aright for us, in historic retrospect, what after ages will probably regard as the most salient feature of the nineteenth century—the conflict of theology with philosophy and science. The pens of our Huxleys, and Tyndalls, and Darwins lie where they fell; there is none left in strength among us to sum up the issues of that struggle with knowledge and sympathy.

In these circumstances it has been thought fitting that we should introduce to English readers the latest work of Professor Haeckel. Germany, as the reader will quickly perceive, is witnessing the same strange reaction of thought that we see about us here in England, yet Die Welträthsel found an immediate and very extensive circle of readers. One of the most prominent zoologists of the century, Professor Haeckel, has a unique claim to pronounce with authority, from the scientific side, on what is known as “the conflict of science and religion.” In the contradictory estimates that are urged on us—for the modern ecclesiastic is as emphatic in his assurance that the conflict has ended favorably to theology as the rationalist is with his counter-assertion—the last words of one of the leading combatants of the second half of the century, still, happily, in full vigor of mind, will be heard with respect and close attention.

A glance at the index of the work suffices to indicate its comprehensive character. The judgment of the distinguished scientist cannot fail to have weight on all the topics included; yet the reader will soon discover a vein of exceptionally interesting thought in the chapters on evolution. The evolution of the human body is no longer a matter of serious dispute. It has passed the first two tribunals—those of theology and of an à priori philosophy—and is only challenged at the third and last—that of empirical proof—by the decorative heads of scientific bodies and a few isolated thinkers.

“Apparent rari nantes in gurgite vasto.”

But the question of the evolution of the human mind, or soul, has been successfully divorced from that of the body. Roman Catholic advanced theologians, whose precise terminology demanded a clear position, admit the latter and deny the former categorically. Other theologians, and many philosophers, have still a vague notion that the evidence for the one does not impair their sentimental objection to the other. Dr. Haeckel’s work summarizes the evidence for the evolution of mind in a masterly and profoundly interesting fashion. It seems impossible to follow his broad survey of the psychic world, from protist to man, without bearing away a conviction of the natural origin of every power and content of the human soul.

Translator.

October, 1900.

[THE RIDDLE OF THE UNIVERSE]

[CHAPTER I]
THE NATURE OF THE PROBLEM

The Condition of Civilization and of Thought at the Close of the Nineteenth Century—Progress of Our Knowledge of Nature, of the Organic and Inorganic Sciences—The Law of Substance and the Law of Evolution—Progress of Technical Science and of Applied Chemistry—Stagnancy in other Departments of Life: Legal and Political Administration, Education, and the Church—Conflict of Reason and Dogma—Anthropism—Cosmological Perspective—Cosmological Theorems—Refutation of the Delusion of Man’s Importance—Number of “World-Riddles”—Criticism of the “Seven” Enigmas—The Way to Solve Them—Function of the Senses and of the Brain—Induction and Deduction—Reason, Sentiment, and Revelation—Philosophy and Science—Experience and Speculation—Dualism and Monism

The close of the nineteenth century offers one of the most remarkable spectacles to the thoughtful observer. All educated people are agreed that it has in many respects immeasurably outstripped its predecessors, and has achieved tasks that were deemed impracticable at its commencement. An entirely new character has been given to the whole of our modern civilization, not only by our astounding theoretical progress in sound knowledge of nature, but also by the remarkably fertile practical application of that knowledge in technical science, industry, commerce, and so forth. On the other hand, however, we have made little or no progress in moral and social life, in comparison with earlier centuries; at times there has been serious reaction. And from this obvious conflict there have arisen, not only an uneasy sense of dismemberment and falseness, but even the danger of grave catastrophes in the political and social world. It is, then, not merely the right, but the sacred duty, of every honorable and humanitarian thinker to devote himself conscientiously to the settlement of that conflict, and to warding off the dangers that it brings in its train. In our conviction this can only be done by a courageous effort to attain the truth, and by the formation of a clear view of the world—a view that shall be based on truth and conformity to reality.

If we recall to mind the imperfect condition of science at the beginning of the century, and compare this with the magnificent structure of its closing years, we are compelled to admit that marvellous progress has been made during its course. Every single branch of science can boast that it has, especially during the latter half of the century, made numerous acquisitions of the utmost value. Both in our microscopic knowledge of the little and in our telescopic investigation of the great we have attained an invaluable insight that seemed inconceivable a hundred years ago. Improved methods of microscopic and biological research have not only revealed to us an invisible world of living things in the kingdom of the protists, full of an infinite wealth of forms, but they have taught us to recognize in the tiny cell the all-pervading “elementary organism” of whose social communities—the tissues—the body of every multicellular plant and animal, even that of man, is composed. This anatomical knowledge is of extreme importance; and it is supplemented by the embryological discovery that each of the higher multicellular organisms is developed out of one simple cell, the impregnated ovum. The “cellular theory,” which has been founded on that discovery, has given us the first true interpretation of the physical, chemical, and even the psychological processes of life—those mysterious phenomena for whose explanation it had been customary to postulate a supernatural “vital force” or “immortal soul.” Moreover, the true character of disease has been made clear and intelligible to the physician for the first time by the cognate science of Cellular Pathology.

The discoveries of the nineteenth century in the inorganic world are no less important. Physics has made astounding progress in every section of its province—in optics and acoustics, in magnetism and electricity, in mechanics and thermo-dynamics; and, what is still more important, it has proved the unity of the forces of the entire universe. The mechanical theory of heat has shown how intimately they are connected, and how each can, in certain conditions, transform itself directly into another. Spectral analysis has taught us that the same matter which enters into the composition of all bodies on earth, including its living inhabitants, builds up the rest of the planets, the sun, and the most distant stars. Astro-physics has considerably enlarged our cosmic perspective in revealing to us, in the immeasurable depths of space, millions of circling spheres larger than our earth, and, like it, in endless transformation, in an eternal rhythm of life and death. Chemistry has introduced us to a multitude of new substances, all of which arise from the combination of a few (about seventy) elements that are incapable of further analysis; some of them play a most important part in every branch of life. It has been shown that one of these elements—carbon—is the remarkable substance that effects the endless variety of organic syntheses, and thus may be considered “the chemical basis of life.” All the particular advances, however, of physics and chemistry yield in theoretical importance to the discovery of the great law which brings them all to one common focus, the “Law of Substance.” As this fundamental cosmic law establishes the eternal persistence of matter and force, their unvarying constancy throughout the entire universe, it has become the pole-star that guides our Monistic Philosophy through the mighty labyrinth to a solution of the world-problem.

Since we intend to make a general survey of the actual condition of our knowledge of nature and its progress during the present century in the following chapters, we shall delay no longer with the review of its particular branches. We would only mention one important advance, which was contemporary with the discovery of the law of substance, and which supplements it—the establishment of the theory of evolution. It is true that there were philosophers who spoke of the evolution of things a thousand years ago; but the recognition that such a law dominates the entire universe, and that the world is nothing else than an eternal “evolution of substance,” is a fruit of the nineteenth century. It was not until the second half of this century that it attained to perfect clearness and a universal application. The immortal merit of establishing the doctrine on an empirical basis, and pointing out its world-wide application, belongs to the great scientist Charles Darwin; he it was who, in 1859, supplied a solid foundation for the theory of descent, which the able French naturalist Jean Lamarck had already sketched in its broad outlines in 1809, and the fundamental idea of which had been almost prophetically enunciated in 1799 by Germany’s greatest poet and thinker, Wolfgang Goethe. In that theory we have the key to “the question of all questions,” to the great enigma of “the place of man in nature,” and of his natural development. If we are in a position to-day to recognize the sovereignty of the law of evolution—and, indeed, of a monistic evolution—in every province of nature, and to use it, in conjunction with the law of substance, for a simple interpretation of all natural phenomena, we owe it chiefly to those three distinguished naturalists; they shine as three stars of the first magnitude amid all the great men of the century.

This marvellous progress in a theoretical knowledge of nature has been followed by a manifold practical application in every branch of civilized life. If we are to-day in the “age of commerce,” if international trade and communication have attained dimensions beyond the conception of any previous age, if we have transcended the limits of space and time by our telegraph and telephone, we owe it, in the first place, to the technical advancement of physics, especially in the application of steam and electricity. If, in photography, we can, with the utmost ease, compel the sunbeam to create for us in a moment’s time a correct picture of any object we like; if we have made enormous progress in agriculture, and in a variety of other pursuits; if, in surgery, we have brought an infinite relief to human pain by our chloroform and morphia, our antiseptics and serous therapeutics, we owe it all to applied chemistry. But it is so well known how much we have surpassed all earlier centuries through these and other scientific discoveries that we need linger over the question no longer.

While we look back with a just pride on the immense progress of the nineteenth century in a knowledge of nature and in its practical application, we find, unfortunately, a very different and far from agreeable picture when we turn to another and not less important province of modern life. To our great regret we must endorse the words of Alfred Wallace: “Compared with our astounding progress in physical science and its practical application, our system of government, of administrative justice, and of national education, and our entire social and moral organization, remain in a state of barbarism.” To convince ourselves of the truth of this grave indictment we need only cast an unprejudiced glance at our public life, or look into the mirror that is daily offered to us by the press, the organ of public sentiment.

We begin our review with justice, the fundamentum regnorum. No one can maintain that its condition to-day is in harmony with our advanced knowledge of man and the world. Not a week passes in which we do not read of judicial decisions over which every thoughtful man shakes his head in despair; many of the decisions of our higher and lower courts are simply unintelligible. We are not referring in the treatment of this particular “world-problem” to the fact that many modern states, in spite of their paper constitutions, are really governed with absolute despotism, and that many who occupy the bench give judgment less in accordance with their sincere conviction than with wishes expressed in higher quarters. We readily admit that the majority of judges and counsel decide conscientiously, and err simply from human frailty. Most of their errors, indeed, are due to defective preparation. It is popularly supposed that these are just the men of highest education, and that on that very account they have the preference in nominations to different offices. However, this famed “legal education” is for the most part rather of a formal and technical character. They have but a superficial acquaintance with that chief and peculiar object of their activity, the human organism, and its most important function, the mind. That is evident from the curious views as to the liberty of the will, responsibility, etc., which we encounter daily. I once told an eminent jurist that the tiny spherical ovum from which every man is developed is as truly endowed with life as the embryo of two, or seven, or even nine months; he laughed incredulously. Most of the students of jurisprudence have no acquaintance with anthropology, psychology, and the doctrine of evolution—the very first requisites for a correct estimate of human nature. They have “no time” for it; their time is already too largely bespoken for an exhaustive study of beer and wine and for the noble art of fencing. The rest of their valuable study-time is required for the purpose of learning some hundreds of paragraphs of law books, a knowledge of which is supposed to qualify the jurist for any position whatever in our modern civilized community.

We shall touch but lightly on the unfortunate province of politics, for the unsatisfactory condition of the modern political world is only too familiar. In a great measure its evils are due to the fact that most of our officials are jurists—that is, men of high technical education, but utterly devoid of that thorough knowledge of human nature which is only obtained by the study of comparative anthropology and the monistic psychology—men without an acquaintance with those social relations of which we find the earlier types in comparative zoology and the theory of evolution, in the cellular theory, and the study of the protists. We can only arrive at a correct knowledge of the structure and life of the social body, the state, through a scientific knowledge of the structure and life of the individuals who compose it, and the cells of which they are in turn composed. If our political rulers and our “representatives of the people” possessed this invaluable biological and anthropological knowledge, we should not find our journals so full of the sociological blunders and political nonsense which at present are far from adorning our parliamentary reports, and even many of our official documents. Worst of all is it when the modern state flings itself into the arms of the reactionary Church, and when the narrow-minded self-interest of parties and the infatuation of short-sighted party-leaders lend their support to the hierarchy. Then are witnessed such sad scenes as the German Reichstag puts before our eyes even at the close of the nineteenth century. We have the spectacle of the educated German people in the power of the ultramontane Centre, under the rule of the Roman papacy, which is its bitterest and most dangerous enemy. Then superstition and stupidity reign instead of right and reason. Never will our government improve until it casts off the fetters of the Church and raises the views of the citizens on man and the world to a higher level by a general scientific education. That does not raise the question of any special form of constitution. Whether a monarchy or a republic be preferable, whether the constitution should be aristocratic or democratic, are subordinate questions in comparison with the supreme question: Shall the modern civilized state be spiritual or secular? Shall it be theocratic—ruled by the irrational formulæ of faith and by clerical despotism—or nomocratic—under the sovereignty of rational laws and civic right? The first task is to kindle a rational interest in our youth, and to uplift our citizens and free them from superstition. That can only be achieved by a timely reform of our schools.

Our education of the young is no more in harmony with modern scientific progress than our legal and political world. Physical science, which is so much more important than all other sciences, and which, properly understood, really embraces all the so-called moral sciences, is still regarded as a mere accessory in our schools, if not treated as the Cinderella of the curriculum. Most of our teachers still give the most prominent place to that dead learning which has come down from the cloistral schools of the Middle Ages. In the front rank we have grammatical gymnastics and an immense waste of time over a “thorough knowledge” of classics and of the history of foreign nations. Ethics, the most important object of practical philosophy, is entirely neglected, and its place is usurped by the ecclesiastical creed. Faith must take precedence over knowledge—not that scientific faith which leads to a monistic religion, but the irrational superstition that lays the foundation of a perverted Christianity. The valuable teaching of modern cosmology and anthropology, of biology and evolution, is most inadequately imparted, if not entirely unknown, in our higher schools; while the memory is burdened with a mass of philological and historical facts which are utterly useless, either from the point of view of theoretical education or for the practical purposes of life. Moreover, the antiquated arrangements and the distribution of faculties in the universities are just as little in harmony with the point we have reached in monistic science as the curriculum of the primary and secondary schools.

The climax of the opposition to modern education and its foundation, advanced natural philosophy, is reached, of course, in the Church. We are not speaking here of ultramontane papistry, nor of the orthodox evangelical tendencies, which do not fall far short of it in ignorance and in the crass superstition of their dogmas. We are imagining ourselves for the moment to be in the church of a liberal Protestant minister, who has a good average education, and who finds room for “the rights of reason” by the side of his faith. There, besides excellent moral teaching, which is in perfect harmony with our own monistic ethics, and humanitarian discussion of which we cordially approve, we hear ideas on the nature of God, of the world, of man, and of life which are directly opposed to all scientific experience. It is no wonder that physicists and chemists, doctors and philosophers, who have made a thorough study of nature, refuse a hearing to such preachers. Our theologians and our politicians are just as ignorant as our philosophers and our jurists of that elementary knowledge of nature which is based on the monistic theory of evolution, and which is already far exceeded in the triumph of our modern learning.

From this opposition, which we can only briefly point out at present, there arise grave conflicts in our modern life which urgently demand a settlement. Our modern education, the outcome of our great advance in knowledge, has a claim upon every department of public and private life; it would see humanity raised, by the instrumentality of reason, to that higher grade of culture, and, consequently, to that better path towards happiness which has been opened out to us by the progress of modern science. That aim, however, is vigorously opposed by the influential parties who would detain the mind in the exploded views of the Middle Ages with regard to the most important problems of life; they linger in the fold of traditional dogma, and would have reason prostrate itself before their “higher revelation.” That is the condition of things, to a very large extent, in theology and philosophy, in sociology and jurisprudence. It is not that the motives of the latter are to be attributed, as a rule, to pure self-interest; they spring partly from ignorance of the facts, and partly from an indolent acquiescence in tradition. The most dangerous of the three great enemies of reason and knowledge is not malice; but ignorance, or, perhaps, indolence. The gods themselves still strive in vain against these two latter influences when they have happily vanquished the first.

One of the main supports of that reactionary system is still what we may call “anthropism.” I designate by this term “that powerful and world-wide group of erroneous opinions which opposes the human organism to the whole of the rest of nature, and represents it to be the preordained end of the organic creation, an entity essentially distinct from it, a godlike being.” Closer examination of this group of ideas shows it to be made up of three different dogmas, which we may distinguish as the anthropocentric, the anthropomorphic, and the anthropolatrous.[4]

I. The anthropocentric dogma culminates in the idea that man is the preordained centre and aim of all terrestrial life—or, in a wider sense, of the whole universe. As this error is extremely conducive to man’s interest, and as it is intimately connected with the creation-myth of the three great Mediterranean religions, and with the dogmas of the Mosaic, Christian, and Mohammedan theologies, it still dominates the greater part of the civilized world.

II. The anthropomorphic dogma is likewise connected with the creation-myth of the three aforesaid religions, and of many others. It likens the creation and control of the world by God to the artificial creation of a talented engineer or mechanic, and to the administration of a wise ruler. God, as creator, sustainer, and ruler of the world, is thus represented after a purely human fashion in his thought and work. Hence it follows, in turn, that man is godlike. “God made man to His own image and likeness.” The older, naïve mythology is pure “homotheism,” attributing human shape, flesh, and blood to the gods. It is more intelligible than the modern mystic theosophy that adores a personal God as an invisible—properly speaking, gaseous—being, yet makes him think, speak, and act in human fashion; it gives us the paradoxical picture of a “gaseous vertebrate.”

III. The anthropolatric dogma naturally results from this comparison of the activity of God and man; it ends in the apotheosis of the human organism. A further result is the belief in the personal immortality of the soul, and the dualistic dogma of the twofold nature of man, whose “immortal soul” is conceived as but the temporary inhabitant of the mortal frame. Thus these three anthropistic dogmas, variously adapted to the respective professions of the different religions, came at length to be vested with an extraordinary importance, and proved the source of the most dangerous errors. The anthropistic view of the world which springs from them is in irreconcilable opposition to our monistic system; indeed, it is at once disproved by our new cosmological perspective.

Not only the three anthropistic dogmas, but many other notions of the dualistic philosophy and orthodox religion, are found to be untenable as soon as we regard them critically from the cosmological perspective of our monistic system. We understand by that the comprehensive view of the universe which we have from the highest point of our monistic interpretation of nature. From that stand-point we see the truth of the following “cosmological theorems,” most of which, in our opinion, have already been amply demonstrated:

(1) The universe, or the cosmos, is eternal, infinite, and illimitable. (2) Its substance, with its two attributes (matter and energy), fills infinite space, and is in eternal motion. (3) This motion runs on through infinite time as an unbroken development, with a periodic change from life to death, from evolution to devolution. (4) The innumerable bodies which are scattered about the space-filling ether all obey the same “law of substance;” while the rotating masses slowly move towards their destruction and dissolution in one part of space others are springing into new life and development in other quarters of the universe. (5) Our sun is one of these unnumbered perishable bodies, and our earth is one of the countless transitory planets that encircle them. (6) Our earth has gone through a long process of cooling before water, in liquid form (the first condition of organic life), could settle thereon. (7) The ensuing biogenetic process, the slow development and transformation of countless organic forms, must have taken many millions of years—considerably over a hundred.[5] (8) Among the different kinds of animals which arose in the later stages of the biogenetic process on earth the vertebrates have far outstripped all other competitors in the evolutionary race. (9) The most important branch of the vertebrates, the mammals, were developed later (during the triassic period) from the lower amphibia and the reptilia. (10) The most perfect and most highly developed branch of the class mammalia is the order of primates, which first put in an appearance, by development from the lowest prochoriata, at the beginning of the Tertiary period—at least three million years ago. (11) The youngest and most perfect twig of the branch primates is man, who sprang from a series of manlike apes towards the end of the Tertiary period. (12) Consequently, the so-called “history of the world”—that is, the brief period of a few thousand years which measures the duration of civilization—is an evanescently short episode in the long course of organic evolution, just as this, in turn, is merely a small portion of the history of our planetary system; and as our mother-earth is a mere speck in the sunbeam in the illimitable universe, so man himself is but a tiny grain of protoplasm in the perishable framework of organic nature.

Nothing seems to me better adapted than this magnificent cosmological perspective to give us the proper standard and the broad outlook which we need in the solution of the vast enigmas that surround us. It not only clearly indicates the true place of man in nature, but it dissipates the prevalent illusion of man’s supreme importance, and the arrogance with which he sets himself apart from the illimitable universe, and exalts himself to the position of its most valuable element. This boundless presumption of conceited man has misled him into making himself “the image of God,” claiming an “eternal life” for his ephemeral personality, and imagining that he possesses unlimited “freedom of will.” The ridiculous imperial folly of Caligula is but a special form of man’s arrogant assumption of divinity. Only when we have abandoned this untenable illusion, and taken up the correct cosmological perspective, can we hope to reach the solution of the “riddles of the universe.”

The uneducated member of a civilized community is surrounded with countless enigmas at every step, just as truly as the savage. Their number, however, decreases with every stride of civilization and of science; and the monistic philosophy is ultimately confronted with but one simple and comprehensive enigma—the “problem of substance.” Still, we may find it useful to include a certain number of problems under that title. In the famous speech which Emil du Bois-Reymond delivered in 1880, in the Leibnitz session of the Berlin Academy of Sciences, he distinguished seven world-enigmas, which he enumerated as follows: (1) The nature of matter and force. (2) The origin of motion. (3) The origin of life. (4) The (apparently preordained) orderly arrangement of nature. (5) The origin of simple sensation and consciousness. (6) Rational thought, and the origin of the cognate faculty, speech. (7) The question of the freedom of the will. Three of these seven enigmas are considered by the orator of the Berlin Academy to be entirely transcendental and insoluble—they are the first, second, and fifth; three others (the third, fourth, and sixth) he considers to be capable of solution, though extremely difficult; as to the seventh and last “world-enigma,” the freedom of the will, which is the one of the greatest practical importance, he remains undecided.

As my monism differs materially from that of the Berlin orator, and as his idea of the “seven great enigmas” has been very widely accepted, it may be useful to indicate their true position at once. In my opinion, the three transcendental problems (1, 2, and 5) are settled by our conception of substance (vide [chap. xii].); the three which he considers difficult, though soluble, (3, 4, and 6), are decisively answered by our modern theory of evolution; the seventh and last, the freedom of the will, is not an object for critical, scientific inquiry at all, for it is a pure dogma, based on an illusion, and has no real existence.

The means and methods we have chosen for attaining the solution of the great enigma do not differ, on the whole, from those of all purely scientific investigation—firstly, experience; secondly, inference. Scientific experience comes to us by observation and experiment, which involve the activity of our sense-organs in the first place, and, secondly, of the inner sense-centres in the cortex of the brain. The microscopic elementary organs of the former are the sense-cells; of the latter, groups of ganglionic cells. The experiences which we derive from the outer world by these invaluable instruments of our mental life are then moulded into ideas by other parts of the brain, and these, in their turn, are united in a chain of reasoning by association. The construction of this chain may take place in two different ways, which are, in my opinion, equally valuable and indispensable: induction and deduction. The higher cerebral operations, the construction of complicated chains of reasoning, abstraction, the formation of concepts, the completion of the perceptive faculty by the plastic faculty of the imagination—in a word, consciousness, thought, and speculation—are functions of the ganglionic cells of the cortex of the brain, just like the preceding simpler mental functions. We unite them all in the supreme concept of reason.[6]

By reason only can we attain to a correct knowledge of the world and a solution of its great problems. Reason is man’s highest gift, the only prerogative that essentially distinguishes him from the lower animals. Nevertheless, it has only reached this high position by the progress of culture and education, by the development of knowledge. The uneducated man and the savage are just as little (or just as much) “rational” as our nearest relatives among the mammals (apes, dogs, elephants, etc.). Yet the opinion still obtains in many quarters that, besides our godlike reason, we have two further (and even surer!) methods of receiving knowledge—emotion and revelation. We must at once dispose of this dangerous error. Emotion has nothing whatever to do with the attainment of truth. That which we prize under the name of “emotion” is an elaborate activity of the brain, which consists of feelings of like and dislike, motions of assent and dissent, impulses of desire and aversion. It may be influenced by the most diverse activities of the organism, by the cravings of the senses and the muscles, the stomach, the sexual organs, etc. The interests of truth are far from promoted by these conditions and vacillations of emotion; on the contrary, such circumstances often disturb that reason which alone is adapted to the pursuit of truth, and frequently mar its perceptive power. No cosmic problem is solved, or even advanced, by the cerebral function we call emotion. And the same must be said of the so-called “revelation,” and of the “truths of faith” which it is supposed to communicate; they are based entirely on a deception, consciously or unconsciously, as we shall see in the [sixteenth chapter].

We must welcome as one of the most fortunate steps in the direction of a solution of the great cosmic problems the fact that of recent years there is a growing tendency to recognize the two paths which alone lead thereto—experience and thought, or speculation—to be of equal value, and mutually complementary. Philosophers have come to see that pure speculation—such, for instance, as Plato and Hegel employed for the construction of their idealist systems—does not lead to knowledge of reality. On the other hand, scientists have been convinced that mere experience—such as Bacon and Mill, for example, made the basis of their realist systems—is insufficient of itself for a complete philosophy. For these two great paths of knowledge, sense-experience and rational thought, are two distinct cerebral functions; the one is elaborated by the sense-organs and the inner sense-centres, the other by the thought-centres, the great “centres of association in the cortex of the brain,” which lie between the sense-centres. (Cf. cc. [vii]. and [x].) True knowledge is only acquired by combining the activity of the two. Nevertheless, there are still many philosophers who would construct the world out of their own inner consciousness, and who reject our empirical science precisely because they have no knowledge of the real world. On the other hand, there are many scientists who still contend that the sole object of science is “the knowledge of facts, the objective investigation of isolated phenomena”; that “the age of philosophy” is past, and science has taken its place.[7] This one-sided over-estimation of experience is as dangerous an error as the converse exaggeration of the value of speculation. Both channels of knowledge are mutually indispensable. The greatest triumphs of modern science—the cellular theory, the dynamic theory of heat, the theory of evolution, and the law of substance—are philosophic achievements; not, however, the fruit of pure speculation, but of an antecedent experience of the widest and most searching character.

At the commencement of the nineteenth century the great idealistic poet, Schiller, gave his counsel to both groups of combatants, the philosophers and the scientists:

“Does strife divide your efforts—no union bless your toil?
Will truth e’er be delivered if ye your forces rend?”

Since then the situation has, happily, been profoundly modified; while both schools, in their different paths, have pressed onward towards the same high goal, they have recognized their common aspiration, and they draw nearer to a knowledge of the truth in mutual covenant. At the end of the nineteenth century we have returned to that monistic attitude which our greatest realistic poet, Goethe, had recognized from its very commencement to be alone correct and fruitful.[8]

All the different philosophical tendencies may, from the point of view of modern science, be ranged in two antagonistic groups; they represent either a dualistic or a monistic interpretation of the cosmos. The former is usually bound up with teleological and idealistic dogmas, the latter with mechanical and realistic theories. Dualism, in the widest sense, breaks up the universe into two entirely distinct substances—the material world and an immaterial God, who is represented to be its creator, sustainer, and ruler. Monism, on the contrary (likewise taken in its widest sense), recognizes one sole substance in the universe, which is at once “God and nature”; body and spirit (or matter and energy) it holds to be inseparable. The extramundane God of dualism leads necessarily to theism; and the intra-mundane God of the monist leads to pantheism.

The different ideas of monism and materialism, and likewise the essentially distinct tendencies of theoretical and practical materialism, are still very frequently confused. As this and other similar cases of confusion of ideas are very prejudicial, and give rise to innumerable errors, we shall make the following brief observations, in order to prevent misunderstanding:

I. Pure monism is identical neither with the theoretical materialism that denies the existence of spirit, and dissolves the world into a heap of dead atoms, nor with the theoretical spiritualism (lately entitled “energetic” spiritualism by Ostwald) which rejects the notion of matter, and considers the world to be a specially arranged group of “energies” or immaterial natural forces.

II. On the contrary, we hold, with Goethe, that “matter cannot exist and be operative without spirit, nor spirit without matter.” We adhere firmly to the pure, unequivocal monism of Spinoza: Matter, or infinitely extended substance, and spirit (or energy), or sensitive and thinking substance, are the two fundamental attributes or principal properties of the all-embracing divine essence of the world, the universal substance. (Cf. [chap. xii].)

[CHAPTER II]
OUR BODILY FRAME

Fundamental Importance of Anatomy—Human Anatomy—Hippocrates, Aristotle, Galen, Vesalius—Comparative Anatomy—Georges Cuvier—Johannes Müller—Karl Gegenbaur—Histology—The Cellular Theory—Schleiden and Schwann—Kölliker—Virchow—Man a Vertebrate, a Tetrapod, a Mammal, a Placental, a Primate—Prosimiæ and Simiæ—The Catarrhinæ—Papiomorphic and Anthropomorphic Apes—Essential Likeness of Man and the Ape in Corporal Structure

All biological research, all investigation into the forms and vital activities of organisms, must first deal with the visible body, in which the morphological and physiological phenomena are observed. This fundamental rule holds good for man just as much as for all other living things. Moreover, the inquiry must not confine itself to mere observation of the outer form; it must penetrate to the interior, and study both the general plan and the minute details of the structure. The science which pursues this fundamental investigation in the broadest sense is anatomy.

The first stimulus to an inquiry into the human frame arose, naturally, in medicine. As it was usually practised by the priests in the older civilizations, we may assume that these highest representatives of the education of the time had already acquired a certain amount of anatomical knowledge two thousand years before Christ, or even earlier. We do not, however, find more exact observations, founded on the dissection of mammals, and applied, by analogy, to the human frame, until we come to the Greek scientists of the sixth and fifth centuries before Christ—Empedocles (of Agrigentum) and Democritus (of Abdera), and especially the most famous physician of classic antiquity, Hippocrates (of Cos). It was from these and other sources that the great Aristotle, the renowned “father of natural history,” equally comprehensive as investigator and philosopher, derived his first knowledge. After him only one anatomist of any consequence is found in antiquity, the Greek physician Claudius Galenus (of Pergamus), who developed a wealthy practice in Rome in the second century after Christ, under the Emperor Marcus Aurelius. All these ancient anatomists acquired their knowledge, as a rule, not by the dissection of the human body itself—which was then sternly forbidden—but by a study of the bodies of the animals which most closely resembled man, especially the apes; they were all, indeed, comparative anatomists.

The triumph of Christianity and its mystic theories meant retrogression to anatomy, as it did to all the other sciences. The popes were resolved above all things to detain humanity in ignorance; they rightly deemed a knowledge of the human organism to be a dangerous source of enlightenment as to our true nature. During the long period of thirteen centuries the writings of Galen were almost the only source of human anatomy, just as the works of Aristotle were for the whole of natural history. It was not until the sixteenth century, when the spiritual tyranny of the papacy was broken by the Reformation, and the geocentric theory, so intimately connected with papal doctrine, was destroyed by the new cosmic system of Copernicus, that the knowledge of the human frame entered upon a new period of progress. The great anatomists, Vesalius (of Brussels), and Eustachius and Fallopius (of Modena), advanced the knowledge of our bodily structure so much by their own thorough investigations that little remained for their numerous followers to do, with regard to the more obvious phenomena, except the substantiation of details. Andreas Vesalius, as courageous as he was talented and indefatigable, was the pioneer of the movement; he completed in his twenty-eighth year (1543) that great and systematic work De humani corporis fabrica; he gave to the whole of human anatomy a new and independent scope and a more solid foundation. On that account he was, at a later date, at Madrid—where he was physician to Charles V. and Philip II.—condemned to death by the Inquisition as a magician. He only escaped by undertaking a pilgrimage to Jerusalem; in returning he suffered shipwreck on the Isle of Zante, and died there in misery and destitution.

The great merit of the nineteenth century, as far as our knowledge of the human frame is concerned, lies in the founding of two new lines of research of immense importance—comparative anatomy and histology, or microscopic anatomy. The former was intimately associated with human anatomy from the very beginning; indeed, it had to supply the place of the latter so long because the dissection of human corpses was a crime visited with capital punishment—that was the case even in the fifteenth century! But the many anatomists of the next three centuries devoted themselves mainly to a more accurate study of the human organism. The elaborate science which we now call comparative anatomy was born in the year 1803, when the great French zoologist Georges Cuvier (a native of Mömpelgard, in Alsace) published his profound Leçons sur l’anatomie comparée, and endeavored to formulate, for the first time, definite laws as to the organism of man and the beasts. While his predecessors—among whom was Goethe in 1790—had mainly contented themselves with comparing the skeleton of man with those of other animals, Cuvier’s broader vision took in the whole of the animal organization. He distinguished therein four great and mutually independent types: Vertebrata, Articulata, Mollusca, and Radiata. This advance was of extreme consequence for our “question of all questions,” since it clearly brought out the fact that man belonged to the vertebral type, and differed fundamentally from all the other types. It is true that the keen-sighted Linné had already, in his Systema Natuae, made a great step in advance by assigning man a definite place in the class of mammals; he had even drawn up the three groups of half-apes, apes, and men (Lemur, simia, and homo) in the order of primates. But his keen, systematic mind was not furnished with that profound empirical foundation, supplied by comparative anatomy, which Cuvier was the first to attain. Further developments were added by the great comparative anatomists of our own century—Friedrich Meckel (Halle), Johannes Müller (Berlin), Richard Owen, T. Huxley, and Karl Gegenbaur (Jena, subsequently Heidelberg). The last-named, in applying the evolutionary theory, which Darwin had just established, to comparative anatomy, raised his science to the front rank of biological studies. The numerous comparative anatomical works of Gegenbaur are, like his well-known Manual of Human Anatomy, equally distinguished by a thorough empirical acquaintance with their immense multitudes of facts, and by a comprehensive control of his material, and its philosophic appreciation in the evolutionary sense. His recent Comparative Anatomy of the Vertebrata establishes the solid foundation on which our conviction of the vertebral character of man in every aspect is chiefly based.

Microscopic anatomy has been developed, in the course of the present century, in a very different fashion from comparative anatomy. At the beginning of the century (1802) a French physician, Bichat, made an attempt to dissect the organs of the human body into their finer constituents by the aid of the microscope, and to show the connection of these various tissues (hista, or tela). This first attempt led to little result, because the scientist was ignorant of the one common element of all the different tissues. This was first discovered (1838) in the shape of the cell, in the plant world, by Matthias Schleiden, and immediately afterwards proved to be the same in the animal world by Theodor Schwann, the pupil and assistant of Johannes Müller at Berlin. Two other distinguished pupils of this great master, who are still living, Albert Kölliker and Rudolph Virchow, took up the cellular theory, and the theory of tissues which is founded on it, in the sixties, and applied them to the human organism in all its details, both in health and disease; they proved that, in man and all other animals, every tissue is made up of the same microscopic particles, the cells, and these “elementary organisms” are the real, self-active citizens which, in combinations of millions, constitute the “cellular state,” our body. All these cells spring from one simple cell, the cytula, or impregnated ovum, by continuous subdivision. The general structure and combination of the tissues are the same in man as in the other vertebrates. Among these the mammals, the youngest and most highly developed class take precedence, in virtue of certain special features which were acquired late. Such are, for instance, the microscopic texture of the hair, of the glands of the skin, and of the breasts, and the corpuscles of the blood, which are quite peculiar to mammals, and different from those of the other vertebrates; man, even in these finest histological relations, is a true mammal.

The microscopic researches of Albert Kölliker and Franz Leydig (at Würzburg) not only enlarged our knowledge of the finer structure of man and the beasts in every direction, but they were especially important in the light of their connection with the evolution of the cell and the tissue; they confirmed the great theory of Carl Theodor Siebold (1845) that the lowest animals, the Infusoria and the Rhizopods, are unicellular organisms.

Our whole frame, both in its general plan and its detailed structure, presents the characteristic type of the vertebrates. This most important and most highly developed group in the animal world was first recognized in its natural unity in 1801 by the great Lamarck; he embraced under that title the four higher animal groups of Linné—mammals, birds, amphibia, and fishes. To these he opposed the two lower classes, insects and worms, as invertebrates. Cuvier (1812) established the unity of the vertebrate type on a firmer basis by his comparative anatomy. It is quite true that all the vertebrates, from the fish up to man, agree in every essential feature; they all have a firm internal skeleton, a framework of cartilage and bone, consisting principally of a vertebral column and a skull; the advanced construction of the latter presents many variations, but, on the whole, all may be reduced to the same fundamental type. Further, in all vertebrates the “organ of the mind,” the central nervous system, in the shape of a spinal cord and a brain, lies at the back of this axial skeleton. Moreover, what we said of its bony environment, the skull, is also true of the brain—the instrument of consciousness and all the higher functions of the mind; its construction and size present very many variations in detail, but its general characteristic structure remains always the same.

We meet the same phenomenon when we compare the rest of our organs with those of the other vertebrates; everywhere, in virtue of heredity, the original plan and the relative distribution of the organs remain the same, although, through adaptation to different environments, the size and the structure of particular sections offer considerable variation. Thus we find that in all cases the blood circulates in two main blood-vessels, of which one—the aorta—passes over the intestine, and the other—the principal vein—passes underneath, and that by the broadening out of the latter in a very definite spot a heart has arisen; this “ventral heart” is just as characteristic of all vertebrates as the “dorsal heart” is of the articulata and mollusca. Equally characteristic of all vertebrates is the early division of the intestinal tube into a “head-gut” (or gill-gut), which serves in respiration, and a “body-gut” (or liver-gut), which co-operates with the liver in digestion; so are, likewise, the ramification of the muscular system, the peculiar structure of the urinary and sexual organs, and so forth. In all these anatomical relations man is a true vertebrate.

Aristotle gave the name of four-footed, or tetrapoda, to all the higher warm-blooded animals which are distinguished by the possession of two pairs of legs. The category was enlarged subsequently, and its title changed into the Latin “quadrupeda,” when Cuvier proved that even “two-legged” birds and men are really “four-footed”; he showed that the internal skeleton of the four legs in all the higher land-vertebrates, from the amphibia up to man, was originally constructed after the same pattern out of a definite number of members. The “arm” of man and the “wing” of bats and birds have the same typical skeleton as the foreleg of the animals which are conspicuously “four-footed.”

The anatomical unity of the fully developed skeleton in the four limbs of all tetrapods is very important. In order to appreciate it fully one has only to compare carefully the skeleton of a salamander or a frog with that of a monkey or a man. One perceives at once that the humeral zone in front and the pelvic zone behind are made up of the same principal parts as in the rest of the quadrupeds. We find in all cases that the first section of the leg proper consists of one strong marrow-bone (the humerus, in the forearm; the femur, behind); the second part, on the contrary, originally always consists of two bones (the ulna and radius, in front; the fibula and tibia, behind). When we further compare the developed structure of the foot proper we are surprised to find that the small bones of which it is made up are also similarly arranged and distributed in every case: in the front limb the three groups of bones of the forefoot (or “hand”) correspond in all classes of the tetrapoda: (1) the carpus, (2) the metacarpus, (3) the five fingers (digiti anteriores); in the rear limb, similarly, we have always the same three osseous groups of the hind foot: (1) the tarsus, (2) the metatarsus, and (3) the five toes (digiti posteriores). It was a very difficult task to reduce all these little bones to one primitive type, and to establish the equivalence (or homology) of the separate parts in all cases; they present extreme variations of form and construction in detail, sometimes being partly fused together and losing their individuality. This great task was first successfully achieved by the most eminent comparative anatomist of our day, Karl Gegenbaur. He pointed out, in his Researches into the Comparative Anatomy of the Vertebrata (1864), how this characteristic “five-toed leg” of the land tetrapods originally (not before the Carboniferous period) arose out of the radiating fin (the breast-fin, or the belly-fin) of the ancient fishes. He had also, in his famous Researches into the Skull of the Vertebrata (1872), deduced the younger skull of the tetrapods from the oldest cranial form among the fishes, that of the shark.

It is especially remarkable that the original number of the toes (five) on each of the four feet, which first appeared in the old amphibia of the Carboniferous period, has, in virtue of a strict heredity, been preserved even to the present day in man. Also, naturally and harmoniously, the typical construction of the joints, ligaments, muscles, and nerves of the two pairs of legs has, in the main, remained the same as in the rest of the “four-footed.” In all these important relations man is a true tetrapod.

The mammals are the youngest and most advanced class of the vertebrates. It is true they are derived from the older class of amphibia, like birds and reptiles: yet they are distinguished from all the other tetrapods by a number of very striking anatomical features. Externally, there is the clothing of the skin with hair, and the possession of two kinds of skin glands—the sweat glands and the sebaceous glands. A local development of these glands on the abdominal skin gave rise (probably during the Triassic period) to the organ which is especially characteristic of the class, and from which it derives its name—the mammarium. This important instrument of lactation is made up of milk glands (mammae) and the “mammar-pouches” (folds of the abdominal skin); in its development the teats appear, through which the young mammal sucks its mother’s milk. In internal structure the most remarkable feature is the possession of a complete diaphragm, a muscular wall which, in all mammals—and only in mammals—separates the thoracic from the abdominal cavity; in all other vertebrates there is no such separation. The skull of mammals is distinguished by a number of remarkable formations, especially in the maxillary apparatus (the upper and lower jaws, and the temporal bones). Moreover, the brain, the olfactory organ, the heart, the lungs, the internal and external sexual organs, the kidneys, and other parts of the body present special peculiarities, both in general and detailed structure, in the mammals; all these, taken collectively, point unequivocally to an early derivation of the mammals from the older groups of the reptiles and amphibia, which must have taken place, at the latest, in the Triassic period—at least twelve million years ago! In all these important characteristics man is a true mammal.

The numerous orders (12-33) which modern systematic zoology distinguishes in the class of mammals had been arranged in 1816 (by Blainville) in three natural groups, which still hold good as sub-classes: (1) the monotrema, (2) the marsupialia, and (3) the placentalia. These three sub-classes not only differ in the important respect of bodily structure and development, but they correspond, also, to three different historical stages in the formation of the class, as we shall see later on. The monotremes of the Triassic period were followed by the marsupials of the Jurassic, and these by the placentals of the Cretaceous. Man belongs to this, the youngest, sub-class; for he presents in his organization all the features which distinguish the placentals from the marsupials and the still older monotremes. First of all, there is the peculiar organ which gives a name to the placentals—the placenta. It serves the purpose of nourishing the young mammal embryo for a long time during its enclosure in the mother’s womb; it consists of blood-bearing tufts which grow out of the chorion surrounding the embryo, and penetrate corresponding cavities in the mucous membrane of the maternal uterus; the delicate skin between the two structures is so attenuated in this spot that the nutriment in the mother’s blood can pass directly into the blood of the child. This excellent contrivance for nourishing the embryo, which makes its first appearance at a somewhat late date, gives the fœtus the opportunity of a longer maintenance and a higher development in the protecting womb; it is wanting in the implacentalia, the two older sub-classes of the marsupials and the monotremes. There are, likewise, other anatomical features, particularly the higher development of the brain and the absence of the marsupial bone, which raise the placentals above all their implacental ancestors. In all these important particulars man is a true placental.

The very varied sub-class of the placentals has been recently subdivided into a great number of orders; they are usually put at from ten to sixteen, but when we include the important extinct forms which have been recently discovered the number runs up to from twenty to twenty-six. In order to facilitate the study of these numerous orders, and to obtain a deeper insight into their kindred construction, it is very useful to form them into great natural groups, which I have called “legions.” In my latest attempt[9] to arrange the advanced system of placentals in phylogenetic order I have substituted eight of these legions for the twenty-six orders, and shown that these may be reduced to four main groups. These, in turn, are traceable to one common ancestral group of all the placentals, their fossil ancestors, the prochoriata of the Cretaceous period. These are directly connected with the marsupial ancestors of the Jurassic period. We will only specify here, as the most important living representatives of these four main groups, the rodentia, the ungulata, the carnivora, and the primates. To the legion of the primates belong the prosimiæ (half-apes), the simiæ (real apes), and man. All the members of these three orders agree in many important features, and are at the same time distinguished by these features from the other twenty-three orders of placentals. They are especially conspicuous for the length of their bones, which were originally adapted to their arboreal manner of life. Their hands and feet are five-fingered, and the long fingers are excellently suited for grasping and embracing the branches of trees; they are provided, either partially or completely, with nails, but have no claws. The dentition is complete, containing all four classes—incisors, canine, premolars, and molars. Primates are also distinguished from all the other placentals by important features in the special construction of the skull and the brain; and these are the more striking in proportion to their development and the lateness of their appearance in the history of the earth. In all these important anatomical features our human organism agrees with that of all the other primates: man is a true primate.

An impartial and thorough comparison of the bodily structure of the primates forces us to distinguish two orders in this most advanced legion of the mammalia—half-apes (prosimiae or hemipitheci) and apes (simiae or pitheci). The former seem in every respect to be the lower and older, the latter to be the higher and younger order. The womb of the half-ape is still double, or two-horned, as it is in all the other mammals. In the true ape, on the contrary, the right and left wombs have completely amalgamated; they blend into a pear-shaped womb, which the human mother possesses besides the ape. In the skull of the apes, just as in that of man, the orbits of the eyes are completely separated from the temporal cavities by an osseous partition; in the prosimiae this is either entirely wanting or very imperfect. Finally, the cerebrum of the prosimia is either quite smooth or very slightly furrowed, and proportionately small; that of the true ape is much larger, and the gray bed especially, the organ of higher psychic activity, is much more developed; the characteristic convolutions and furrows appear on its surface exactly in proportion as the ape approaches to man. In these and other important respects, particularly in the construction of the face and the hands, man presents all the anatomical marks of a true ape.

The extensive order of apes was divided by Geoffroi, in 1812, into two sub-orders, which are still universally accepted in systematic zoology—New World and Old World monkeys, according to the hemisphere they respectively inhabit. The American “New World” monkeys are called Platyrrhinae (flat-nosed); their nose is flat, and the nostrils divergent, with a broad partition. The “Old World” monkeys, on the contrary, are called collectively Catarrhinae (narrow-nosed); their nostrils point downward, like man’s, and the dividing cartilage is narrow. A further difference between the two groups is that the tympanum is superficial in the platyrrhinae, but lies deeper, inside the petrous bone, in the catarrhinae; in the latter a long and narrow bony passage has been formed, while in the former it is still short and wide, or even altogether wanting. Finally, we have a much more important and decisive difference between the two groups in the circumstance that all the Old World monkeys have the same teeth as man—i. e., twenty deciduous and thirty-two permanent teeth (two incisors, one canine, two premolars, and three molars in each half of the jaw). The New World monkeys, on the other hand, have an additional premolar in each half-jaw, or thirty-six teeth altogether. The fact that these anatomical differences of the two simian groups are universal and conspicuous, and that they harmonize with their geographical distribution in the two hemispheres, fully authorizes a sharp systematic division of the two, as well as the phylogenetic conclusion that for a very long period (for more than a million years) the two sub-orders have been developing quite independently of each other in the western and eastern hemispheres. That is a most important point in view of the genealogy of our race; for man bears all the marks of a true catarrhina; he has descended from some extinct member of this sub-order in the Old World.

The numerous types of catarrhinae which still survive in Asia and Africa have been formed into two sections for some time—the tailed, doglike apes (the cynopitheci) and the tailless, manlike apes (the anthropomorpha). The latter are much nearer to man than the former, not only in the absence of a tail and in the general build of the body (especially of the head), but also on account of certain features which are unimportant in themselves but very significant in their constancy. The sacrum of the anthropoid ape, like that of man, is made up of the fusion of five vertebræ; that of the cynopithecus consists of three (more rarely four) sacral vertebræ. The premolar teeth of the cynopitheci are greater in length than breadth; those of the anthropomorpha are broader than they are long; and the first molar has four protuberances in the former, five in the latter. Furthermore, the outer incisor of the lower jaw is broader than the inner one in the manlike apes and man; in the doglike ape it is the smaller. Finally, there is a special significance in the fact, established by Selenka in 1890, that the anthropoid apes share with man the peculiar structure of the discoid placenta, the decidua reflexa, and the pedicle of the allantois. In fact, even a superficial comparison of the bodily structure of the anthropomorpha which still survive makes it clear that both the Asiatic (the orang-outang and the gibbous ape) and the African (the gorilla and chimpanzee) representatives of this group are nearer to man in build than any of the cynopitheci. Under the latter group we include the dog-faced papiomorpha, the baboon, and the long-tailed monkey, at a very low stage. The anatomical difference between these low papiomorpha and the most highly developed anthropoid apes is greater in every respect, whatever organ we take for comparison, than the difference between the latter and man. This instructive fact was established with great penetration by the anatomist Robert Hartmann, in his work on The Anthropoid Apes;[10] he proposed to divide the order of Simiae in a new way—namely, into the two great groups of primaria (man and the anthropoid ape) and the simiae proper, or pitheci (the rest of the catarrhinæ and all the platyrrhinæ). In any case, we have a clear proof of the close affinity of man and the anthropoid ape.

Thus comparative anatomy proves to the satisfaction of every unprejudiced and critical student the significant fact that the body of man and that of the anthropoid ape are not only peculiarly similar, but they are practically one and the same in every important respect. The same two hundred bones, in the same order and structure, make up our inner skeleton; the same three hundred muscles effect our movements; the same hair clothes our skin; the same groups of ganglionic cells build up the marvellous structure of our brain; the same four chambered heart is the central pulsometer in our circulation; the same thirty-two teeth are set in the same order in our jaws; the same salivary, hepatic, and gastric glands compass our digestive process; the same reproductive organs insure the maintenance of our race.

It is true that we find, on close examination, certain minor differences in point of size and shape in most of the organs of man and the ape; but we discover the same, or similar, differences between the higher and lower races of men, when we make a careful comparison—even, in fact, in a minute comparison of the various individuals of our own race. We find no two persons who have exactly the same size and form of nose, ears, eyes, and so forth. One has only to compare attentively these special features in many different persons in any large company to convince one’s self of the astonishing diversity of their construction and the infinite variability of specific forms. Not infrequently even two sisters are so much unlike as to make their origin from the same parents almost incredible. Yet all these individual variations do not weaken the significance of the fundamental similarity of structure; they are traceable to certain minute differences in the growth of the individual features.

[CHAPTER III]
OUR LIFE

Development of Physiology in Antiquity and the Middle Ages: Galen—Experiment and Vivisection—Discovery of the Circulation of the Blood by Harvey—Vitalism: Haller—Teleological and Vitalistic Conception of Life—Mechanical and Monistic View of the Physiological Processes—Comparative Physiology in the Nineteenth Century: Johannes Müller—Cellular Physiology: Max Verworn—Cellular Pathology: Virchow—Mammal Physiology—Similarity of all Vital Activity in Man and the Ape

It is only in the nineteenth century that our knowledge of human life has attained the dignity of a genuine, independent science; during the course of the century it has developed into one of the highest, most interesting, and most important branches of knowledge. This “science of the vital functions,” physiology, had, it is true, been regarded at a much earlier date as a desirable, if not a necessary, condition of success in medical treatment, and had been constantly associated with anatomy, the science of the structure of the body. But it was only much later, and much more slowly, than the latter that it could be thoroughly studied, as it had to contend with much more serious difficulties.

The idea of life, as the opposite of death, naturally became the subject of speculation at a very early age. In the living man, just as in other living animals, there were certain peculiar changes, especially movements, which were wanting in lifeless nature: spontaneous locomotion, the beat of the heart, the drawing of the breath, speech, and so forth. But the discrimination of such “organic movements” from similar phenomena in inorganic bodies was by no means easy, and was frequently impossible; the flowing stream, the flickering flame, the rushing wind, the falling rock, seemed to man to exhibit the same movements. It was quite natural that primitive man should attribute an independent life to these “dead” bodies. He knew no more of the real sources of movement in the one case than in the other.

We find the earliest scientific observations on the nature of man’s vital functions (as well as on his structure) in the Greek natural philosophers and physicians of the sixth and fifth centuries before Christ. The best collection of the physiological facts which were known at that time is to be found in the Natural History of Aristotle; a great number of his assertions were probably taken from Democritus and Hippocrates. The school of the latter had already made attempts to explain the mystery; it postulated as the ultimate source of life in man and the beasts a volatile “spirit of life” (Pneuma); and Erasistratus (280 B.C.) already drew a distinction between the lower and the higher “spirit of life,” the pneuma zoticon in the heart and the pneuma psychicon in the brain.

The credit of gathering these scattered truths into unity, and of making the first attempt at a systematic physiology, belongs to the great Greek physician Galen; we have already recognized in him the first great anatomist of antiquity (cf. [p. 23]). In his researches into the organs of the body he never lost sight of the question of their vital activity, their functions; and even in this direction he proceeded by the same comparative method, taking for his principal study the animals which approach nearest to man. Whatever he learned from these he applied directly to man. He recognized the value of physiological experiment; in his vivisection of apes, dogs, and swine he made a number of interesting experiments. Vivisection has been made the object of a violent attack in recent years, not only by the ignorant and narrow-minded, but by theological enemies of knowledge and by perfervid sentimentalists; it is, however, one of the indispensable methods of research into the nature of life, and has given us invaluable information on the most important questions. This was recognized by Galen seventeen hundred years ago.

Galen reduces all the different functions of the body to three groups, which correspond to the three forms of the pneuma, or vital spirit. The pneuma psychicon—the soul—which resides in the brain and nerves, is the cause of thought, sensation, and will (voluntary movement); the pneuma zoticon—the heart—is responsible for the beat of the heart, the pulse, and the temperature; the pneuma physicon, seated in the liver, is the source of the so-called vegetative functions, digestion and assimilation, growth and reproduction. He especially emphasized the renewal of the blood in the lungs, and expressed a hope that we should some day succeed in isolating the permanent element in the atmosphere—the pneuma, as he calls it—which is taken into the blood in respiration. More than fifteen centuries elapsed before this pneuma—oxygen—was discovered by Lavoisier.

In human physiology, as well as in anatomy, the great system of Galen was for thirteen centuries the Codex aureus, the inviolable source of all knowledge. The influence of Christianity, so fatal to scientific culture, raised the same insuperable obstacles in this as in every other branch of secular knowledge. Not a single scientist appeared from the third to the sixteenth century who dared to make independent research into man’s vital activity, and transcend the limits of the Galenic system. It was not until the sixteenth century that experiments were made in that direction by a number of distinguished physicians and anatomists (Paracelsus, Servetus, Vesalius, and others). In 1628 Harvey published his great discovery of the circulation of the blood, and showed that the heart is a pump, which drives the red stream unceasingly through the connected system of arteries and veins by a rhythmic, unconscious contraction of its muscles. Not less important were Harvey’s researches into the procreation of animals, as a result of which he formulated the well-known law: “Every living thing comes from an egg” (omne vivum ex ovo).

The powerful impetus which Harvey gave to physiological observation and experiment led to a great number of discoveries in the sixteenth and seventeenth centuries. These were co-ordinated for the first time by the learned Albrecht Haller about the middle of the last century; in his great work, Elementa Physiologiae, he established the inherent importance of the science, independently of its relation to practical medicine. In postulating, however, a special “sensitive force or sensibility” for neural action, and a special “irritability” for muscular movement, Haller gave strong support to the erroneous idea of a specific “vital force” (vis vitalis).

For more than a century afterwards, from the middle of the eighteenth until the middle of the nineteenth century, medicine and (especially) physiology were dominated by the old idea that a certain number of the vital processes may be traced to physical and chemical causes, but that others are the outcome of a special vital force which is independent of physical agencies. However much scientists differed in their conceptions of its nature and its relation to the “soul,” they were all agreed as to its independence of, and essential distinction from, the chemico-physical forces of ordinary “matter”; it was a self-contained force (archaeus), unknown in inorganic nature, which compelled ordinary forces into its service. Not only the distinctly psychical activity, the sensibility of the nerves and the irritability of the muscles, but even the phenomena of sense activity, of reproduction, and of development seemed so wonderful and so mysterious in their sources that it was impossible to attribute them to simple physical and chemical processes. As the free activity of the vital force was purposive and conscious, it led, in philosophy, to a complete teleology; especially did this seem indisputable when even the “critical” philosopher Kant had acknowledged, in his famous critique of the teleological position, that, though the mind’s authority to give a mechanical interpretation of all phenomena is theoretically unlimited, yet its actual capacity for such interpretation does not extend to the phenomena of organic life; here we are compelled to have recourse to a purposive—therefore supernatural—principle. This divergence of the vital phenomena from the mechanical processes of life became, naturally, more conspicuous as science advanced in the chemical and physical explanation of the latter. The circulation of the blood and a number of other phenomena could be traced to mechanical agencies; respiration and digestion were attributable to chemical processes like those we find in inorganic nature. On the other hand, it seemed impossible to do this with the wonderful performances of the nerves and muscles, and with the characteristic life of the mind; the co-ordination of all the different forces in the life of the individual seemed also beyond such a mechanical interpretation. Hence there arose a complete physiological dualism—an essential distinction was drawn between inorganic and organic nature, between mechanical and vital processes, between material force and life force, between the body and the soul. At the beginning of the nineteenth century this vitalism was firmly established in France by Louis Dumas, and in Germany by Reil. Alexander Humboldt had already published a poetical presentation of it in 1795, in his narrative of the Legend of Rhodes; it is repeated, with critical notes, in his Views of Nature.

In the first half of the seventeenth century the famous philosopher Descartes, starting from Harvey’s discovery of the circulation of the blood, put forward the idea that the body of man, like that of other animals, is merely an intricate machine, and that its movements take place under the same mechanical laws as the movements of an automaton of human construction. It is true that Descartes, at the same time, claimed for man the exclusive possession of a perfectly independent, immaterial soul, and held that its subjective experience, thought, was the only thing in the world of which we have direct and certain cognizance (“Cogito, ergo sum”). Yet this dualism did not prevent him from doing much to advance our knowledge of the mechanical life processes in detail. Borelli followed (1660) with a reduction of the movements of the animal body to purely physical laws, and Sylvius endeavored, about the same time, to give a purely chemical explanation of the phenomena of digestion and respiration; the former founded the iatromechanical, the latter the iatrochemical, school of medicine. However, these rational tendencies towards a natural, mechanical explanation of the phenomena of life did not attain to a universal acceptance and application; in the course of the eighteenth century they fell entirely away before the advance of teleological vitalism. The final disproof of the latter and a return to mechanism only became possible with the happy growth of the new science of comparative physiology in the forties of the present century.

Our knowledge of the vital functions, like our knowledge of the structure of the human body, was originally obtained, for the most part, not by direct observation of the human organism itself, but by a study of the more closely related animals among the vertebrates, especially the mammals. In this sense the very earliest beginning of human anatomy and physiology was “comparative.” But the distinct science of “comparative physiology,” which embraces the whole sphere of life phenomena, from the lowest animal up to man, is a triumph of the nineteenth century. Its famous creator was Johannes Müller, of Berlin (born, the son of a shoemaker, at Coblentz, in 1801). For fully twenty-five years—from 1833 to 1858—this most versatile and most comprehensive biologist of our age evinced an activity at the Berlin University, as professor and investigator, which is only comparable with the associated work of Haller and Cuvier. Nearly every one of the great biologists who have taught and worked in Germany for the last sixty years was, directly or indirectly, a pupil of Johannes Müller. Starting from the anatomy and physiology of man, he soon gathered all the chief groups of the higher and lower animals within his sphere of comparison. As, moreover, he compared the structure of extinct animals with the living, and the healthy organism with the diseased, endeavoring to bring together all the phenomena of life in a truly philosophic fashion, he attained a biological knowledge far in advance of his predecessors.

The most valuable fruit of these comprehensive studies of Johannes Müller was his Manual of Human Physiology. This classical work contains much more than the title indicates; it is the sketch of a comprehensive “comparative biology.” It is still unsurpassed in respect of its contents and range of investigation. In particular, we find the methods of observation and experiment applied in it as masterfully as the philosophic processes of induction and deduction. Müller was originally a vitalist, like all the physiologists of his time. Nevertheless, the current idea of a vital force took a novel form in his speculations, and gradually transformed itself into the very opposite. For he attempted to explain the phenomena of life mechanically in every department of physiology. His “transfigured” vital force was not above the physical and chemical laws of the rest of nature but entirely bound up with them. It was, in a word, nothing more than life itself—that is, the sum of all the movements which we perceive in the living organism. He sought especially to give them the same mechanical interpretation in the life of the senses and of the mind as in the working of the muscles; the same in the phenomena of circulation, respiration, and digestion as in generation and development. Müller’s success was chiefly due to the fact that he always began with the simplest life phenomena of the lowest animals, and followed them step by step in their gradual development up to the very highest, to man. In this his method of critical comparison proved its value both from the physiological and from the anatomical point of view. Johannes Müller is, moreover, the only great scientist who has equally cultivated these two branches of research, and combined them with equal brilliancy. Immediately after his death his vast scientific kingdom fell into four distinct provinces, which are now nearly always represented by four or more chairs—human and comparative anatomy, pathological anatomy, physiology, and the history of evolution. This sudden division of Müller’s immense realm of learning in 1858 has been compared to the dissolution of the empire which Alexander the Great had consolidated and ruled.

Among the many pupils of Johannes Müller who, either during his lifetime or after his death, labored hard for the advancement of the various branches of biology, one of the most fortunate—if not the most important—was Theodor Schwann. When the able botanist Schleiden, in 1838, indicated the cell as the common elementary organ of all plants, and proved that all the different tissues of the plant are merely combinations of cells, Johannes Müller recognized at once the extraordinary possibilities of this important discovery. He himself sought to point out the same composition in various tissues of the animal body—for instance, in the spinal cord of vertebrates—and thus led his pupil, Schwann, to extend the discovery to all the animal tissues. This difficult task was accomplished by Schwann in his Microscopic Researches into the Accordance in the Structure and Growth of Plants and Animals (1839). Thus was the foundation laid of the “cellular theory,” the profound importance of which, both in physiology and anatomy, has become clearer and more widely recognized in each subsequent year. Moreover, it was shown by two other pupils of Johannes Müller that the activity of all organisms is, in the ultimate analysis, the activity of the components of their tissues, the microscopic cells—these were the able physiologist Ernst Brücke, of Vienna, and the distinguished histologist Albert Kölliker, of Würzburg. Brücke correctly denominated the cells the “elementary organisms,” and showed that, in the body of man and of all other animals, they are the only actual, independent factors of the life process. Kölliker earned special distinction, not only in the construction of the whole science of histology, but particularly by showing that the animal ovum and its products are simple cells.

Still, however widely the immense importance of the cellular theory for all biological research was acknowledged, the “cellular physiology” which is based on it only began an independent development very recently. In this Max Verworn (of Jena) earned a twofold distinction. In his Psycho-physiological Studies of the Protistae (1889) he showed, as a result of an ingenious series of experimental researches, that the “theory of a cell-soul” which I put forward in 1866[11] is completely established by an accurate study of the unicellular protozoa, and that “the psychic phenomena of the protistæ form the bridge which unites the chemical processes of inorganic nature with the mental life of the highest animals.” Verworn has further developed these views, and based them on the modern theory of evolution, in his General Physiology. This distinguished work returns to the comprehensive point of view of Johannes Müller, in opposition to the one-sided and narrow methods of those modern physiologists who think to discover the nature of the vital phenomena by the exclusive aid of chemical and physical experiments. Verworn showed that it is only by Müller’s comparative method and by a profound study of the physiology of the cell that we can reach the higher stand-point which will give us a comprehensive survey of the wonderful realm of the phenomena of life. Only thus do we become convinced that the vital processes in man are subject to the same physical and chemical laws as those of all other animals.

The fundamental importance of the cellular theory for all branches of biology was made clear in the second half of the nineteenth century, not only by the rapid progress of morphology and physiology, but also by the entire reform of that biological science which has always been deemed most important on account of its relation to practical medicine—pathology, or the science of disease. Many even of the older physicians were convinced that human diseases were natural phenomena, like all other manifestations of life, and should be studied scientifically, like other vital functions. Particular schools of medicine—the Iatrophysical and the Iatrochemical—had already, in the seventeenth century, attempted to trace the sources of disease to certain physical and chemical changes. However, the imperfect condition of science at that period precluded any lasting results of these efforts. Many of the older theories, which sought the nature of disease in supernatural and mystical causes, were almost universally accepted down to the middle of the nineteenth century.

It was then that Rudolf Virchow, another pupil of Müller, conceived the happy idea of transferring the cellular theory from the healthy to the diseased organism; he sought in the more minute metamorphoses of the diseased cells and the tissues they composed the true source of those larger changes which, in the form of disease, threaten the living organism with peril and death. Especially during the seven years of his professorship at Würzburg (1849-56) Virchow pursued his great task with such brilliant results that his Cellular Pathology (published in 1858) turned, at one stroke, the whole of pathology and the dependent science of practical medicine into new and eminently fruitful paths. This reform of medicine is significant for our present purpose in that it led us to a monistic and purely scientific conception of disease. In sickness, no less than in health, man is subject to the same eternal “iron laws” of physics and chemistry as all the rest of the organic world.

Among the numerous classes of animals which modern zoology distinguishes the mammals occupy a pre-eminent position, not only on morphological grounds, but also for physiological reasons. As man belongs to the class of mammals (see [p. 27]) by every portion of his frame, we must expect him to share his characteristic functions with the rest of the mammals. Such we find to be the case. The circulation of the blood and respiration are accomplished in man under precisely the same laws and in the same manner as in all the other mammals—and in these alone; they are determined by the peculiar structure of their heart and lungs. In mammals only is all the arterial blood conducted from the left ventricle of the heart to the body by one, the left, branch of the aorta, while in birds it passes along the right branch, and in reptiles along both branches. The blood of mammals is distinguished from that of any other vertebrate by the circumstance that its red cells have lost their nucleus (by reversion). The respiratory movements are effected largely by the diaphragm in this class of animals alone, because only in them does it form a complete partition between the pectoral and abdominal cavities. Special importance, however, in this highest class of animals, attaches to the production of milk in the breasts (mammae), and to the peculiar method of the rearing of the young, which entails the supplying of the offspring with the mother’s milk. As this nutritive process reacts most powerfully on the other vital functions, and the maternal affection of mammals must have arisen from this intimate form of rearing, the name of the class justly reminds us of its great importance. In millions of pictures, most of them produced by painters of the highest rank, the “madonna with the child” is revered as the purest and noblest type of maternal love—the instinct which is found in its extreme form in the exaggerated tenderness of the mother-ape.

As the apes approach nearest to man of all the mammals in point of structure, we shall expect to hear the same of their vital functions; and that we find to be the case. Everybody knows how closely the habits, the movements, the sense activity, the mental life, and the parental customs of apes resemble those of man. Scientific physiology proves the same significant resemblance in other less familiar processes, particularly in the working of the heart, the division of the breasts, and the sexual life. In the latter connection it is especially noteworthy that the mature females of many kinds of apes suffer a periodical discharge of blood from the womb, which corresponds to the menstruation of the human female. The secretion of the milk in the glands and the suctorial process also take place in the female ape in precisely the same fashion as in women.

Finally, it is of especial interest that the speech of apes seems on physiological comparison to be a stage in the formation of articulate human speech. Among living apes there is an Indian species which is musical; the hylobates syndactylus sings a full octave in perfectly pure, harmonious half-tones. No impartial philologist can hesitate any longer to admit that our elaborate rational language has been slowly and gradually developed out of the imperfect speech of our Pliocene simian ancestors.

[CHAPTER IV]
OUR EMBRYONIC DEVELOPMENT

The Older Embryology—The Theory of Preformation—The Theory of Scatulation: Haller and Leibnitz—The Theory of Epigenesis: C. F. Wolff—The Theory of Germinal Layers: Carl Ernst Baer—Discovery of the Human Ovum: Remak, Kölliker—The Egg-Cell and the Sperm-Cell—The Theory of the Gastræa—Protozoa and Metazoa—The Ova and the Spermatozoa: Oscar Hertwig—Conception—Embryonic Development in Man—Uniformity of the Vertebrate Embryo—The Germinal Membranes in Man—The Amnion, the Serolemma, and the Allantois—The Formation of the Placenta and the “After-Birth”—The Decidua and the Funiculus Umbilicalis—The Discoid Placenta of Man and the Ape

Comparative ontogeny, or the science of the development of the individual animal, is a child of the nineteenth century in even a truer sense than comparative anatomy and physiology. How is the child formed in the mother’s womb? How do animals evolve from ova? How does the plant come forth from the seed? These pregnant questions have occupied the thoughtful mind for thousands of years. Yet it is only seventy years since the embryologist Baer pointed out the correct means and methods for penetrating into the mysteries of embryonic life; it is only forty years since Darwin, by his reform of the theory of descent, gave us the key which should open the long-closed door, and lead to a knowledge of embryonic agencies. As I have endeavored to give a complete, popular presentation of this very interesting but difficult study in the first section of my Anthropogeny, I will confine myself here to a brief survey and discussion of the most important phenomena. Let us first cast a historical glance at the older ontogeny, and the theory of preformation which is connected with it.

The classical works of Aristotle, the many-sided “father of science,” are the oldest known scientific sources of embryology, as we found them to be for comparative anatomy. Not only in his great natural history, but also in a special small work, Five Books on the Generation and Development of Animals, the great philosopher gives us a host of interesting facts, adding many observations on their significance; it was not until our own days that many of them were fully appreciated, and, indeed, we may say, discovered afresh. Naturally, many fables and errors are mixed up with them; it was all that was known at that time of the hidden growth of the human germ. Yet during the long space of the next two thousand years the slumbering science made no further progress. It was not until the commencement of the seventeenth century that there was a renewal of activity. In 1600 the Italian anatomist Fabricius ab Aquapendente published at Padua the first pictures and descriptions of the embryos of man and some of the higher animals; in 1687 the famous Marcello Malpighi, of Bologna, a distinguished pioneer alike in zoology and botany, published the first consistent exposition of the growth of the chick in the hatched egg.

All these older scientists were possessed with the idea that the complete body, with all its parts, was already contained in the ovum of animals, only it was so minute and transparent that it could not be detected; that, therefore, the whole development was nothing more than a growth, or an “unfolding,” of the parts that were already “infolded” (involutae). This erroneous notion, almost universally accepted until the beginning of the present century, is called the “preformation theory”; sometimes it is called the “evolution theory” (in the literal sense of “unfolding”); but the latter title is accepted by modern scientists for the very different theory of “transformation.”

Closely connected with the preformation theory, and as a logical consequence of it, there arose in the last century a further theory which keenly interested all thoughtful biologists—the curious “theory of scatulation.” As it was thought that the outline of the entire organism, with all its parts, was present in the egg, the ovary of the embryo had to be supposed to contain the ova of the following generation; these, again, the ova of the next, and so on in infinitum! On that basis the distinguished physiologist Haller calculated that God had created together, 6000 years ago—on the sixth day of his creatorial labors—the germs of 200,000,000,000 men, and ingeniously packed them all in the ovary of our venerable mother Eve. Even the gifted philosopher Leibnitz fully accepted this conclusion, and embodied it in his monadist theory; and as, on his theory, soul and body are in eternal, inseparable companionship, the consequence had to be accepted for the soul; “the souls of men have existed in organized bodies in their ancestors from Adam downward—that is, from the very beginning of things.”

In the month of November, 1759, a young doctor of twenty-six years, Caspar Friedrich Wolff (son of a Berlin tailor), published his dissertation for the degree at Halle, under the title, Theoria Generationis. Supported by a series of most laborious and painstaking observations, he proved the entire falsity of the dominant theories of preformation and scatulation. In the hatched egg there is at first no trace of the coming chick and its organs; instead of it we find on top of the yolk a small, circular, white disk. This thin “germinal disk” becomes gradually round, and then breaks up into four folds, lying upon each other, which are the rudiments of the four chief systems of organs—the nervous system above, the muscular system underneath, the vascular system (with the heart), and, finally, the alimentary canal. Thus, as Wolff justly remarked, the embryonic development does not consist in an unfolding of the preformed organs, but in a series of new constructions; it is a true epigenesis. One part arises after another, and all make their appearance in a simple form, which is very different from the later structure. This only appears after a series of most remarkable formations. Although this great discovery—one of the most important of the eighteenth century—could be directly proved by a verification of the facts Wolff had observed, and although the “theory of generation” which was founded on it was in reality not a theory at all, but a simple fact, it met with no sympathy whatever for half a century. It was particularly retarded by the high authority of Haller, who fought it strenuously with the dogmatic assertion that “there is no such thing as development: no part of the animal body is formed before another; all were created together.” Wolff, who had to go to St. Petersburg, was long in his grave before the forgotten facts he had observed were discovered afresh by Oken at Jena, in 1806.

After Wolff’s “epigenesis theory” had been established by Oken and Neckel (whose important work on the development of the alimentary canal was translated from Latin into German), a number of young German scientists devoted themselves eagerly to more accurate embryological research. The most important and successful of these was Carl Ernst Baer. His principal work appeared in 1828, with the title, History of the Development of Animals: Observations and Reflections. Not only the phenomena of the formation of the germ are clearly illustrated and fully described in it, but it adds a number of very pregnant speculations. In particular, the form of the embryo of man and the mammals is correctly presented, and the vastly different development of the lower invertebrate animals is also considered. The two leaflike layers which appear in the round germ disk of the higher vertebrates first divide, according to Baer, into two further layers, and these four germinal layers are transformed into four tubes, which represent the fundamental organs—the skin layer, the muscular layer, the vascular layer, and the mucous layer. Then, by very complicated evolutionary processes, the later organs arise, in substantially the same manner, in man and all the other vertebrates. The three chief groups of invertebrates, which in their turn differ widely from each other, have a very different development.

One of the most important of Baer’s many discoveries was the finding of the human ovum. Up to that time the little vesicles which are found in great numbers in the human ovary and in that of all other mammals had been taken for the ova. Baer was the first to prove, in 1827, that the real ova are enclosed in these vesicles—the “Graafian follicles”—and much smaller, being tiny spheres 1-120th inch in diameter, visible to the naked eye as minute specks under favorable conditions. He discovered likewise that from this tiny ovum of the mammal there develops first a characteristic germ globule, a hollow sphere with liquid contents, the wall of which forms the slender germinal membrane, or blastoderm.

Ten years after Baer had given a firm foundation to embryological science by his theory of germ layers a new task confronted it on the establishment of the cellular theory in 1838. What is the relation of the ovum and the layers which arise from it to the tissues and cells which compose the fully developed organism? The correct answer to this difficult question was given about the middle of this century by two distinguished pupils of Johannes Müller—Robert Remak, of Berlin, and Albert Kölliker, of Würzburg. They showed that the ovum is at first one simple cell, and that the many germinal globules, or granules, which arise from it by repeated segmentation, are also simple cells. From this mulberry-like group of cells are constructed first the germinal layers, and subsequently by differentiation, or division of labor, all the different organs. Kölliker has the further merit of showing that the seminal fluid of male animals is also a mass of microscopic cells. The active pin-shaped “seed-animalcules,” or spermatozoa, in it are merely ciliated cells, as I first proved in the case of the seed-filaments of the sponge in 1866. Thus it was proved that both the materials of generation, the male sperm and the female ova, fell in with the cellular theory. That was a discovery of which the great philosophic significance was not appreciated until a much later date, on a close study of the phenomena of conception in 1875.

All the older studies in embryonic development concern man and the higher vertebrates, especially the embryonic bird, since hens’ eggs are the largest and most convenient objects for investigation, and are plentiful enough to facilitate experiment; we can hatch them in the incubator, as well as by the natural function of the hen, and so observe from hour to hour, during the space of three weeks, the whole series of formations, from the simple germ cell to the complete organism. Even Baer had only been able to gather from such observations the fact that the different classes of vertebrates agreed in the characteristic form of the germ layers and the growth of particular organs. In the innumerable classes of invertebrates, on the other hand—that is, in the great majority of animals—the embryonic development seemed to run quite a different course, and most of them seemed to be altogether without true germinal layers. It was not until about the middle of the century that such layers were found in some of the invertebrates. Huxley, for instance, found them in the medusæ in 1849, and Kölliker in the cephalopods in 1844. Particularly important was the discovery of Kowalewsky (1886) that the lowest vertebrate—the lancelot, or amphioxus—is developed in just the same manner (and a very original fashion it is) as an invertebrate, apparently quite remote, tunicate, the sea-squirt, or ascidian. Even in some of the worms, the radiata and the articulata, a similar formation of the germinal layers was pointed out by the same observer. I myself was then (since 1886) occupied with the embryology of the sponges, corals, medusæ, and siphonophoræ, and, as I found the same formation of two primary germ layers everywhere in these lowest classes of multicellular animals, I came to the conclusion that this important embryonic feature is common to the entire animal world. The circumstance that in the sponges and the cnidaria (polyps, medusæ, etc.) the body consists for a long time, sometimes throughout life, merely of two simple layers of cells, seemed to me especially significant. Huxley had already (1849) compared these, in the case of the medusæ, with the two primary germinal layers of the vertebrates. On the ground of these observations and comparisons I then, in 1872, in my Philosophy of the Calcispongiae, published the “theory of the gastræa,” of which the following are the essential points:

I. The whole animal world falls into two essentially different groups, the unicellular primitive animals (Protozoa) and the multicellular animals with complex tissues (Metazoa). The entire organism of the protozoon (the rhizopods of the infusoria) remains throughout life a single simple cell (or occasionally a loose colony of cells without the formation of tissue, a coenobium). The organism of the metazoon, on the contrary, is only unicellular at the commencement, and is subsequently built up of a number of cells which form tissues.

II. Hence the method of reproduction and development is very different in each of these great categories of animals. The protozoa usually multiply by non-sexual means, by fission, gemmation, or spores; they have no real ova and no sperm. The metazoa, on the contrary, are divided into male and female sexes, and generally propagate sexually, by means of true ova, which are fertilized by the male sperm.

III. Hence, further, true germinal layers, and the tissues which are formed from them, are found only in the metazoa; they are entirely wanting in the protozoa.

IV. In all the metazoa only two primary layers appear at first, and these have always the same essential significance; from the outer layer the external skin and the nervous system are developed; from the inner layer are formed the alimentary canal and all the other organs.

V. I called the germ, which always arises first from the impregnated ovum, and which consists of these two primary layers, the “gut-larva,” or the gastrula; its cup-shaped body with the two layers encloses originally a simple digestive cavity, the primitive gut (the progaster or archenteron), and its simple opening is the primitive mouth (the prostoma or blastoporus). These are the earliest organs of the multicellular body, and the two cell layers of its enclosing wall, simple epithelia, are its earliest tissues; all the other organs and tissues are a later and secondary growth from these.

VI. From this similarity, or homology, of the gastrula in all classes of compound animals I drew the conclusion, in virtue of the biogenetic law ([p. 81]), that all the metazoa come originally from one simple ancestral form, the gastraea, and that this ancient (Laurentian), long-extinct form had the structure and composition of the actual gastrula, in which it is preserved by heredity.

VII. This phylogenetic conclusion, based on the comparison of ontogenetic facts, is confirmed by the circumstance that there are several of these gastræades still in existence (gastraemaria, cyemaria, physemaria, etc.), and also some ancient forms of other animal groups whose organization is very little higher (the olynthus of the sponges, the hydra, or common fresh-water polyp, of the cnidaria, the convoluta and other cryptocæla, or worms of the simplest type, of the platodes).

VIII. In the further development of the various tissue-forming animals from the gastrula we have to distinguish two principal groups. The earlier and lower types (the coelenteria or acoelomia) have no body cavity, no vent, and no blood; such is the case with the gastræades, sponges, cnidaria, and platodes. The later and higher types (the caelomaria or bilateria), on the other hand, have a true body cavity, and generally blood and a vent; to these we must refer the worms and the higher types of animals which were evolved from these later on, the echinodermata, mollusca, articulata, tunicata, and vertebrata.

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.

[CHAPTER V]
THE HISTORY OF OUR SPECIES

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.

[CHAPTER VI]
THE NATURE OF THE SOUL

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.

The most neglected of all psychological methods, even up to the present day, is the evolution of the soul; yet this little-frequented path is precisely the one that leads us most quickly and securely through the gloomy primeval forest of psychological prejudices, dogmas, and errors, to a clear insight into many of the chief psychic problems. As I did in the other branch of organic evolution, I again put before the reader the two great branches of the science which I differentiated in 1866—ontogeny and phylogeny. The ontogeny, or embryonic development, of the soul, individual or biontic psychogeny, investigates the gradual and hierarchic development of the soul in the individual, and seeks to learn the laws by which it is controlled. For a great part of the life of the mind a good deal has been done in this direction for centuries; rational pedagogy must have set itself the task at an early date of the theoretical study of the gradual development and formative capacity of the young mind that was committed to it for education and formation. Most pedagogues, however, were idealistic or dualistic philosophers, and so they went to work with all the prejudices of the spiritualistic psychology. It is only in the last few decades that this dogmatic tendency has been largely superseded even in the school by scientific methods; we now find a greater concern to apply the chief laws of evolution even in the discussion of the soul of the child. The raw material of the child’s soul is already qualitatively determined by heredity from parents and ancestors; education has the noble task of bringing it to a perfect maturity by intellectual instruction and moral training—that is, by adaptation. Wilhelm Preyer was the first to lay the foundation of our knowledge of the early psychic development in his interesting work on The Mind of the Child. Much is still to be done in the study of the later stages and metamorphoses of the individual soul, and once more the correct, critical application of the biogenetic law is proving a guiding star to the scientific mind.

A new and fertile epoch of higher development dawned for psychology and all other biological sciences when Charles Darwin applied the principles of evolution to them forty years ago. The seventh chapter of his epoch-making work on The Origin of Species is devoted to instinct. It contains the valuable proof that the instincts of animals are subject, like all other vital processes, to the general laws of historic development. The special instincts of particular species were formed by adaptation, and the modifications thus acquired were handed on to posterity by heredity; in their formation and preservation natural selection plays the same part as in the transformation of every other physiological function. Darwin afterwards developed this fundamental thought in a number of works, showing that the same laws of “mental evolution” hold good throughout the entire organic world, not less in man than in the brute, and even in the plant. Hence the unity of the organic world, which is revealed by the common origin of its members, applies also to the entire province of psychic life, from the simplest unicellular organism up to man.

To George Romanes we owe the further development of Darwin’s psychology and its special application to the different sections of psychic activity. Unfortunately, his premature decease prevented the completion of the great work which was to reconstruct every section of comparative psychology on the lines of monistic evolution. The two volumes of this work which were completed are among the most valuable productions of psychological literature. For, conformably to the principles of our modern monistic research, his first care was to collect and arrange all the important facts which have been empirically established in the field of comparative psychology in the course of centuries; in the second place, these facts are tested with an objective criticism, and systematically distributed; finally, such rational conclusions are drawn from them on the chief general questions of psychology as are in harmony with the fundamental principles of modern monism. The first volume of Romanes’s work bears the title of Mental Evolution in the Animal World; it presents, in natural connection, the entire length of the chain of psychic evolution in the animal world, from the simplest sensations and instincts of the lowest animals to the elaborate phenomena of consciousness and reason in the highest. It contains also a number of extracts from a manuscript which Darwin left “on instinct,” and a complete collection of all that he wrote in the province of psychology.

The second and more important volume of Romanes’s work treats of “Mental evolution in man and the origin of human faculties.” The distinguished psychologist gives a convincing proof in it “that the psychological barrier between man and the brute has been overcome.” Man’s power of conceptual thought and of abstraction has been gradually evolved from the non-conceptual stages of thought and ideation in the nearest related mammals. Man’s highest mental powers—reason, speech, and conscience—have arisen from the lower stages of the same faculties in our primate ancestors (the simiæ and prosimiæ). Man has no single mental faculty which is his exclusive prerogative. His whole psychic life differs from that of the nearest related mammals only in degree, and not in kind; quantitatively, not qualitatively.

I recommend those of my readers who are interested in these momentous questions of psychology to study the profound work of Romanes. I am completely at one with him and Darwin in almost all their views and convictions. Wherever an apparent discrepancy is found between these authors and my earlier productions, it is either a case of imperfect expression on my part or an unimportant difference in application of principle. For the rest, it is characteristic of this “science of ideas” that the most eminent philosophers hold entirely antagonistic views on its fundamental notions.

[CHAPTER VII]
PSYCHIC GRADATIONS

Psychological Unity of Organic Nature—Material Basis of the Soul: Psychoplasm—Scale of Sensation—Scale of Movement—Scale of Reflex Action—Simple and Compound Reflex Action—Reflex Action and Consciousness—Scale of Perception—Unconscious and Conscious Perception—Scale of Memory—Unconscious and Conscious Memory—Association of Perceptions—Instinct—Primary and Secondary Instincts—Scale of Reason—Language—Emotion and Passion—The Will—Freedom of the Will

The great progress which psychology has made, with the assistance of evolution, in the latter half of the century culminates in the recognition of the psychological unity of the organic world. Comparative psychology, in co-operation with the ontogeny and phylogeny of the psyche, has enforced the conviction that organic life in all its stages, from the simplest unicellular protozoon up to man, springs from the same elementary forces of nature, from the physiological functions of sensation and movement. The future task of scientific psychology, therefore, is not, as it once was, the exclusively subjective and introspective analysis of the highly developed mind of a philosopher, but the objective, comparative study of the long gradation by which man has slowly arisen through a vast series of lower animal conditions. This great task of separating the different steps in the psychological ladder, and proving their unbroken phylogenetic connection, has only been seriously attempted during the last ten years, especially in the splendid work of Romanes. We must confine ourselves here to a brief discussion of a few of the general questions which that gradation has suggested.

All the phenomena of the psychic life are, without exception, bound up with certain material changes in the living substance of the body, the protoplasm. We have given to that part of the protoplasm which seems to be the indispensable substratum of psychic life the name of psychoplasm (the “soul-substance,” in the monistic sense); in other words, we do not attribute any peculiar “essence” to it, but we consider the psyche to be merely a collective idea of all the psychic functions of protoplasm. In this sense the “soul” is merely a physiological abstraction like “assimilation” or “generation.” In man and the higher animals, in accordance with the division of labor of the organs and tissues, the psychoplasm is a differentiated part of the nervous system, the neuroplasm of the ganglionic cells and their fibres. In the lower animals, however, which have no special nerves and organs of sense, and in the plants, the psychoplasm has not yet reached an independent differentiation. Finally, in the unicellular protists, the psychoplasm is identified either with the whole of the living protoplasm of the simple cell or with a portion of it. In all cases, in the lowest as well as the highest stages of the psychological hierarchy, a certain chemical composition and a certain physical activity of the psychoplasm are indispensable before the “soul” can function or act. That is equally true of the elementary psychic function of the plasmatic sensation and movement of the protozoa, and of the complex functions of the sense-organs and the brain in the higher animals and man. The activity of the psychoplasm, which we call the “soul,” is always connected with metabolism.

All living organisms, without exception, are sensitive; they are influenced by the condition of their environment, and react thereon by certain modifications in their own structure. Light and heat, gravity and electricity, mechanical processes and chemical action in the environment, act as stimuli on the sensitive psychoplasm, and effect changes in its molecular composition. We may distinguish the following five chief stages of this sensibility:

I. At the lowest stage of organization the whole psychoplasm, as such, is sensitive, and reacts on the stimuli from without; that is the case with the lowest protists, with many plants, and with some of the most rudimentary animals.

II. At the second stage very simple and undiscriminating sense-organs begin to appear on the surface of the organism, in the form of protoplasmic filaments and pigment spots, the forerunners of the nerves of touch and the eyes; these are found in some of the higher protists and in many of the lower animals and plants.

III. At the third stage specific organs of sense, each with a peculiar adaptation, have arisen by differentiation out of these rudimentary processes: there are the chemical instruments of smell and taste, and the physical organs of touch, temperature, hearing, and sight. The “specific energy” of these sense-organs is not an original inherent property of theirs, but has been gained by functional adaptation and progressive heredity.

IV. The fourth stage is characterized by the centralization or integration of the nervous system, and, consequently, of sensation; by the association of the previously isolated or localized sensations presentations arise, though they still remain unconscious. That is the condition of many both of the lower and the higher animals.

V. Finally, at the fifth stage, the highest psychic function, conscious perception, is developed by the mirroring of the sensations in a central part of the nervous system, as we find in man and the higher vertebrates, and probably in some of the higher invertebrates, notably the articulata.

All living organisms without exception have the faculty of spontaneous movement, in contradistinction to the rigidity and inertia of unorganized substances (e.g., crystals); in other words, certain changes of place of the particles occur in the living psychoplasm from internal causes, which have their source in its own chemical composition. These active vital movements are partly discovered by direct observation and partly only known indirectly, by inference from their effects. We may distinguish five stages of them.

I. At the lowest stage of organic life, in the chromacea, and many protophyta and lower metaphyta, we perceive only those movements of growth which are common to all organisms. They are usually so slow that they cannot be directly observed; they have to be inferred from their results—from the change in size and form of the growing organism.

II. Many protists, particularly unicellular algæ of the groups of diatomacea and desmidiacea, accomplish a kind of creeping or swimming motion by secretion, by ejecting a slimy substance at one side.

III. Other organisms which float in water—for instance, many of the radiolaria, siphonophora, ktenophora, and others—ascend and descend by altering their specific gravity, sometimes by osmosis, sometimes by the separation or squeezing-out of air.

IV. Many plants, especially the sensitive plants (mimosa) and other papilionacea, effect movements of their leaves or other organs by change of pressure—that is, they alter the strain of the protoplasm, and, consequently, its pressure on the enclosing elastic walls of the cells.

V. The most important of all organic movements are the phenomena of contraction—i.e., changes of form at the surface of the organism, which are dependent on a twofold displacement of their elements; they always involve two different conditions or phases of motion—contraction and expansion. Four different forms of this plasmatic contraction may be enumerated:

(a) Amœboid movement (in rhizopods, blood-cells, pigment-cells, etc.).

(b) A similar flow of protoplasm within enclosed cells.

(c) Vibratory motion (ciliary movements) in infusoria, spermatozoa, ciliated epithelial cells.

(d) Muscular movement (in most animals).

The elementary psychic activity that arises from the combination of sensation and movement is called reflex (in the widest sense), reflective function, or reflex action. The movement—no matter what kind it is—seems in this case to be the immediate result of the stimulus which evoked the sensation; it has, on that account, been called stimulated motion in its simplest form (in the protists). All living protoplasm has this feature of irritability. Any physical or chemical change in the environment may, in certain circumstances, act as a stimulus on the psychoplasm, and elicit or “release” a movement. We shall see later on how this important physical concept of “releasing” directly connects the simplest organic reflex actions with similar mechanical phenomena of movement in the inorganic world (for instance, in the explosion of powder by a spark, or of dynamite by a blow). We may distinguish the following seven stages in the scale of reflex action:

I. At the lowest stage of organization, in the lowest protists, the stimuli of the outer world (heat, light, electricity, etc.) cause in the indifferent protoplasm only those indispensable movements of growth and nutrition which are common to all organisms, and are absolutely necessary for their preservation. That is also the case in most of the plants.

II. In the case of many freely moving protists (especially the amœba, the heliozoon, and the rhizopod) the stimuli from without produce on every spot of the unprotected surface of the unicellular organism external movements which take the form of changes of shape, and sometimes changes of place (amœboid movement, pseudopod formation, the extension and withdrawal of what look like feet); these indefinite, variable processes of the protoplasm are not yet permanent organs. In the same way, general organic irritability takes the form of indeterminate reflex action in the sensitive plants and the lowest metazoa; in many multicellular organisms the stimuli may be conducted from one cell to another, as all the cells are connected by fine fibres.

III. Many protists, especially the more highly developed protozoa, produce on their unicellular body two little organs of the simplest character—an organ of touch and an organ of movement. Both these instruments are direct external projections of protoplasm; the stimulus, which alights on the first, is immediately conducted to the other by the psychoplasm of the unicellular body, and causes it to contract. This phenomenon is particularly easy to observe, and even produce experimentally, in many of the stationary infusoria (for instance, the poteriodendron among the flagellata, and the vorticella among the ciliata). The faintest stimulus that touches the extremely sensitive hairs, or cilia, at the free end of the cells, immediately causes a contraction of a thread-like stalk at the other, fixed end. This phenomenon is known as a “simple reflex arch.”

IV. These phenomena of the unicellular organism of the infusoria lead on to the interesting mechanism of the neuro-muscular cells, which we find in the multicellular body of many of the lower metazoa, especially in the cnidaria (polyps and corals). Each single neuro-muscular cell is a “unicellular reflex organ”; it has on its surface a sensitive spot, and a motor muscular fibre inside at the opposite end; the latter contracts as soon as the former is stimulated.

V. In other cnidaria, notably in the free swimming medusæ—which are closely related to the stationary polyps—the simple neuro-muscular cell becomes two different cells, connected by a filament; an external sense-cell (in the outer skin) and an internal muscular cell (under the skin). In this bicellular reflex organ the one cell is the rudimentary organ of sensation, the other of movement; the connecting bridge of the psychoplasmic filament conducts the stimulus from one to the other.

VI. The most important step in the gradual construction of the reflex mechanism is the division into three cells; in the place of the simple connecting bridge we spoke of there appears a third independent cell, the soul-cell, or ganglionic cell; with it appears also a new psychic function, unconscious presentation, which has its seat in this cell. The stimulus is first conducted from the sensitive cell to this intermediate presentative or psychic cell, and then issued from this to the motor muscular cell as a mandate of movement. These tricellular reflex organs are preponderantly developed in the great majority of the invertebrates.

VII. Instead of this arrangement we find in most of the vertebrates a quadricellular reflex organ, two distinct “soul-cells,” instead of one, being inserted between the sensitive cell and the motor cell. The external stimulus, in this case, is first conducted centripetally to the sensitive cell (the sensible psychic cell), from this to the will-cell (the motor psychic cell), and from this, finally, to the contractile muscular cell. When many such reflex organs combine and new psychic cells are interposed we have the intricate reflex mechanism of man and the higher vertebrates.

The important distinction which we make, in morphology and physiology, between unicellular and multicellular organisms holds good for their elementary psychic activity, reflex action. In the unicellular protists (both the plasmodomous primitive plants, or protophyta, and the plasmophagous primitive animals, or protozoa) the whole physical process of reflex action takes place in the protoplasm of one single cell; their “cell-soul” seems to be a unifying function of the psychoplasm of which the various phases only begin to be seen separately when the differentiation of special organs sets in.

The second stage of psychic activity, compound reflex action, begins with the cenobitic protists (v.g., the volvox and the carchesium). The innumerable social cells, which make up this cell-community or cœnobium, are always more or less connected, often directly connected by filamentous bridges of protoplasm. A stimulus that alights on one or more cells of the community is communicated to the rest by means of the connecting fibres, and may produce a general contraction. This connection is found, also, in the tissues of the multicellular animals and plants. It was erroneously believed at one time that the cells of vegetal tissue were completely isolated from each other, but we have now discovered fine filaments of protoplasm throughout, which penetrate the thick membranes of the cells, and maintain a material and psychological communication between their living plasmic contents. That is the explanation of the mimosa: when the tread of the passer-by shakes the root of the plant, the stimulus is immediately conveyed to all the cells, and causes a general contraction of its tender leaves and a drooping of the stems.

An important and universal feature of all reflex phenomena is the absence of consciousness. For reasons which we shall give in the [tenth chapter] we only admit the presence of consciousness in man and the higher animals, not in plants, the lower animals, and the protists; consequently all stimulated movements in the latter must be regarded as reflex—that is, all movements which are not spontaneous, not the outcome of internal causes (impulsive and automatic movements).[14] It is different with the higher animals which have developed a centralized nervous system and elaborate sense-organs. In these cases consciousness has been gradually evolved from the psychic reflex activity, and now conscious, voluntary action appears, in opposition to the still continuing reflex action below. However, we must distinguish two different processes, as we did in the question of instinct—primary and secondary reflex action. Primary reflex actions are those which have never reached the stage of consciousness in phyletic development, and thus preserve the primitive character (by heredity from lower animal forms). Secondary reflex actions are those which were conscious, voluntary actions in our ancestors, but which afterwards became unconscious from habit or the lapse of consciousness. It is impossible to draw a hard and fast line in such cases between conscious and unconscious psychic function.

Older psychologists (Herbart, for instance) considered “presentation” to be the fundamental psychic phenomenon, from which all the others are derived. Modern comparative psychology endorses this view in so far as it relates to the idea of unconscious presentation; but it considers conscious presentation to be a secondary phenomenon of mental life, which is entirely wanting in plants and the lower animals, and is only developed in the higher animals. Among the many contradictory definitions which psychologists have given of “presentation,” we think the best is that which makes it consist in an internal picture of the external object which is given us in sensation—an “idea,” in the broader sense. We may distinguish the following four stages in the rising scale of presentative function:

I. Cellular presentation.—At the lowest stages we find presentation to be a general physiological property of psychoplasm; even in the simplest unicellular protist sensations may leave a permanent trace in the psychoplasm, and these may be reproduced by memory. In more than four thousand kinds of radiolaria, which I have described, every single species is distinguished by special, hereditary skeletal structure. The construction of this specific, and often highly elaborate, skeleton by a cell of the simplest description (generally globular) is only intelligible when we attribute the faculty of presentation, and, indeed, of a special reproduction of the plastic “feeling of distance,” to the constructive protoplasm—as I have pointed out in my Psychology of the Radiolaria.[15]

II. Histionic presentation.—In the cœnobia or cell-colonies of the social protists, and still better in the tissues of plants and lower, nerveless animals (sponges, polyps, etc.), we find the second stage of unconscious presentation, which consists of the common psychic activity of a number of closely connected cells. If a single stimulus may, instead of simply spending itself in the reflex movement of an organ (the leaf of a plant, for instance, or the arm of a polyp), leave a permanent impression, which can be spontaneously reproduced later on, we are bound to assume, in explaining the phenomenon, a histionic presentation, dependent on the psychoplasm of the associated tissue-cells.

III. Unconscious presentation in the ganglionic cells.—This third and higher stage of presentation is the commonest form the function takes in the animal world; it seems to be a localization of presentation in definite “soul-cells.” In its simplest form it appears at the sixth stage of reflex action, when the tricellular reflex organ arises: the seat of presentation is then the intermediate psychic cell, which is interposed between the sensitive cell and the muscular cell. With the increasing development of the animal nervous system and its progressive differentiation and integration, this unconscious presentation also rises to higher stages.

IV. Conscious presentation in the cerebral cells.—With the highest stage of development of the animal organization consciousness arises, as a special function of a certain central organ of the nervous system. As the presentations are conscious, and as special parts of the brain arise for the association of these conscious presentations, the organism is qualified for those highest psychic functions which we call thought and reflection, intellect and reason. Although the tracing of the phyletic barrier between the older, unconscious, and the younger, conscious, presentation is extremely difficult, we can affirm, with some degree of probability, that the evolution of the latter from the former was polyphyletic; because we find conscious and rational thought, not only in the highest forms of the vertebrate stem (man, mammals, birds, and a part of the lower vertebrates), but also in the most highly developed representatives of other animal groups (ants and other insects, spiders and the higher crabs among the articulata, cephalopods among the mollusca).

The evolutionary scale of memory is closely connected with that of presentation; this extremely important function of the psychoplasm—the condition of all further psychic development—consists essentially in the reproduction of presentations. The impressions in the bioplasm, which the stimulus produced as sensations, and which became presentations in remaining, are revived by memory; they pass from potentiality to actuality. The latent potential energy of the psychoplasm is transformed into kinetic energy. We may distinguish four stages in the upward development of memory, corresponding to the four stages of presentation.

I. Cellular memory.—Thirty years ago Ewald Hering showed “memory to be a general property of organized matter” in a thoughtful work, and indicated the great significance of this function, “to which we owe almost all that we are and have.” Six years later, in my work on The Perigenesis of the Plastidule, or the Undulatory Origin of the Parts of Life: an Experiment in the Mechanical Explanation of Elementary Evolutionary Processes, I developed these ideas, and endeavored to base them on the principles of evolution. I have attempted to show in that work that unconscious memory is a universal and very important function of all plastidules; that is, of those hypothetical molecules, or groups of molecules, which Naegeli has called micellae, others bioplasts, and so forth. Only living plastidules, as individual molecules of the active protoplasm, are reproductive, and so gifted with memory; that is the chief difference between the organic and inorganic worlds. It might be stated thus: “Heredity is the memory of the plastidule, while variability is its comprehension.” The elementary memory of the unicellular protist is made up of the molecular memory of the plastidules or micellae, of which its living cell-body is constructed. As regards the extraordinary performances of unconscious memory in these unicellular protists, nothing could be more instructive than the infinitely varied and regular formation of their defensive apparatus, their shells and skeletons; in particular, the diatomes and cosmaria among the protophytes, and the radiolaria and thalamophora among the protozoa, afford an abundance of most interesting illustrations. In many thousand species of these protists the specific form which is inherited is relatively constant, and proves the fidelity of their unconscious cellular memory.

II. Histionic memory.—Equally interesting examples of the second stage of memory, the unconscious memory of tissues, are found in the heredity of the individual organs of plants and the lower, nerveless animals (sponges, etc.). This second stage seems to be a reproduction of the histionic presentations, that association of cellular presentations which sets in with the formation of cœnobia in the social protists.

III. In the same way we must regard the third stage, the unconscious memory of those animals which have a nervous system, as a reproduction of the corresponding “unconscious presentations” which are stored up in certain ganglionic cells. In most of the lower animals all memory is unconscious. Moreover, even in man and the higher animals, to whom we must ascribe consciousness, the daily acts of unconscious memory are much more numerous and varied than those of the conscious faculty; we shall easily convince ourselves of that if we make an impartial study of a thousand unconscious acts we perform daily out of habit, and without thinking of them, in walking, speaking, writing, eating, and so forth.

IV. Conscious memory, which is the work of certain brain-cells in man and the higher animals, is an “internal mirroring” of very late development, the highest outcome of the same psychic reproduction of presentations which were mere unconscious processes in the ganglionic cells of our lower animal ancestors.

The concatenation of presentations—usually called the association of ideas—also runs through a long scale, from the lowest to the highest stages. This, too, is originally and predominantly unconscious (“instinct”); only in the higher classes of animals does it gradually become conscious (“reason”). The psychic results of this “association of ideas” are extremely varied; still, a very long, unbroken line of gradual development connects the simplest unconscious association of the lowest protist with the elaborate conscious chain of ideas of the civilized man. The unity of consciousness in man is given as its highest consequence (Hume, Condillac). All higher mental activity becomes more perfect in proportion as the normal association extends to more numerous presentations, and in proportion to the order which is imposed on them by the “criticism of pure reason.” In dreams, where this criticism is absent, the association of the reproduced impressions often takes the wildest forms. Even in the work of the poetic imagination, which constructs new groups of images by varying the association of the impressions received, and in hallucinations, etc., they are often most unnaturally arranged, and seem to the prosaic observer to be perfectly irrational. This is especially true of supernatural “forms of belief,” the apparitions of spiritism, and the fantastic notions of the transcendental dualist philosophy; though it is precisely these abnormal associations of “faith” and of “revelation” that have often been deemed the greatest treasures of the human mind (cf. [chap. xvi].).

The antiquated psychology of the Middle Ages (which, however, still numbers many adherents) considered the mental life of man and that of the brute to be two entirely different phenomena; the one it attributed to “reason,” the other to “instinct.” In harmony with the traditional story of creation, it was assumed that each animal species had received a definite, unconscious psychic force from the Creator at its formation, and that this instinct of each species was just as unchangeable as its bodily structure. Lamarck proved the untenableness of this error in 1809 by establishing the theory of Descent, and Darwin completely demolished it in 1859. He proved the following important theses with the aid of his theory of selection:

1. The instincts of species show individual differences, and are just as subject to modification under the law of adaptation as the morphological features of their bodily structure.

2. These modifications (generally arising from a change of habits) are partly transmitted to offspring by heredity, and thus accumulate and are accentuated in the course of generations.

3. Selection, both artificial and natural, singles out certain of these inherited modifications of the psychic activity; it preserves the most useful and rejects the least adaptive.

4. The divergence of psychic character which thus arises leads, in the course of generations, to the formation of new instincts, just as the divergence of morphological character gives rise to new species.

Darwin’s theory of instinct is now accepted by most biologists; Romanes has treated it so ably, and so greatly expanded it in his distinguished work on Mental Evolution in the Animal World, that I need merely refer to it here. I will only venture the brief statement that, in my opinion, there are instincts in all organisms—in all the protists and plants as well as in all the animals and in man; though in the latter they tend to disappear in proportion as reason makes progress at their expense.

The two chief classes of instincts to be differentiated are the primary and secondary. Primary instincts are the common lower impulses which are unconscious and inherent in the psychoplasm from the commencement of organic life; especially the impulses to self-preservation (by defence and maintenance) and to the preservation of the species (by generation and the care of the young). Both these fundamental instincts of organic life, hunger and love, sprang up originally in perfect unconsciousness, without any co-operation of the intellect or reason. It is otherwise with the secondary instincts. These were due originally to an intelligent adaptation, to rational thought and resolution, and to purposive conscious action. Gradually, however, they became so automatic that this “other nature” acted unconsciously, and, even through the action of heredity, seemed to be “innate” in subsequent generations. The consciousness and deliberation which originally accompanied these particular instincts of the higher animals and man have died away in the course of the life of the plastidules (as in “abridged heredity”). The unconscious purposive actions of the higher animals (for instance, their mechanical instincts) thus come to appear in the light of innate impulses. We have to explain in the same way the origin of the “à priori ideas” of man; they were originally formed empirically by his predecessors.[16]

In the superficial psychological treatises which ignore the mental activity of animals and attribute to man only a “true soul,” we find him credited also with the exclusive possession of reason and consciousness. This is another trivial error (still to be found in many a manual, nevertheless) which the comparative psychology of the last forty years has entirely dissipated. The higher vertebrates (especially those mammals which are most nearly related to man) have just as good a title to “reason” as man himself, and within the limits of the animal world there is the same long chain of the gradual development of reason as in the case of humanity. The difference between the reason of a Goethe, a Kant, a Lamarck, or a Darwin, and that of the lowest savage, a Veddah, an Akka, a native Australian, or a Patagonian, is much greater than the graduated difference between the reason of the latter and that of the most “rational” mammals, the anthropoid apes, or even the papiomorpha, the dog, or the elephant. This important thesis has been convincingly proved by the thoroughly critical comparative work of Romanes and others. We shall not, therefore, attempt to cover that ground here, nor to enlarge on the distinction between the reason and the intellect; as to the meaning and limits of these concepts philosophic experts give the most contradictory definitions, as they do on so many other fundamental questions of psychology. In general it may be said that the process of the formation of concepts, which is common to both these cerebral functions, is confined to the narrower circle of concrete, proximate associations in the intellect, but reaches out to the wider circle of abstract, more comprehensive groups of associations in the work of reason. In the long gradation which connects the reflex actions and the instincts of the lower animals with the reason of the highest, intellect precedes the latter. And there is the fact, of great importance to our whole psychological treatise, that even these highest of our mental faculties are just as much subject to the laws of heredity and adaptation as are their respective organs; Flechsig pointed out in 1894 that the “organs of thought,” in man and the higher mammals, are those parts of the cortex of the brain which lie between the four inner sense-centres (cf. chapters [x]. and [xi].).

The higher grade of development of ideas, of intellect and reason, which raises man so much above the brute, is intimately connected with the rise of language. Still here also we have to recognize a long chain of evolution which stretches unbroken from the lowest to the highest stages. Speech is no more an exclusive prerogative of man than reason. In the wider sense, it is a common feature of all the higher gregarious animals, at least of all the articulata and the vertebrates, which live in communities or herds; they need it for the purpose of understanding each other and communicating their impressions. This is effected either by touch or by signs, or by sounds having a definite meaning. The song of the bird or of the anthropoid ape (hylobates), the bark of the dog, the neigh of the horse, the chirp of the cricket, the cry of the cicada, are all specimens of animal speech. Only in man, however, has that articulate conceptual speech developed which has enabled his reason to attain such high achievements. Comparative philology, one of the most interesting sciences that has arisen during the century, has shown that the numerous elaborate languages of the different nations have been slowly and gradually evolved from a few simple primitive tongues (Wilhelm Humboldt, Bopp, Schleicher, Steinthal, and others). August Schleicher, of Jena, in particular, has proved that the historical development of language takes place under the same phylogenetic laws as the evolution of other physiological faculties and their organs. Romanes (1893) has expanded this proof, and amply demonstrated that human speech, also, differs from that of the brute only in degree of development, not in essence and kind.

The important group of psychic activities which we embrace under the name of “emotion” plays a conspicuous part both in theoretical and practical psychology. From our point of view they have a peculiar importance from the fact that we clearly see in them the direct connection of cerebral functions with other physiological functions (the beat of the heart, sense-action, muscular movement, etc.); they, therefore, prove the unnatural and untenable character of the philosophy which would essentially dissociate psychology from physiology. All the external expressions of emotional life which we find in man are also present in the higher animals (especially in the anthropoid ape and the dog); however varied their development may be, they are all derived from the two elementary functions of the psyche, sensation and motion, and from their combination in reflex action and presentation. To the province of sensation, in a wide sense, we must attribute the feeling of like and dislike which determines the emotion; while the corresponding desire and aversion (love and hatred), the effort to attain what is liked and avoid what is disliked, belong to the category of movement. “Attraction” and “repulsion” seem to be the sources of will, that momentous element of the soul which determines the character of the individual. The passions, which play so important a part in the psychic life of man, are but intensifications of emotion. Romanes has recently shown that these also are common to man and the brute. Even at the lowest stage of organic life we find in all the protists those elementary feelings of like and dislike, revealing themselves in what are called their tropisms, in the striving after light and darkness, heat or cold, and in their different relations to positive and negative electricity. On the other hand, we find at the highest stage of psychic life, in civilized man, those finer shades of emotion, of delight and disgust, of love and hatred, which are the mainsprings of civilization and the inexhaustible sources of poetry. Yet a connecting chain of all conceivable gradations unites the most primitive elements of feeling in the psychoplasm of the unicellular protist with the highest forms of passion that rule in the ganglionic cells of the cortex of the human brain. That the latter are absolutely amenable to physical laws was proved long ago by the great Spinoza in his famous Statics of Emotion.

The notion of will has as many different meanings and definitions as most other psychological notions—presentation, soul, mind, and so forth. Sometimes will is taken in the widest sense as a cosmic attribute, as in the “World as will and presentation” of Schopenhauer; sometimes it is taken in its narrowest sense as an anthropological attribute, the exclusive prerogative of man—as Descartes taught, for instance, who considered the brute to be a mere machine, without will or sensation. In the ordinary use of the term, will is derived from the phenomenon of voluntary movement, and is thus regarded as a psychic attribute of most animals. But when we examine the will in the light of comparative physiology and evolution, we find—as we do in the case of sensation—that it is a universal property of living psychoplasm. The automatic and the reflex movements which we observe everywhere, even in the unicellular protists, seem to be the outcome of inclinations which are inseparably connected with the very idea of life. Even in the plants and lowest animals these inclinations, or tropisms, seem to be the joint outcome of the inclinations of all the combined individual cells.

But when the “tricellular reflex organ” arises (page 115), and a third independent cell—the “psychic,” or “ganglionic,” cell—is interposed between the sense-cell and the motor cell, we have an independent elementary organ of will. In the lower animals, however, this will remains unconscious. It is only when consciousness arises in the higher animals, as the subjective mirror of the objective, though internal, processes in the neuroplasm of the psychic cells, that the will reaches that highest stage which likens it in character to the human will, and which, in the case of man, assumes in common parlance the predicate of “liberty.” Its free dominion and action become more and more deceptive as the muscular system and the sense-organs develop with a free and rapid locomotion, entailing a correlative evolution of the brain and the organs of thought.

The question of the liberty of the will is the one which has more than any other cosmic problem occupied the time of thoughtful humanity, the more so that in this case the great philosophic interest of the question was enhanced by the association of most momentous consequences for practical philosophy—for ethics, education, law, and so forth. Emil du Bois-Reymond, who treats it as the seventh and last of his “seven cosmic problems,” rightly says of the question: “Affecting everybody, apparently accessible to everybody, intimately involved in the fundamental conditions of human society, vitally connected with religious belief, this question has been of immeasurable importance in the history of civilization. There is probably no other object of thought on which the modern library contains so many dusty folios that will never again be opened.” The importance of the question is also seen in the fact that Kant put it in the same category with the questions of the immortality of the soul and belief in God. He called these three great questions the indispensable “postulates of practical reason,” though he had already clearly shown them to have no reality whatever in the light of pure reason.

The most remarkable fact in connection with this fierce and confused struggle over the freedom of the will is, perhaps, that it has been theoretically rejected, not only by the greatest critical philosophers, but even by their extreme opponents, and yet it is still affirmed to be self-evident by the majority of people. Some of the first teachers of the Christian Churches—such as St. Augustine and Calvin—rejected the freedom of the will as decisively as the famous leaders of pure materialism, Holbach in the eighteenth and Büchner in the nineteenth century. Christian theologians deny it, because it is irreconcilable with their belief in the omnipotence of God and in predestination. God, omnipotent and omniscient, saw and willed all things from eternity—he must, consequently, have predetermined the conduct of man. If man, with his free will, were to act otherwise than God had ordained, God would not be all-mighty and all-knowing. In the same sense Leibnitz, too, was an unconditional determinist. The monistic scientists of the last century, especially Laplace, defended determinism as a consequence of their mechanical view of life.

The great struggle between the determinist and the indeterminist, between the opponent and the sustainer of the freedom of the will, has ended to-day, after more than two thousand years, completely in favor of the determinist. The human will has no more freedom than that of the higher animals, from which it differs only in degree, not in kind. In the last century the dogma of liberty was fought with general philosophic and cosmological arguments. The nineteenth century has given us very different weapons for its definitive destruction—the powerful weapons which we find in the arsenal of comparative physiology and evolution. We now know that each act of the will is as fatally determined by the organization of the individual and as dependent on the momentary condition of his environment as every other psychic activity. The character of the inclination was determined long ago by heredity from parents and ancestors; the determination to each particular act is an instance of adaptation to the circumstances of the moment wherein the strongest motive prevails, according to the laws which govern the statics of emotion. Ontogeny teaches us to understand the evolution of the will in the individual child. Phylogeny reveals to us the historical development of the will within the ranks of our vertebrate ancestors.

[CHAPTER VIII]
THE EMBRYOLOGY OF THE SOUL

Importance of Ontogeny to Psychology—Development of the Child-Soul—Commencement of Existence of the Individual Soul—The Storing of the Soul—Mythology of the Origin of the Soul—Physiology of the Origin of the Soul—Elementary Processes in Conception—Coalescence of the Ovum and the Spermatozoon—Cell-Love—Heredity of the Soul from Parents and Ancestors—Its Physiological Nature as the Mechanics of the Protoplasm—Blending of Souls (Psychic Amphigony)—Reversion, Psychological Atavism—The Biogenetic Law in Psychology—Palingenetic Repetition and Cenogenetic Modification—Embryonic and Post-Embryonic Psychogeny

The human soul—whatever we may hold as to its nature—undergoes a continual development throughout the life of the individual. This ontogenetic fact is of fundamental importance in our monistic psychology, though the “professional” psychologists pay little or no attention to it. Since the embryology of the individual is, on Baer’s principle—and in accordance with the universal belief of modern biologists—the “true torch-bearer for all research into the organic body,” it will afford us a reliable light on the momentous problems of its psychic activity.

Although, however, this “embryology of the soul” is so important and interesting, it has hitherto met with the consideration it deserves only within a very narrow circle. Until recently teachers were almost the only ones to occupy themselves with a part of the problem; since their avocation compelled them to assist and supervise the formation of the psychic activity in the child, they were bound to take a theoretical interest, also, in the psychogenetic facts that came under their notice. However, these teachers, for the most part, both in recent and in earlier times, were dominated by the current dualistic psychology—in so far as they reflected at all; and they were totally ignorant of the important facts of comparative psychology, and unacquainted with the structure and function of the brain. Moreover, their observations only extended to children in their school-days, or in the years immediately preceding. The remarkable phenomena which the individual psychogeny of the child offers in its earliest years, and which are the joy and admiration of all thoughtful parents, were scarcely ever made the subject of serious scientific research. Wilhelm Preyer was the pioneer of this study in his interesting work on The Mind of the Child (1881). To obtain a perfectly clear knowledge of the matter, however, we must go further back still; we must commence at the first appearance of the soul in the impregnated ovum.

The origin of the human individual—body and soul—was still wrapped in complete mystery at the beginning of the nineteenth century. Caspar Friedrich Wolff had, it is true, discovered the true character of embryonic development in 1759, in his theoria generationis, and proved with the confidence of a critical observer that there is a true epigenesis—i.e., a series of very remarkable formative processes—in the evolution of the fœtus from the simple ovum. But the physiologists of the time, with the famous Albert Haller at their head, flatly refused to entertain these empirical truths, which may be directly proved by microscopic observation, and clung to the old dogma of “preformation.” This theory assumed that in the human ovum—and in the egg of all other animals—the organism was already present, or “preformed,” in all its parts; the “evolution” of the embryo consisted literally in an “unfolding” (evolutio) of the folded organs. One curious consequence of this error was the theory of scatulation, which we have mentioned on [p. 55]; since the ovary had to be admitted to be present in the embryo of the woman, it was also necessary to suppose that the germs of the next generation were already formed in it, and so on in infinitum. Opposed to this dogma of the “Ovulists” was the equally erroneous notion of the “Animalculists”; the latter held that the germ was not really in the female ovum, but in the paternal element, and that the store of succeeding generations was to be sought in the spermatozoa.

Leibnitz consistently applied this theory of scatulation to the human soul; he denied that either soul or body had a real development (epigenesis), and said in his Theodicy: “Thus I consider that the souls which are destined one day to become human exist in the seed, like those of other species; that they have existed in our ancestors as far back as Adam—that is, since the beginning of the world—in the forms of organized bodies.” Similar notions prevailed in biology and philosophy until the third decade of the present century, when the reform of embryology by Baer gave them their death blow. In the province of psychology, however, they still find many adherents; they form one group of the many curious mystical ideas which give us a living illustration of the ontogeny of the soul.

The more accurate knowledge which we have recently obtained, through comparative ethnology, of the various forms of myths of ancient and modern uncivilized races, is also of great interest in psychogeny. Still, it would take us too far from our purpose if we were to enter into it with any fulness here; we must refer the reader to Adalbert Svoboda’s excellent work on Forms of Faith (1897). In respect of their scientific and poetical contents, we may arrange all pertinent psychogenetic myths in the following five groups:

I. The myth of transmigration.—The soul lived formerly in the body of another animal, and passed from this into a human body. The Egyptian priests, for instance, taught that the human soul wandered through all the species of animals after the death of the body, returning to a human frame after three thousand years of transmigration.

II. The myth of the in-planting of the soul.—The soul existed independently in another place—a psychogenetic store, as it were (in a kind of embryonic slumber or latent life); it was taken out by a bird (sometimes represented as an eagle, generally as a white stork), and implanted in the human body.

III. The myth of the creation of the soul.—God creates the souls, and keeps them stored—sometimes in a pond (living in the form of plankton), according to other myths in a tree (where they are conceived as the fruit of a phanerogam); the Creator takes them from the pond or tree, and inserts them in the human germ during the act of conception.

IV. The myth of the scatulation of the soul (the theory of Leibnitz which we have given above).

V. The myth of the division of the soul (the theory of Rudolph Wagner [1855] and of other physiologists).—In the act of procreation a portion is detached from both the (immaterial) souls of the parents; the maternal contribution passes in the ovum, the paternal in the spermatozoa; when these two germinal cells coalesce, the two psychic fragments that accompany them also combine to form a new (immaterial) soul.

Although the poetic fancies we have mentioned as to the origin of the individual human soul are still widely accepted, their purely mythological character is now firmly established. The deeply interesting and remarkable research which has been made in the course of the last twenty-five years into the more minute processes of the impregnation and germination of the ovum has made it clear that these mysterious phenomena belong entirely to the province of cellular physiology (cf. [p. 48]). Both the female element, the ovum, and the male fertilizing body, the sperma or spermatozoa, are simple cells. These living cells possess a certain sum of physiological properties to which we give the title of the “cell-soul,” just as we do in the permanently unicellular protist (see [p. 48]). Both germinal cells have the faculty of movement and sensation. The young ovum, or egg-cell, moves after the manner of an amœba; the minute spermatozoa, of which there are millions in every drop of the seminal fluid, are ciliated cells, and swim about as freely in the sperm, by means of their lashes or cilia, as the ordinary ciliated infusoria (the flagellata).

When the two cells meet as a result of copulation, or when they are brought into contact through artificial fertilization (in the fishes, for instance), they attract each other and become firmly attached. The main cause of this cellular attraction is a chemical sensitive action of the protoplasm, allied to smell or taste, which we call “erotic chemicotropism”; it may also be correctly (both in the chemical and the romantic sense) termed “cellular affinity” or “sexual cell-love.” A number of the ciliated cells in the sperm swim rapidly towards the stationary egg-cell and seek to penetrate into it. As Hertwig showed in 1875, as a rule only one of the suitors is fortunate enough to reach the desired goal. As soon as this favored spermatozoon has pierced into the body of the ovum with its head (the nucleus of the cell), a thin mucous layer is detached from the ovum which prevents the further entrance of spermatozoa. The formation of this protective membrane was only prevented when Hertwig kept the ovum stiff with cold by lowering the temperature, or benumbed it with narcotics (chloroform, morphia, nicotine, etc.); then there was “super-impregnation” or “poly-spermy”—a number of sperm-threads pierced into the body of the unconscious ovum. This remarkable fact proved that there is a low degree of “cellular instinct” (or, at least, of specific, lively sensation) in the sexual cells just as effectively as do the important phenomena that immediately follow in their interior. Both nuclei—that of the ovum and of the spermatozoon—attract each other, approach, and, on contact, completely fuse together. Thus from the impregnated ovum arises the important new cell which we call the “stem-cell” (cytula), from the repeated segmentation of which the whole polycellular organism is evolved.

The psychological information which is afforded by these remarkable facts of impregnation, which have only been properly observed during the last twenty-five years, is supremely important; its vast significance has hitherto been very far from appreciated. We shall condense the main conclusions of research in the following five theses:

I. Each human individual, like every other higher animal, is a single simple cell at the commencement of his existence.

II. This “stem-cell” (cytula) is formed in the same manner in all cases—that is, by the blending or copulation of two separate cells of diverse origin, the female ovum and the male spermatozoon.

III. Each of these sexual cells has its own “cell-soul”—that is, each is distinguished by a peculiar form of sensation and movement.

IV. At the moment of conception or impregnation, not only the protoplasm and the nuclei of the two sexual cells coalesce, but also their “cell-souls”; in other words, the potential energies which are latent in both, and inseparable from the matter of the protoplasm, unite for the formation of a new potential energy, the “germ-soul” of the newly constructed stem-cell.