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THE SKULL THEORY AND THE APE THEORY.

Inasmuch as Virchow persists in treating the theory of descent as an “unproved hypothesis,” inasmuch as he ignores all the forcible evidences of that hypothesis, he deprives himself of the right of speaking a decisive word in this, the most important scientific dispute of the present day. Virchow is, in fact, simply incompetent in the great question of evolution, as he is deficient in the greater part of that knowledge more especially morphological knowledge which is indispensable to forming a judgment upon it. Hence on the turning-point of the whole matter viz., the problem as to the origin of species he can have no opinion, as he has never turned his attention to the systematic treatment of species: those transitions of one species into another, which he asks to see, abound on all sides, as is well known to every systematic naturalist. Only consider, for example, the genera of Rubus and Salix among the living plants of the present period, and the Ammonites and Brachiopoda among extinct animals. Hence, too, Virchow can have no independent views as to the historical development of the higher from the lower animals, because the abundant living forms of the lower animals are almost unknown to him, and because he has hardly any conception of the marvellous strides which hundreds of industrious workers have made in this very department within the last twenty years. But there can be no doubt, indeed it is already universally acknowledged, that it is precisely the comparative anatomy of the lower nay, of the very lowest animals that has solved the greatest riddles of life, and removed the greatest obstacles from the path of the doctrine of descent. He simply ignores the fact that true Monads actually exist, and have been positively identified by many different observers as structureless “organisms without organs,” and he turns out the poor Bathybius with a kick. And yet I believe that in “Kosmos" I have conclusively proved that Monads must retain their vast elementary importance whether the Bathybius actually exists or not.

But even as regards the higher animals nay, even as to the comparative anatomy of the highest next to man, the apes Virchow stands apart, not understanding the views of modern morphology.

We must here examine more closely into this, because it is precisely in this department that Virchow’s only morphological experiments have been made; viz., his investigations as to the skulls of apes and of men. This is precisely the one only point on which he has sought a closer acquaintance with morphology, and precisely here it is most clearly to be seen how little he is acquainted with the recent advances our science has made, and that he has hardly any conception of the extraordinary importance to that science of the theory of descent.

The skull theory, as is well known, has for a long time been a very favourite theme, not only with prominent naturalists, but also with talented amateurs. Undoubtedly the skull, viewed as the bony capsule which encloses our most important organ of sense, our brain, has a special claim to morphological importance; for the general conformation of the skull corresponds on the whole to the development of the brain, and its inner surface gives an approximate idea of the outer surface of the brain. In this correspondence lies the only sound kernel of the sickly, overgrown fancies of phrenology. The various development of the skull allows of an approximate inference as to the various degrees of development of the brain and of the mental faculties. The comparative study of the skulls of the vertebrate animals had excited the lively interest of morphologists by the end of the last century, when comparative anatomy was beginning to constitute a special science; and the genetic inquiry as to the morphological significance and development of the skull soon grew out of it. It was no less a man than our greatest German poet who first answered this question, and propounded the theory that the skull was neither more nor less than the modified foremost end of the vertebral column, and that the separate groups of bones which lie behind one another in the human skull, as in that of all the higher vertebrata, answer to the separate modified vertebrae. This “vertebral theory” of the skull, which Von Goethe and Oken simultaneously and independently attempted to prove, aroused universal interest and maintained its ground for seventy years, while many attempts were made to improve and enlarge upon it in detail.

A quite new light was thrown on this, as on every other morphological question, as soon as Darwin in 1859 had once more put into our hands the torch of the doctrine of descent. The inquiry as to the origin of the skull now assumed a real and tangible form. Since all vertebrate animals, from fishes up to man, agree so completely as to their essential internal structure that they can be rationally conceived of no otherwise than as branches of one stock and as descendants of one parent-form, the distinctly formulated question as to the skull theory which now started into prominence was this: “How, historically, has the skull of man and of the higher animals originated from that of the lower animals? How is the development of the bones of the skull from the vertebrae to be proved?” The answer to these difficult questions was supplied by the first comparative anatomist of the present day, by Carl Gegenbaur. After Huxley had pointed out that the ontogenesis or individual development of the skull by no means favoured the older hypothesis of Goethe and Oken, Gegenbaur brought forward evidence that the fundamental idea of that theory was correct; that the skull does in fact correspond to a series of coalescent vertebrae, but that the separate bones of the skull are not to be regarded as representing parts of such modified vertebrae. The skull-bones of all recent vertebrate animals are rather, for the most part, dermal bones, which have come into closer connection as supplementary to the cartilaginous primitive skull. We can even now trace the number and position of the original vertebrae, from which this primitive skull originated, by the number of the vertebral arches (gill-arches) which are attached to it, as well as by the number and position of those vertebrae, from nine to ten. Of all the recent vertebrata, the cartilaginous fishes, or Selachians, have most nearly preserved the form and structure of this primordial skull. These Selachians, the Rays and Sharks, are on the whole the creatures which throw the clearest light on the history of the lineage of the vertebrata and on the organisation of our primeval fish-natured ancestors. It is one of the particular merits of Gegenbaur that he clearly and firmly established the place in nature of the Selachians as the common ancestors of all vertebrate animals from fish up to man.

None but those who have thoroughly studied the comparative morphology of the vertebrata, who have sought the genetic issue from that labyrinth of intricate morphological problems at the hands of the theory of descent, can duly value the immeasurable service which Gegenbaur has done by this and other “Investigations into the Comparative Anatomy of the Vertebrata.” These investigations are as much distinguished by a profound knowledge and careful working out of the wonderfully-extensive empirical materials for the subject, as by their critical acumen and philosophic grasp. At the same time they set in the clearest light the immeasurable value of the theory of descent in the causal explanation of the most difficult morphological problems. Gegenbaur might, therefore, with perfect right, enunciate this axiom in the Introduction to his “Comparative Anatomy.” “The theory of descent will at once find a touchstone of proof in comparative anatomy. Up to this time no experience in comparative anatomy has transpired which contradicts that theory; on the contrary, they all lead up to it. Thus it will receive back from science that which it has given to scientific method: clearness and certainty.” In point of fact we can adduce no morphological investigations which better support this declaration than those very phylogenetic researches “as to the cranium of the Selachians, as a basis for the critical examination of the genesis of the cranium of the vertebrata,” 1872. As Virchow had formerly thoroughly studied the old skull-hypothesis, and in his admirable discourse on “Goethe as a Naturalist,” 1861, had given an excellent exposition of it; as moreover he had produced most valuable contributions to the normal and pathological anatomy of the human skull, we might have expected that he would have received Gegenbaur’s grand reform of the theory of the skull, and historical solution of the skull-problem, with the greatest interest, and have made it the clue to his own further researches. But we seek in vain through Virchow’s latest contributions to the study of the human skull, for any indication of his knowing or appreciating Gegenbaur’s investigations. On the contrary, we see him persistently moving, without any clear goal in view, on that trodden and devious path of investigation which finds the highest aim of craniological science in the measuring of skulls, or craniometry.

We are far from undervaluing the full significance of the results of exact and careful descriptions and measurements of various conformations of the skull as an empirical basis for a true and scientific study of the skull i.e., for comparative and genetic craniology. But still we must say that the way and method by which this skull measurement has, for ten years now, been pursued by numerous craniologists can never yield corresponding scientific results; on the contrary, though it is cried up as the “exact morphology” of the skull, it simply loses itself in the domains of harmless trifling. A large amount of time has in the last ten years been squandered in disputes as to the best method of measuring skulls, while the craniologists concerned have not, in the first place, answered the obviously most important question: What end they propose to gain by this specialist measuring, what proposition they mean to prove by it? Most of those numerous skull measurers know nothing beyond the perfect human skull, or at most the skulls of a few other mammalia, while the comparative morphology and historical development of the crania of the lower vertebrata are wholly unknown to them; and yet these last contain the true key to the comprehension of the others. One single month devoted by these “exact skull measurers” to the study of Gegenbaur’s theory of the skull, and to testing the hypothesis by the skulls of Selachians, would have yielded them more fruit and have given them more light than long years of describing and measuring human skulls, however various.

Virchow himself affords the most striking example of the usual results of this so-called “exact method” of studying skulls. In his popular essay on “The Skulls of Men and Apes,” 1870, he concludes with this notable proposition: “It is therefore self-evident that Man can never by any progressive development have originated from the Apes.” Every evolutionist who is familiar with the surprising facts of comparative morphology will draw from them the opposite conclusion: “It is self-evident that Man could only have originated from the progressive development of the Ape (organism).”

This brings us to that question which, in the popular treatment of the theory of descent, is justly considered as its most important outcome and as the keystone of the evolutionist edifice to the well-known proposition, “Man is descended from the Ape.” While we simply ignore all the misrepresentation, distortion, and misinterpretation which this ape, or pithecoid hypothesis, has met with on all sides, we will only remark that this fundamental proposition, in the sense of our modern doctrine of evolution, can rationally have only this plain meaning: that the human species as a whole was long since developed from the order of apes, indeed actually from one (or perhaps more) long since extinct form of ape; the immediate progenitors of man in the long series of his vertebrate ancestry were apes or ape-like animals. Of course none of the now surviving species of apes is to be regarded as the unaltered posterity of that primeval parent-form. Virchow, however, understanding the “ape question” in this sense, answers it, as Bastian also does, with the most positive contradiction. “We cannot teach the doctrine that man is descended from apes or from any other animal, for we cannot regard it as a real acquisition of science” . Although I myself, in direct opposition to this view, and in agreement with almost all my professional colleagues, look upon the descent of man from apes as one of the surest of phylogenetic hypotheses, I will here expressly admit that the relative certainty of this, as of all other historical hypotheses of descent, is not comparable with the absolute certainty of the general theory of descent. It is now ten years since I first explicitly stated (in my “Natural History of Creation,” vol. ii. : “The pedigree of the human race, like that of every animal or plant, remains in detail a more or less approximate general hypothesis. This, however, in no way affects the application of the theory of descent to man. In this, as in all researches into the derivation of our organism, we must distinguish between the general theory of descent and the specific hypothesis of descent. The general theory of descent claims full and permanent value, because it is inductively based on the whole range of common biological phenomena and on their internal causal connection. Each special hypothesis of descent, on the other hand, is conditional as to its specific value on the existing state of our biological information, and on the extent of those objective empirical grounds on which we deductively found the hypothesis, by our subjective inferences.” And I must here emphatically add that I have on every opportunity repeated that reservation, and have always insisted on the difference which exists between the absolute certainty of transmutation in general and the relative certainty of each individual specific pedigree. So that when Semper and others of my opponents assert that I teach my specific genealogies as “infallible dogmas,” it is simply false. I have, on the contrary, pointed out on all occasions that I regard them only as heuristic or provisional hypotheses, and as a means of investigating the actual relations of cognate races of organic forms more and more approximately.

Since the conception of the natural animal system as a hypothetical genealogical tree, and the phylogenetic interpretation of morphological affinity which that conception involves, afford in fact the only rational interpretation of that affinity in general, my first genealogical attempts soon found many imitators, and at the present time numerous industrious labourers in the different departments of systematic zoology are endeavouring to find in the construction of such hypothetical genealogies the shortest and completest expression of the modern conception of structural affinity. If Virchow had not been as ignorant of the true significance and method of systematic morphology as he is of its progress and scientific contents, he must certainly have known this, and then he would surely have withheld his mockery of all these grave phylogenetic studies as “personal crotchets” and worthless dreams.

What mighty strides towards a mechanical morphology we have made by this phylogenetic working out of the system, and how much light and life it has at once thrown into the system that before was dead and cold, can only be known to those who have long and deeply studied specific systematisation and the grouping of species; Virchow has not the remotest suspicion of it. Moreover, these attempts have now proceeded so far, that a large proportion of the phylogenetic hypotheses are regarded as very nearly certain, and can hardly undergo any further essential modifications; while the greater number of them are still in an unfixed state, and one systematist tries to improve them in this direction, and another in that.

The following phylogenetic hypotheses are held to be almost certain: The descent of many-celled animals from single-celled, of the Medusae from the hydroid Polyps, of the jointed from the unjointed worms, of the sucking from the gnawing insects, of amphibious animals from fishes, of birds from reptiles, of the placental mammalia from the marsupials, and so forth. I personally consider the descent of man from the apes as equally certain; nay, I regard this most important and pregnant genealogical hypothesis as one of those which, up to the present time, rest on the best empirical basis.

Huxley, in particular, fifteen years ago, in his celebrated “Man’s Place in Nature,” 1863, so admirably proved the undoubted “descent of man from apes,” and so clearly discussed all the relations that had to be taken into consideration, that very little was left to others to do. The result of his comparative morphological investigations is contained in this proposition ” If we take up a system of organs, be it which we will, the comparison of its modifications throughout the series of apes leads us to the same conclusion: that in every single visible character man differs less from the higher apes than these do from the lower members of the same order.” It is therefore impossible for any objective zoologist, according to the principles of comparative systematisation, to ascribe to man any other place in the animal world than in the order of apes; and it is quite immaterial whether we designate this individual group as the Order of Apes, or, with Linnaeus, as the Primates. For the phylogenetic construction of the system, the common descent of man and of apes from one common parent-form, necessarily follows from this inevitable grouping, and on this proposition only all the general inferences of the “ape-hypothesis” depend. As to what that common parent-form of men and apes may have been, very different views might probably be brought on opposite sides; but any one who knows the collected facts that bear upon the matter, and estimates them impartially, must, in conclusion, arrive at the certain conviction that that hypothetical and long-since extinct parent-form can only have been genuine apes; that is to say, of the placental mammalian type, such as when we see them now living before our eyes we unhesitatingly class, on the ground of their zoological characters, as true apes, in the order of Apes or Primates.

In this, and all other sound phylogenetic hypotheses, we may most easily attain to a conviction of their truth by taking into consideration and comparison the other possible hypotheses. But in fact no single opponent of the ape-hypothesis has been able to combat it with any other phylogenetic hypothesis that has the faintest glimmer of probability. Not one opponent has suggested, or can suggest, any other animal form that can serve as our nearest ancestor than the ape. No one has ever reproached me by saying that Mother Nature has endowed me with too little imagination; on the contrary, I am often accused of having a superfluity of that gift of the gods; but I have often and repeatedly exerted my imagination to picture to myself any known or unknown animal-form as the nearest parent-form to man in the place of the apes, and have always found myself under the necessity of falling back upon the stock of apes. Let me conceive of the outward conformation and the internal structure of the nearest mammalian ancestors of men as I will, I am always forced to acknowledge that this hypothetical parent-form ranges under the zoologically-conceived order of apes, and cannot possibly be separated from the Simiadoe or Primates. If, in spite of this, any one chooses, out of a “personal crotchet,” to accept some other series of unknown animal ancestors of man that have nothing to do with apes, that is but a mere empty hypothesis floating in the air. Our ape-hypothesis, on the other hand, is objectively and thoroughly proved by the essential agreement of the internal bodily structure of man and of apes, and by the identity of their embryonic development, as I have fully shown in my “Evolution of Man” (chaps. xix. and xxvi.) The mode and manner in which he here puts palaeontology in the foreground, and throws on the theory of descent the task of producing an unbroken gradation of fossil transitional forms between the apes and man, is very indicative of Virchow’s ignorance of this zoological question in which I, as a professional zoologist, must decisively declare his incompetence. The reasons why such a solution of the problem is not to be expected, the extraordinary imperfection of the palaeontological record, the natural impediments to the palaeontological evidence of the genealogical table, have been so lucidly unfolded by Darwin himself (chaps. ix. and x. of the “Origin of Species”) that I am obliged once more to come to the conclusion that Virchow has never read it with any attention.

Besides, long before Darwin, the gifted Lyell, the great originator of modern geology, showed clearly and convincingly how, for many reasons, the greater part of the fossil series must remain most imperfect, and these reasons were at a later period so often and so fully discussed (by myself among others, in chap. xv. of the “History of Creation,” vol. ii. pp. 24-32) that it is wholly superfluous once more and in this place to state these well-known and time-worn questions. It only shows how little Virchow was acquainted with geology and palaeontology, and what a limited judgment he can form of these historical causal relations.