Smithsonian Report/1898/On Our Present Knowledge of the Origin of Man
ON OUR PRESENT KNOWLEDGE OF THE ORIGIN OF MAN.[1]
By Ernst Haeckel.
At the close of the nineteenth century we look with just pride on the mighty and incomparable advances which human science and culture have made during its course—the natural sciences excelling all others. These facts are characteristically expressed by the statement we often hear that this is the "great" century, or the "age of natural science." Every single science that concerns itself with the knowledge and history of nature claims for itself that it can show the greatest advances and excels all others, and it can also show good ground for such an opinion. But a nonpartisan and unprejudiced philosopher who should survey the entire field would award the first prize of victory to our zoology above all others; for it was in her bosom that was born evolution, or the theory of descent, that powerful branch of the theory of development for which John Lamarck, in 1809, laid the foundation, and which fifty years later Charles Darwin brought to general attention.
It is not my task to lay before you now the fundamental significance and the priceless worth of the theory of descent. Indeed, the entire science of biology is to-day interpenetrated with it. No great and general question in zoology and botany, in anatomy and physiology, can be discussed and solved without the question of origins, "the genesis of the generated," presenting itself before everything else. This question was, however, quite unknown when Charles Darwin, the great reformer of biology, began his academic studies in Cambridge, and, indeed, as a student of divinity. This occurred in that memorable year, 1828, in which Carl Ernst von Baer published in Germany his classical "History of the development of animals," the first successful attempt to explain by "observation and reflection" the genesis of the animal body, and to investigate the "history of the growing individual in every relation," from the simplest germ throughout the completed cycle. Darwin knew nothing of these mighty advances, and he could have had no presentiment that this history of germs, embryology, or ontogeny, was, fifty years later, to be the most important basis of the work upon which he was to spend his life, the most secure support of that doctrine of descent which was founded by Lamarck in the very year of Darwin's birth, and which was at that time received with warm approbation by his grandfather, Erasmus Darwin.
Of all the naturalists of the nineteenth century Charles Darwin has certainly had the greatest success and the most powerful influence. We often, indeed, call the last forty years the "Darwinian age." And if we investigate more closely the causes of this unexampled success we will see, as I have repeatedly said, that they depend upon three important services rendered: (1) The total reform of the theory of descent or doctrine of Lamarck; (2) the founding of the new theory of natural selection, the special Darwinian theory; and (3) the development of the science of the evolution of man, that most important deduction from the theory of descent, which far exceeds in significance all other problems of the doctrine of evolution.
I shall to-day, before this zoological congress, speak only of the last-named service of Darwin, and do this for the especial purpose of showing critically the certainty to which we have attained in our present knowledge of the origin of man and of the various branches of his genealogical tree. That this is one of the most important of all scientific questions is to-day no longer disputed. For all other problems which the human mind can investigate and understand are conditioned chiefly by the psychological theory of perception, and this again depends upon the animal nature of man, upon his origin, his development, and his mental powers. With good reason, then, did the greatest zoologist of our century, Thomas Huxley, characterize this problem as the "question of questions for mankind," as the "problem which underlies all others and is more deeply interesting than any other." This was done in 1863 in the second of those three masterly essays which for the first time thoroughly examined the "Evidence as to man's place in nature" in the light of the Darwinian theory; the first, treating of the anthropoid apes, the second of the relations of man to the next lower animals, the third of some fossil human remains. Darwin himself, in 1859, in his principal work, On the Origin of Species, had purposely avoided referring to these consequences of his doctrine except in a brief, significant, passing allusion that, by its means, light would be thrown on the origin of man and his history. Later (1871) in his famous work on The Descent of Man, and Selection in Relation to Sex, Darwin brought forward in a most able manner, both the morphological and the historical as well as the physiological and psychological side of this problem.
I had myself, in 1886, in my Generelle Morphologie, estimated the importance of the "history of the development of organisms as bearing upon anthropology," and especially remarked that the fundamental biogenetic law held good for man also; with him, as with all other organisms, there is the most intimate causal connection between ontogeny and phylogeny founded upon progressive inheritance, between the history of the development of the germ of the individual and the history of the development of his ancestral stock. In the latter, I, at that time, distinguished ten different principal degrees within the vertebrate stock. I dwelt especially, however, upon the logical connection between the evolution of man and the theory of modification by descent; if the latter is true it gives absolute validity to the former^ "The proposition that man has developed from the lower vertebrates and, indeed, immediately from the true apes is a special deduction which must necessarily result from the general induction made by the law of the theory of descent." I showed the further development and results of this conception in the various editions of my Naturliche Schöpfungs-Geschichte (first edition, 1868; ninth edition, 1898), and my Anthropogenie (first edition, 1874; fourth edition, 1891), its firm foundation was shown in the third part of my Systematische Phylogenie (1895).
It is well known that in the course of the forty years which have passed since the first publication of Darwin's theory an extensive polemical literature has appeared, relating to both its general significance and to the evolution of man, its most important result. That the latter is indissolubly connected with the former is now generally recognized, and it is exactly this intimate connection that explains the stubborn resistance that has been shown to the entire theory of evolution by all mystical and orthodox schools, by all men who have not been able to free themselves from the traditional anthropocentric superstition. In the sharp fight that has ensued on this subject the most varied weapons have been used. We can refer here only to certain exceptions based upon empirical biological grounds; we must disregard all those numerous assaults based upon metaphysical and mystical speculations made by those ignorant of the empirical but well established facts of biology. The most important part of our task will, therefore, be the critical examination of the three evidential sciences which we place at the base of all phylogenetic researches: paleontology, comparative anatomy, and ontogeny. We must cast a glance upon the advances made during the last ten years in these three auxiliaries of the science of the evolution of man and thus ascertain the degree of certainty to which a knowledge of his origin has attained by reason of these advances.
First, we have to examine the position which modern zoology, supported by comparative anatomy, gives to man in the natural system of the animal kingdom. For the aim of the natural system itself is to establish the hypothetical family tree and all the single groups, greater or smaller, which we distinguish as classes, legions, orders, families, genera, and species in the same stock are only different twigs and branches of this tree. Now, the systematic place which should be assigned to man in consideration of all the details of his bodily structure remained for a long time doubtful. When the great Lamarck, at the beginning of this century, grouped together us vertebrates the four higher of the six classes of animals, he immediately assigned to man a position at their head. Linnæus himself had already, in 1735, in his fundamental "Systema Naturæ," placed man at the head of the mammals, grouping him with the apes and the lemurs in the "Anthropomorpha," or man-like creatures; later he called these dominant animals, or "Primates," the "lords of creation."
Man possesses in his bodily structure all the marks by which mammals are separated from other vertebrates, and there has, therefore, never been any controversy about his belonging to this class. On the contrary, there are, even to this day, differences of opinion as to the place to which man should be assigned in one of the orders of mammals. Cuvier, when he made a new scientific classification of animals (1817), followed the precedent of Blumenbach and created for man the special order Bimana, or two-handed animals, in opposition to the apes and lemurs, who were known as the Quadrumana, or four-handed animals. This arrangement was retained for half a century in most text-books. It first became untenable when Huxley showed, in 1863, that it was based upon an anatomical error, and that the apes were in truth as much two handed as man. Thereupon the order of Primates in the Linnæan sense was again restored.
Most authors in the last thirty years have separated the Primates into three suborders: (1) the lemurs (Prosimiæ); (2) the apes (Simiæ), and (3) men (Anthropi). Again, other zoologists assign to man only the rank of another family in the order of apes. The polymorphic group of true apes (Simiæ or Pitheca) falls into two natural divisions that are geographically quite distinct and have developed entirely independent of each other in the western and the eastern hemispheres. The American or western apes (Hesperopitheca) are distinguished by a short, bony, auditory passage and a broad nasal septum. They are therefore called the flat-nosed apes (Platyrrhinæ). On the contrary, the apes that inhabit Asia and Africa (in early times Europe also) have, like man, a long auditory passage and a narrow nasal septum. They are therefore called Old-World apes (Eopitheca), or also narrow-nosed apes (Catarrhinæ). As man has in the rest of his bodily structure the morphological characters of the Old-World apes and is, like them, thus distinguished from the apes of the New World, certain zoologists have assigned to him a situation within the former group. Undoubtedly this suborder of the catarrhines is an entirely natural division, whose numerous living and extinct species are clearly united by many important characters of bodily structure, but it embraces, nevertheless, a long series of very different structural stages. The lowest dog-apes (Cynopitheca), especially the baboons (Papiomorpha), appear like a repulsive caricature of the noble human form. They remain at a very low stage of development and are allied to the older platyrrhines and prosimians. On the other hand, the tailless apes (Anthropomorpha) rise to a height of organization that makes clear as day the immediate transition to the human form. For that reason one of the most profound students of the anatomy of primates, Robert Hartmann, went so far as to separate the entire order of primates into three families: (1) Primarii (man and the anthropoid apes); (2) Simiæ, or apes proper (catarrhines and platyrrhines); (3) Prosimiæ (lemurs). This arrangement seems justified by the interesting statement made by Selenka (1890) that the quite peculiar formation of the placenta of man is found in the anthropoids, but not in the other apes.
Decisive for the question as to which of these various classifications we should prefer was the proposition advanced by Huxley, in 1863, after a careful and critical examination of all the anatomical relations within the order of primates, and which I have called in his honor "Huxley's law," or Huxley's pithecometric proposition: "Whatever system of organs be studied, the comparison of their modifications in the ape series leads us to one and the same result—that the structural differences which separate man from the gorilla and the chimpanzee are not so great as those which separate the gorilla from the lower apes." Thereupon it becomes necessary for every unprejudiced taxonomist to give man a systematic place within the order of the apes. By the most conscientious testing of each difference, and by the most severe logical inference, we can, however, go a step further and instead of using the wider term apes (Simiæ), use the more restricted one of Old-World apes (Catarrhinæ). The standard pithecometric proposition would then be worded in this more exact way: "The comparative auatomy of all organs within the catarrhine group leads us to one and the same result—the morphological differences between man and the anthropomorphous Old-World apes are not so great as those which separate these anthropoids from the papiomorphous baboons, the lowest of the catarrhines."
We can now immediately utilize this incontestable pithecometric proposition, both for firmly establishing the basis of the systematic classification of the primates and for the genealogy of man. For the natural system is, within the order of the primates, an expression of genealogical relationship, just as it is in every other group of the animal and vegetable kingdoms. Hence result the following important inferences as to the genealogical tree of man: (1) The primates form a natural monophyletic group; all "dominant animals," lemurs, apes, and man himself sprang from a common original stem form, a hypothetical Archiprimas. (2) Of the two orders of the legion of the primates the lemurs are the lowest and oldest; from them, later, the true apes (Simiæ) first developed. (3) Among these latter the Old-World apes form a natural monophyletic group; their common hypothetical stem form (Archipithecus) is, directly or indirectly, derived from a branch of the lemurs, no matter what relation they may be assumed to have to the New-World apes. (4) Man is descended from a series of extinct Old-World apes; the more recent ancestors of this series belonged to the group of tailless anthropoid apes with five sacral vertebræ (Anthropoides), the older ancestors to the group of the tailed baboons with three or four sacral vertebræ (Cynopitheca). These four propositions are, according to our conviction, unalterably settled, no matter what further anatomical or palæontological discoveries may later do to clear up the particulars of the many steps of the phyletic evolution of man.
Comparative anatomy, which, with critical penetration, examines analytically on the one hand the structural differences of separate species of animals, and on the other systematically groups them in natural order according to their common characters, has completely demonstrated the validity of our pithecometric proposition and its significant inferences. Not less important than these morphological considerations are the physiological ones that are taught us by that instructive but hitherto, alas! too much neglected science, comparative physiology. For an unprejudiced comparison of all the activities of life teaches us that in this department also there is nowhere any radical distinction between man and apes. Our entire nutrition, secretion and circulation, breathing and digestion, are performed by the same physical and chemical processes as with the anthropoid apes. It is the same with the isolated processes of sexual activities and propagation. It is the same also for the animal functions of movement and sensation. Our mental ability results from the same physical and chemical laws as does that of the apes. The mechanics of our bony frame and the movements our muscles impart to this arrangement of levers are in no way different in man and the anthropoid apes. It was formerly thought that walking erect was a special attribute of man. We now know that this can sometimes be done by the gorilla and the chimpanzee, and especially by the gibbon.
It is quite the same with human speech. The various sounds by which apes express their sensations and their wishes, their affection, and aversion must by comparative physiology be considered as speech, just as much as are the similarly imperfect sounds that children make when learning to talk, and as the manifold tones by means of which social mammals and birds impart to each other their ideas. The modulated song of the singing bird belongs to speech just as much as the similar song of man. Besides, there exists a musical anthropoid. The singing gibbon or siamang (Hylobates syndactylus) begins with the fundamental tone E and goes upward through the entire chromatic scale, a full octave, in pure and sonorous half tones. The old doctrine that only man is endowed with speech and reason is still to-day held by some authoritative philologists, as, for example, Max Müller at Oxford. It is high time that this erroneous impression, resting on a lack of zoological information, should be abandoned.
Our pithecometric proposition met with the greatest difficulties and the most violent opposition in an isolated department of neurophysiology, namely, that of psychology. The wonderful "soul of man" was thought to be a peculiar "being," and it today seems to many impossible that it should have been historically developed from the "soul of the ape." But in the first place the wonderful discoveries of comparative anatomy during the last ten years inform us for the first time that the minute as well as the gross structure of the brain of man is the same as that of the anthropoid apes, the unimportant difference in shape and size of single parts that exists between the two being less than the corresponding difference between the anthropoid and the lowest apes of the Old World, especially such as the baboons. Secondly, comparative ontogeny teaches us that the very highly complex brain of man has developed out of the same rudimentary form as that of all other vertebrate animals—out of five cerebral vesicles of the embryo that lie one behind the other. The special way and method by which the peculiar form of the primate brain is developed out of this extremely simple rudiment is found to be exactly the same in man as in the anthropoid apes. Thirdly, comparative physiology shows us by observation and experiment that the total functions of the brain, even consciousness and the so-called higher mental faculties, together with reflex acts, are in man preceded by the same physical and chemical phenomena as in all other mammals. Fourthly and lastly, we learn through comparative pathology that all so-called "mental diseases" in man are determined by material changes in the material of the brain, just as they are in the nearest related mammals.
An unprejudiced critical comparison confirms here also Huxley's law: the psychological differences between man and the anthropoid apes are less than the corresponding differences between the anthropoid and the lowest apes. And this physiological fact corresponds exactly with the results of an anatomical examination of the differences found in the structure of the cortex of the brain, the most important "organ of the soul." The deep significance of this information will be clearer to us when we consider the extraordinary differences in mental capacity that exist within the human species itself. There we see, high above, a Goethe and a Shakespeare, a Darwin and a Lamarck, a Spinoza and an Aristotle, and then, far below, a Veddah and an Akkah, a Bushman and a Patagonian. The enormous difference in mental capacity between these highest and lowest representatives of the human race is much greater than between the latter and the anthropoid apes.
Since, in spite of this, we find that the soul of man is to-day regarded in the widest circles as an especial, "being" and as the most important witness against the decried doctrine of the descent of man from apes, we explain it on the one hand by the wretched condition of the so-called "psychology," on the other by the widespread superstition concerning the immortality of the soul. The science which to-day in most textbooks and from most academic chairs is taught as "psychology" is not a true empirical science of the mind, not the physiology of the mental organs, but rather a fantastic metaphysics, compounded of one-sided introspective observation of self and of uncritical comparisons, of mis- understood data and incomplete experiments, of speculative errors and religious dogmas. Most of the so-called psychologists know nothing at all of the brain and organs of special sense, that wonderful and incomparably complex apparatus which solely and alone is the organ of the mental faculties in man and in animals. Most psychologists possess today no knowledge of the significant problems of modern experimental physiology and psychiatry, or they purposely ignore them; indeed, they know nothing at all of the actual localization of the separate mental faculties or their concurrence in the normal workings of the single portions of the brain.
The surprising disclosures which the minute anatomy and ontogeny of the human brain, assisted by experimental physiology and pathology, have made during the last four years are among the most important discoveries of the nineteenth century. Indeed, these have not hitherto been widely known, which is explained on the one hand by the great difficulty of the subject which deals with the extremely complicated structure of our brain, and on the other hand by the passive stiff-necked resistance of the dominant school of psychology. The localization of the higher mental faculties upon the cortex of the brain was effected ten years ago by the suggestive researches of Goltz, Munk, Wernicke, Edinger, and others. But recently (1894) Paul Flechsig has succeeded in marking out the single parts of this region in a definite manner; he has pointed out that in the gray cortical zone of the brain mantle there are four clearly defined regions for the central sense organs, or four "sensory spheres"—the sphere for general bodily sensibility, in the parietal lobe; the sphere for smell, in the frontal lobe; that for vision, in the occipital lobe, and that for hearing, in the temporal lobe. Between these four "seats of sensation" lie the four great seats of thought or "association centers"—the real organs of intellectual life. They are the highest apparatus of the mental faculty, on which thought and consciousness depend. In front the frontal brain, or "frontal association center;" behind and above the parietal brain or "parietal association center;" "behind and below the principal brain, or "great occipitotemporal association center" (the most important of all), and finally, deep underneath, in the interior, is placed the insula brain, or "island of Reil," the "insular association center." These four seats of thought, distinguished by peculiar and highly complicated nerve structure from the intermediate seats of sensation, are the real "organs of thought," the only true apparatus of our mental life. * * *
The next question now is, What has paleontology to say regarding these important results of comparative anatomy and their application to the system of the primates and to phylogeny? For it is the petrifactious that are the true "footprints of the Creator," the immediate testimonials of the historical succession of the numerous groups of forms which have peopled this earthly hall for so many millions of years. Do petrifactions of the primates give us any determinate points of support for the above-mentioned pithecometric law? Do they directly confirm the much disputed "descent of man from apes"? According to our view, this question must be undoubtedly answered in the affirmative. Certainly the negative gaps which we here, as elsewhere, find in paleontological knowledge are very much to be regretted, and immediately in the primate stem they are, since most of these animals lived upon trees, greater than in any other groups of animals. But to offset these wide, empty spaces we have on the other hand a continually increasing number of positive facts, and these recently discovered petrifactions have a phylogenetic value that can not be overestimated. The most important and interesting of these petrifactions of the primates is the renowned Pithecanthropus erectus, which Eugène Dubois found in Java in 1894. As this pliocene ape man brought out a lively discussion at the last zoological congress held three years ago at Leyden, I may be permitted to say a few words in criticism of it.
From the proceedings of the congress at Leyden (at which I was not present), I learn that the most distinguished anatomists and zoologists expressed different views as to the nature of this remarkable Pithecanthropus. Its remains, a skullcap, a femur, and some teeth, were so incomplete that it was not possible to arrive at a conclusive judgment regarding them. The final result of the long and spirited debate held on this subject was that among twelve distinguished authorities three declared the fossil remains to be those of a man, three that they were those of an ape. Six or more other zoologists, on the contrary, stated what I believe to be the real fact, that they are the fossil remains of a form intermediate between ape and man. In fact the ordinary rules of logic seem to me to justify this conclusion. The Pithecanthropus erectus of Dubois is in fact a relic of that extinct group intermediate between man and ape to which as long ago as in 1886 I gave the name of Pithecanthropus. He is the long-sought "missing link" in the chain of the highest primates.
The able discoverer of Pithecanthropus erectus, Eugène Dubois, has not only convincingly pointed out his high significance as a "missing link," but has also shown in a very acute manner the relations which this intermediate form has on the one side to the lower races of man- kind, on the other hand to the various known races of anthropoid apes, as well as to the hypothetical stem form common to this entire group of Primaria or Anthropomorpha. This common stem form Dubois calls Protohylobates (primitive gibbon). It has essentially the same structure as we find in the gibbon of today (Hylobates) in southern Asia, and as the fossil Pliopithecus, whose petrified remains have been found in the Mid-Tertiary mountains of middle Europe (in the Upper Miocene of France, Switzerland, and Styria). This in turn is derived from an older, generalized ape form which lived in the older Miocene period, and which maybe regarded as the common ancestor of the Old World apes, both the tailed Cynopitheci and the tailless Anthropomorpha. Among the latter we now recognize the two living species of the gibbon which stand very near to Pliopithecus, as well as fossil anthropoid apes that lead directly to Pithecanthropus. Such an intermediate stem form is Pliopithecus sivalensis, whose skeleton was found in the early Tertiary layers of eastern India in the Pliocene Siwalik strata.
For forming a correct judgment concerning this important Pithecanthropus and its immediate position between the anthropoids and man, two features are especially valuable; first, the close resemblance of the femur to that of man, and second, the relative size of the brain. Among the few anthropoid apes yet living the gibbons appear to be the lowest and oldest, standing nearest the stem-form of all the Anthropomorpha; they are also the most generalized and appear especially adapted to illustrate the "transformation of apes into man." The gibbons more than the other anthropoids have the habit of voluntarily assuming the upright position, whereby they walk upon the entire sole of the foot and use their long arms as balancing poles. The other modern apes (orang, chimpanzee, and gorilla) seek the upright position, and when they use it do not tread upon the entire sole but upon the outer edge of the foot; they also have in other respects more specialized characters, adapted especially to their tree climbing life. It is thus explained why it is that it is exactly the femur, in Hylobates and Pithecanthropus, that is much more human in form than that of the gorilla, the orang, and the chimpanzee.
But also the skull, that "mysterious vessel" of the organ of the soul, approaches nearest the human proportions both in Pithecanthropus and in the gibbon in important particulars—the rough, bony crests which the skulls of the other anthropoids show are wanting. The relative size of the brain (in proportion to that of the entire body) is in the latter only half as great as it is in the gibbon. The capacity of the skull of Pithecanthropus is from 900 to 1,000 c.c., therefore about two-thirds the capacity of an average human skull. On the other hand: the largest living anthropoids show a capacity half as high as this—500 c.c. So the capacity of the skull and consequently the size of the brain is in Pithecanthropus exactly midway between that of the anthropoid apes and the lower races of mankind; and the same is also true for the characteristic profile line of the face. In this respect compare the skulls of the lowest and most pithecoid races of man. Among these the still living pygmies, the little Veddahs of Ceylon and the Akkas of Central Africa, are of great interest. An unprejudiced comparison of all these anatomical facts shows in no ambiguous manner the character of Pithecanthropus as a true intermediate form between anthropoid apes and man; he is the long sought for and much discussed "missing link" in the chain of our primate ancestors, by many regarded as of the highest importance.
To this momentous interpretation, which is now accepted by nearly all naturalists, the renowned pathologist of Berlin, Robert Virchow, set up the most obstinate opposition. He went to Leyden for the special purpose of contradicting the idea that the Pithecanthropus is a transitional form, but met with little success. His contention that the skull and the femur of Pithecanthropus could not have belonged together, that the first belonged to an ape and the second to a man, was rejected at once by the expert paleontologists present, who declared unanimously that, in view of the extremely careful and conscientious account of the discovery "there could not the slightest doubt exist that the remains belonged to one and the same individual." Virchow further asserted that a pathological exostosis in the femur of Pithecanthropus likewise testified to its human character, for only by the most careful attention by human hands can such disorders be cured. Immediately thereupon the famous paleontologist Marsh showed a number of similar exostoses upon the leg bones of wild apes, who had had no "nursing care," and yet had recovered. Every great osteological collection contains similar specimens; experienced hunters know that fractures and inflammations of bones in foxes, hares, harts, roebucks, etc., are often healed quite well, without the intervention of man, while those animals are in a state of freedom. Finally, Virchow asserted that the deep notch between the orbital edge and the low skullcap of Pithecanthropus—a sign of a very deep conformation of the temporal fossa—were decisive for the ape-like character of the skull, and that such a formation never occurs in man. A few weeks later the paleontologist Nehring (who from the beginning had supported the just conclusion of Dubois) showed that exactly the same formation was presented by a human skull from Santos, in Brazil.
Virchow had formerly the same want of success with his "patho- logical significance of the skulls of the lower races of man." The famous skulls of Neanderthal, of Spy, of Moulin-Quignon, of La Naulette, etc.—which taken together are the interesting isolated remains of an extinct lower race of man standing between Pithecanthropus and the races of the present day—these were all declared by Virchow to be pathological products; indeed, the sagacious pathologist at last made the incredible assertion that "all organic variations are pathological;" that they are only produced through disease. According to this all our noblest cultivated products, our hunting hounds and our horses, our noble grains and our fine table fruit, are, alas! diseased natural objects that have arisen by pathological changes from the wild original forms that alone are "healthy."
In order to make this strange assertion of Virchow intelligible, it must be remembered that for more than thirty years he has regarded it as his especial duty as a scientist to oppose the Darwinian theory and the doctrine of evolution necessarily connected with it. With the greatest obstinacy he has maintained the doctrine of the constancy of species, which is now abandoned by all naturalists of good judgment; but in what now consists the essential idea of a "true species" he can no more tell than any other opponent of evolution. The most important conclusion from the latter, the "descent of man from the ape," Virchow is well known to attack with zeal and energy. "It is quite certain that man did not descend from the apes." This assertion of the Berlin pathologist has been for twenty years past repeated innumerable times in religious and other periodicals—cited as the decisive judgment of the very highest authority—not caring in the least that now almost all experts of good judgment hold the opposite conviction. According to Virchow the ape-man is a mere "figment of a dream;" the petrified remains of Pithecanthropus are the palpable contradiction of such an unfounded theoretical assertion.
How directly fruitful the great advances in paleontology for the last thirty years also are for our pithecoid theory can best be shown by the example of the legion of the primates itself. Cuvier, the founder of scientific paleontology, asserted up to the time of his death (1832) that there were no petrifactions of apes; the only fossil lemur whose skull he described (Adapis) he erroneously took for a hoofed animal.
The first petrified remains of apes were discovered in India, in 1836, in 1838 the Mesopithecus penthelicus was discovered near Athens, and in 1862 further remains of lemurs. But within the last twenty years so numerous remains of extinct primates have become known to us through the discoveries of Gaudry, Filhol, Schlosser, and especially by the rich finds of the American paleontologists Marsh, Cope, Leidy, Osborn, Ameghino, and others, that we have now obtained a satisfactory general insight into the rich development of this highest legion of mammals during the Tertiary period. With great admiration I have recently seen in London the instructive series of fossil primates which is displayed in the noble paleontological section of the museum of natural history in South Kensington, in which there is a gigantic fossil lemur which was nearly as large as a man, and which Forsyth Major recently discovered upon the island of Madagascar (Megaladapis madagascariensis).
Now, as in Cuvier's time, the most important differences between the two principal groups of true apes consists in the characters of the teeth. Man, like the Old-World apes, possesses thirty-two teeth of very characteristic structure and arrangement. The New-World apes have, on the contrary, thirty-six teeth, namely, one more premolar in each half Of either jaw. Comparative odontology is authorized to state on phylogenetic grounds that this number has arisen by reduction from a higher dental formula, from forty-four teeth; for this typical form of dentition (in each half jaw, above and below, three incisors, one canine, four premolars, and three molars) is common to all those older mammals of the Eocene period which we regard as stem-forms of the principal groups of chorion animals (Placentalia): Lemuravida, Condylarthra, Esthonychida, and Ictopsida. These form old Tertiary stem-forms of the primates. The ungulates, the rodents, and the carnivores resemble each other so much in bodily structure that we may bring them all together as a single common stem-group of the placental mammals, the primitive chorion animals (Prochoriata). With great probability we may now connect with this the further monophyletic hypothesis that all chorion or placental animals—from the lowest Prochoriata up to man—arose from a common unknown stem-form in the Cretaceous period, and that this oldest of the chorion animals arose from a marsupial group living in the Jurassic period.
But in fact we now possess among those numerous fossil lemurs that have been found for the first time during the last twenty years all the intermediate forms desired, all the "missing links" that are required by phyletic odontology. The oldest Prosimia of the Tertiary period, the pachylemurs (or Hyopsodines) of the old Eocene, have yet the original forty-four teeth of the placental stem-group; in every half jaw, above and below, three incisors, one canine, four premolars, and three molars. The necrolemurs (or Adapides) with forty teeth followed them; they have lost an incisor on each side above and below. Next come the younger autolemurs (or Stenopides) with thirty-six teeth (one premolar less); they have therefore already the same dental formula as the platyrrhines or American apes. The dentition of the catarrhines has arisen from this through loss of a second premolar. These relations are so clear and go so evidently hand in hand with the formation of the entire skull and the stronger development of the typical primate form that we may say: The general elementary features of the primate genealogical tree from the oldest Eocene lemur up to man lie clearly before our eyes within the Tertiary age; there is no longer any "missing- link." The phyletic unity of the primate stock from the oldest lemur up to man is now an historical fact.
It is quite different, however, when we leave the Tertiary and in the Mesozoic period attempt to discover the oldest ancestral series of the mammals. There we meet everywhere with painful gaps in our paleontological record, and the comparatively few remains of Mesozoic mammals (especially scanty in the chalk) are insufficient to enable us to form any definite conclusions as to the systematic placing of the mammalia in question. However, comparative anatomy and ontogeny compel us to the conclusion that the Cretaceous Placentalia arose from Jurassic marsupials, and these from Triassic monotremes. We may also further suppose that among the unknown Placentalia of the chalk there were found Lemuravida and other Prochoriata; that the Amphitheriidæ of the Jurassic were ancestors of the marsupials, and that the monotreme ancestors of the latter are to be sought among the Pantotheria of the Trias. But paleontology does not at this time offer us any secure foundation for these phyletie hypotheses. Only one important piece of information is given us, that the oldest mammals of the Mesozoic age, the Pantotheria and Allotheria of the Trias, were small, lowly organized, for the most part insect eating animals that represent the derivation from older vertebrates, reptiles or amphibia. There is nothing in this to contradict the idea that the entire class of mammals, from the oldest monotremes to man, is monophyletic; that all members of it can be traced back to a single common stem-form.
This positive conviction of the phyletic unity of the class of mammals, because of its common origin from a single extinct stem-group, is now shared by all expert zoologists, and I hold it to be one of the greatest advances of modern zoology. No matter what system of organs we compare in the various mammalian orders, we everywhere find this typical agreement in the essential characters of their structure, both minute and gross. Only among mammals is the skin covered with true hairs, from which fact Oken named this class the "hairy animals." Only in this class is generally found that remarkable kind of nurture, the nourishment of the newborn child with the milk of the mother. Here lies the physiological source of that highest form of maternal love which has exercised such a significant influence upon the family life of various mammals, as well as upon the culture and higher mental life of man. The poet Chamisso justly says of this:
Only the loving mother, only she
Who nurtures from its birth the child she bears,
Knows the true joy that we call happiness,
Created by the love she never spares.
If the Madonna seems to us the most sublime and pure prototype of this human maternal love, yet we perceive on the other hand in the "ape love," in the excessive tenderness of the ape mother, the counterpart of the same maternal instinct. The slow development of this, in the course of many millions of years, from the Trias period to the present, goes hand in hand with an important series of transformations. For the adaptation of the new-born mammal to suckling involved a series of changes not only in its own body but in that of its mother. While in the skin of the mother the mammary glands developed through the irritation and differentiation of a group of ordinary skin glands, there was formed in the mouth of the child, by the act of sucking, the soft palate and afterwards the epiglottis—two organs of the throat that occur only in mammals. In connection with this the mechanism of breathing was changed; this is shown not only in the minute structure of the lungs, but also in the formation of a complete diaphragm. Only in mammals does the muscular diaphragm form a complete partition between the throax and the abdomen. In all other vertebrates the two cavities remain openly connected. Also in the bony framework of the body, and especially in the skull, do we find results of these important transformations. Much the most important of these is the transformation of the articulation of the mandible or lower jaw, which in mammals is quite strikingly different from that of all other vertebrates. This joint, by which the lower jaw moves upon the temporal bone, is in mammals a temporal joint, while the original joint of its reptilian and amphibian ancestors was a quadrate joint. The latter is, in the mammalia, taken up into the tympanic cavity and there represented by the articulation of two of the special bones of the ear, the malleus and the incus; the malleus was formed from the original joint piece of the lower jaw, while the incus is the quadrate bone or jaw pedicel of the reptilian ancestors.
But apart from these and other anatomical peculiarities which all mammals have in common, and which elevate them above all other vertebrates, in order to recognize their difference it will only be necessary to look at a single drop of blood under a microscope. "Blood is a very peculiar juice." The small red-blood corpuscles which, heaped up by millions, occasion the red color of the blood of vertebrates were all originally elliptical disks, thicker in the middle (biconvex), as it was here that the nucleus lay. Only in the mammals have these lost their nucleus, then appearing thinner in the middle (biconcave), as small circular disks. These and other important peculiarities occur, without exception, among all mammals, and separate them from all other vertebrates. From their peculiar combination and mutual relations they can only have been acquired once in the course of descent, and only from one stem-form can they have been transmitted by inheritance to all members of the class.
The older portion of the genealogical history of the human species leads us still farther back into the domain of the lower vertebrates, into that dark, immeasurably long age of the Paleozoic era, which with its uncounted millions of years (according to recent estimations, at least a thousand) was certainly much longer than the succeeding Mesozoic age. Here we first come upon the important fact that in the earliest portion of the Paleozoic period, in the Permian age, no mammals yet existed, but instead lung-breathing reptiles, as the oldest amnion animals. They belong partly to the Tocosauria, the oldest and lowest group of reptiles, partly to the strange Theromera, which by many characters approach the mammals. These reptiles are preceded in the lower Carboniferous period by true amphibia, such as the armored stegocephali. Such Carboniferous armored amphibia, like small crocodiles, are the oldest vertebrates, who by their creeping method of loco- motion adapted themselves to the firm ground, and in whom the fins of swimming fishes and the paddles of swimming amphibians (Dipneusta) had been modified into the typical five fingered extremity of a four-footed animal (Tetrapoda or Quadrupeda).
We only need to compare carefully the skeleton of the four legs of our salamanders and frogs with the bony framework of our own four limbs to convince ourselves that with these amphibians the same characteristic and peculiar structure had arisen as they handed down by inheritance to all the sauropsida and mammalia; there is the same shoulder girdle and pelvic girdle, the same simple hollow bones in the upper arm and upper leg, the same pair of bones in the forearm and lower leg, the same complicated union of bones in the wrist and ankle, the same typical arrangement of five fingers and five toes. This striking agreement in the assembling of the bony framework in all the higher four-footed vertebrates struck many thoughtful observers more than a hundred years ago; among others it led our greatest poet and thinker, Goethe, to those remarkable observations on the morphology of animals that we may consider the direct precursors of the modern ideas of Darwin.
We can, in fact, show, as a certain sign of the derivation of man from the oldest five-toed or pentadactylate amphibians, the fact that we possess to-day on our hand five fingers and on our foot five toes. Man and most primates (not all) show in this and in other respects that through conservative inheritance they have preserved the original plan of structure much more closely than have the majority of other mammals, especially the ungulates. Among others the one-toed horse on the one side and the two-toed ruminants on the other, are much more modified and specialized than are the primates.
The oldest amphibia of the Carboniferous period, the armored Stegocephali (and especially the remarkable Branchiosauria discovered by Credner), are now quite justly considered by all discriminating zoologists as the undoubted common stem group whence were derived all four-footed animals (Tetrapoda or Quadrupeda), all amphibia and amniota. But what was the origin of this important group itself? To this question also the great advances of palæontology afford a satisfactory answer which harmonizes excellently with the older solutions given by comparative anatomy and ontogeny. Already in Jena, forty-four years ago, the first master of comparative anatomy, Carl Gegenbaur, in a series of classical essays, pointed out that the most important parts in the vertebrate skeleton, particularly the skull and the bones of the limbs, reveal to us to-day, in the succession of classes of living vertebrates, a coherent scale of phyletic steps of development. Apart from the more lowly organized Cyclostomatait is especially the true fishes, and among them again the primitive fishes or Selachians (sharks and rays), which have proved most constant to the original form in the essential relations of their bodily structure. To the Selachians are closely allied the ganoids or enamel fishes, especially the Crossopterygii which take us farther back to the Dipneusta. Among these last the Australian fish Ceratodus has recently become of great interest, its anatomy and ontology having been carefully investigated by Günther and Semon. By this transition group of Dipneusta' or amphibious fishes—that is to say, fishes with lungs, but also with fins, with pentadactylate limbs—is the morphological bridge to the early amphibians easy to find. But this anatomical chain corresponds exactly with the paleontological facts; selachians and ganoids are already found in the Silurian formations, dipneusta in the Devonian, amphibia in the Carboniferous, reptiles in the Permian, mammalia in the Trias.
These are historical facts of the first rank. They attest in the most gratifying manner the successive steps of the development of vertebrates, as they have been made out by the comparative researches of Cuvier and Meckel, of Johannes Müller and Gegenbaur, of Owen, Huxley, and Flower. The historical succession of the principal steps in the vertebrate stock is thereby definitely established, and this success is much more important for an understanding of the human family tree than if we had succeeded in placing, in a hundred fossil skeletons of lemurs and apes, the entire series of our Tertiary primate ancestors in coherent succession before our eyes.
Much more difficult and dark is the oldest history of our stock, the derivation of a vertebrate stem from an invertebrate ancestry. As none of these possessed any hard and petrifiable parts of the skeleton (resembling in this respect the lowest vertebrates, the cyclostomata and acrania) the evidence of paleontology entirely fails us here; we must rely alone upon the other two records of our family history, upon comparative anatomy and ontogeny. To be sure, their value is here so great in many respects that for every expert and discriminating zoologist they throw the clearest light upon many great features of our older phylogeny. Of the greatest value are these far-reaching inferences which modern comparative ontogeny has drawn during the last thirty years by the aid of the fundamental biogenetic law. Already the older embryology has made clear the elements of vertebrate development by the thorough work of Baer and of Bischoff, of Remak and Kölliker. Then, in 1866, came the important discoveries of Kowalevsky, which confirmed the suspicion of Goodsir and pointed to the close relationship of vertebrates and tunicates; the comparative anatomy of Amphioxus and of the ascidians has since that time been the constant starting point for all further investigations concerning our invertebrate predecessors.
Five years investigation of the structure and development of the chalk-sponges (1867–1872) had led me at that time to a reform of the theory of the germinal layers and to advance the gastræa theory. It first appeared in 1872 in my monograph on the chalk-sponges or Calcispongidæ. These views obtained the most earnest support and the most fruitful development by the excellent comparative researches of many other embryologists, especially those of E. Ray-Lankester and Francis Balfour, as well as those of the brothers Oscar and Richard Hertwig. I had already then concluded from these comparative researches that the first step of development in all Metazoa, or tissue-building animals, is essentially the same, and that we may from this obtain definite insight into the common origin and the older ancestral series of the same. The unicellular ovum repeats the unicellular condition of our protozoan ancestors. The blastula germ form corresponds to a Volvox or Magosphæra, a similar ancestral form; the Gastrula is the inherited repetition of the Gastræa, the common stem form of the entire series of Metazoa. All these typical ancestral forms man shares with all the other Metazoa, that is to say with all other animals except the unicellular Protozoa. Every man, without exception, begins his individual existence in the form of a spherical egg cell, barely visible to the naked eye, as a very small dot, and the special characters of this egg-cell are exactly the same in man as in all other mammals.
The most obscure portion of the genealogical history of man is that part which lies between Gastræa and Amphioxus. Amphioxus itself, that famous lancelet, or lancet animal, whose fundamental significance had already been recognized by its first exact describer, the great Johannes Müller, is the most precious document of vertebrate phylogeny. We should not indeed consider it as a stem ancestor to vertebrates, but rather as a near relation to such, and as a unique living relic of the class of acrania. Had the amphioxus accidentally perished, like so many other links in our ancestral chain, we would hardly be in a position to obtain any satisfactory insight into the older steps that led to the formation of vertebrates. Above amphioxus stand its near relations, the Cyclostomata or round-mouths. These are the oldest Craniota or skulled animals, the first vertebrates that succeeded in obtaining a skull and brain. These Cyclostomata (among whom the well-known lamprey, Petromyzon, belongs) are, at the same time, the presilurian forerunners of fishes. Below amphioxus we find that the agreement between the ontogeny of amphioxus and the ascidians points to an unknown older group of chorda animals, the Prochordonia, from which have developed on the one hand the tunicates, on the other the vertebrates. We may derive these prochordonia, or primitive chorda animals, from the Frontonia, a twig of the Vermalia, or true worms. The isolated Balanoglossus and the old Nemertina are probably closely related to these. There certainly existed, in the Cambrian and Laurentian periods, between these worms and the stem group of the Gastræades, a long series of intermediate forms, and we suppose that the older Rotatoria and Turbellaria belonged in this series. But we can not at this time form any well-grounded hypothesis on this point, and there is indeed here a wide empty space in our genealogical history.
But contrasted with these and other obscure portions of our family history stand out clearly and significantly the conclusions which the rich results of comparative anatomy, ontogeny, and palæontology have given in the investigation of the vertebrate stock, and especially of that of its highest class of mammals. All reliable recent researches have here unanimously confirmed the proposition which Lamarck, Darwin, and Huxley declared to be the most important result of the theory of evolution—the proposition that the immediate placental ancestors of man were a series of tertiary primates, and the next nearest were the anthropoid apes, the anthropomorphous catarrhines. The careful critical comparisons which the two zoologists, Paul and Fritz Sarasin, have accomplished in their fine work. Researches in Ceylon (1893) shows that the Veddahs of to-day, the dwarfish aborigines of Ceylon, approach nearest to the anthropoid apes in the primitive relations of their bodily structure, and that among the latter the chimpanzee on the one side and the gorilla on the other stand nearest to man. The gibbon again, as a lower and less specialized form, shows the closest agreement with the common miocene ancestors of all the Anthropomorpha. This direct family relationship is much clearer and easier to settle than that of any other mammal. Far more obscure and enigmatical is, for example, that of the elephants, the sirenia, the cetacea, the edentates (armadillos and pangolins) in both hemispheres. Not only in his pentadactylate hands and feet, but also in other anatomical features does man show the characteristic inherited features of his stock more clearly than many other mammals, as, for example, ungulates, cetaceans, and bats.
The immeasurable significance which this secure knowledge of the primate origin of man possesses for the entire range of human science lies clear before the eyes of every unprejudiced and logical thinker. No one among the philosophers has more thoroughly based his authoritative influence upon a contemplation of the entire universe than has the great English thinker Herbert Spencer, one of the few learned men of the present day who unites the most profound scientific training with the deepest philosophical speculation. Spencer belongs to those older nature philosophers who already before Darwin recognized in the monistic theory of evolution the magic key which would unlock the riddle of the world. He belongs also to those evolutionists who justly lay the greatest stress upon progressive inheritance, upon the "transmission of acquired characters." Like myself, Spencer has, from the beginning, fought in the most resolute manner the germ-plasm theory of Weismann, which denies the most important factor in the theory of descent and wishes to explain the same chiefly through the omnipotence of natural selection.
In England the theory of Weismann has been received with much approval, and is also known as neo-Darwinism, in opposition to older views which are known as neo-Lamarckism. This designation is entirely incorrect, for Charles Darwin was just as firmly convinced of the fundamental significance of progressive inheritance as was his great predecessor Jean Lamarck and as is Herbert Spencer.
I had three times the pleasure of visiting Darwin at Down, and each time we discussed this important question upon which we completely agreed. I share the conviction of Herbert Spencer that progressive inheritance is an indispensable factor of the monistic theory of evolution and one of its most important elements. To deny it, as Weismann does, is to fly to mysticism, and it is better to accept the mysterious creations of separate species. The genesis of man affords innumerable illustrations of it.
When we regard the science of the genesis of man from the most general point of view, and bring together all the empirical arguments for it, then we may say to-day with perfect justice that the descent of man from an extinct tertiary primate chain is no longer a vague hypothesis, but an historical fact. Naturally this fact can not be exactly demonstrated; we can not point out the innumerable physical and chemical processes which in the course of a hundred million years have gradually led up from the simplest moner and the unicellular egg-form to the gorilla and to man. But the same thing is true of all other historical facts. We all believe that Linnæus and Laplace, Newton, and Luther, Malpighi and Aristotle once lived, although this can not be exactly demonstrated in the sense of modern physical science. We firmly believe in the existence of these and of many other heroic minds because we know the works they have left behind, and because we see the powerful influence they have had upon the history of civilization. But these indirect arguments have no more conclusive force than those which we have put forward for the vertebrate history of man.
Of many Mesozoic animals of the Jurassic period we know but a single bone, the under jaw, and Huxley has very finely explained the cause of this strange phenomenon. We all consider it settled that these animals had also upper jaws as well as other bones, although we can not certainly demonstrate it. Yet the "exact school," which considers the evolution of species as an undemonstrated hypothesis, must regard the lower jaw as the only bone in the body of these remarkable animals.
Let us now in conclusion take a hasty glance into the immediate future. I am entirely convinced that the science of the twentieth century will not only accept our doctrine of development, but will celebrate it as the most significant intellectual achievement of our time, for the illuminating beams of this sun have scattered the heavy clouds of ignorance and superstition which hitherto shrouded in impenetrable darkness the most important of all scientific problems, that of the origin of man, of his true essence, and of his place in nature. The incalculable influence of the science of the development of man upon all other branches of science, and especially upon culture, will bear the most blessed fruits. The great work which was in our century begun by Lamarck and finished by Darwin will for all time remain one of the most significant achievements of the human mind, and the monistic philosophy which we found upon its theory of evolution will not only powerfully further the perception of the truths of nature, but also their practical worth in the service of the beautiful and the good. This monism is, however, based upon the empirical data furnished by modern phylogenetic zoology.
- ↑ A discourse delivered at the Fourth International Congress of Zoologists at Cambridge, England, August 26, 1898. Translated from the author's edition in German; printed at Bonn, 1898.