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All the common phenomena of Morphology and Physiology, of Chorology and Oekology, of Ontology and Paleontology, can be explained by the theory of descent, and referred to simple mechanical causes. It is precisely in this, viz., that the primary simple causes of all these complex aggregates of phenomena are common to them all, and that other mechanical causes for them are unthinkable it is in this that, to us, the guarantee of their certainty consists. For this reason all these vast and manifold aggregates of facts are so many evidences of the doctrine of descent. This fundamental relation of facts has been so often expounded that I need dwell no farther on it in this place; those who wish for any closer discussion of it are referred to my “General Morphology” (vol. ii. chap. xix.), or “The History of Creation," or “The Evolution of Man” (vol. i. .

And where is yet farther proof of the truth of the theory of descent to be found? Neither Virchow, nor any one of the clerical opponents and the dualistic philosophers who are perpetually reiterating this cry for more certain evidence, anywhere indicate where possibly such evidence is to be sought. Where in all the world can we discover “facts” which will speak more plainly or significantly for the truth of transmutation than the facts of comparative morphology and physiology; than the facts of the rudimentary organs and of embryonic development; than the facts revealed by fossils and the geographical distribution of organisms in short, than the collective recognised facts of the most diverse provinces of biological science?

But I am in error the certain proof that Virchow demands in order to be perfectly satisfied with the evidence, is to be supplied by “experiment, the test as well as the highest means of evidence.” This demand, that the doctrine of descent should be grounded on experiment, is so perverse and shows such ignorance of the very essence of our theory, that though we have never been surprised at hearing it continually repeated by ignorant laymen, from the lips of a Virchow it has positively astounded us. What can in this case be proved by experiment, and what can experiment prove?

“The variability of species, the transformation of species, the transition of a species into one or more new varieties,” is the answer. Now, so far as these facts can be proved by experiment, they actually have long since been experimentally proved in the completest manner. For what are the numberless trials of artificial selection for breeding purposes which men have practised for thousand of years in breeding domestic animals and cultivated plants, but physiological experiments which prove the transformation of species? As an example we may refer to the different races of horses and pigeons. The swift race-horse and the heavy pack-horse, the graceful carriage-horse and the sturdy cart-horse, the huge dray-horse and the dwarfed pony these and many other “races” are so different from each other, that if we had found them wild we should certainly have described them as quite different varieties of one species, or even representatives of different species. Undoubtedly, these so-called “races” and “sports” of the horse tribe differ from each other in a much greater degree than do the zebra, the quagga, the mountain horse, and the other wild varieties of the horse, which every zoologist distinguishes as “bonae species.” And yet all these artificial varieties, which man has designedly produced by selection, are descended from a single common parent-form, from one wild “true variety.” The same is the case with the numerous and highly differing varieties of pigeons. Domestic pigeons and carrier-pigeons, turbits and cropper-pigeons, fantail pigeons and owls, tumblers and pouters, trumpeters and laughing pigeons (or Indian doves), and the rest, are all, as Darwin has convincingly proved, descendants of a single wild variety, the rock-pigeon (Columba livia). And how wonderfully various they are, not only in general form, size, and colouring, but in the particular form of the skull, the beak, the feet, and so forth! They differ much more in every respect each from the others than the numerous wild varieties which, in systems of ornithology, are recognised as true varieties, and even as true species. It is the same with the different artificial varieties of apples, pears, pansies, dahlias, and so on; in short, of almost all the domestic varieties of animals and plants. We would lay particular stress on the fact that these artificial species which man has produced or created by artificial breeding and through experimental transformation out of one original species, differ far more one from another in physiological as well as in morphological conditions than the natural species in a wild state. With these it is self-evident that any proof by experiment of a common origin is wholly impossible. For, so soon as we subject any wild variety of animal or plant to such an experiment, we bring it under the conditions of artificial breeding.

That the morphological conception of a Species is not a positive but only a relative conception, and that it has no other absolute or positive value than those other similar system-categories sports, varieties, races, tribes, families, classes is now acknowledged by every systematiser who forms an honest and unprejudiced judgment of the practical systematic distinction of species. From the very nature of the case there are no limits to arbitrary discretion in this department, and there are no two systematists who are at one in every instance; this one separating forms as true varieties which that one does not. (Compare on this point “History of Creation,” vol. i., .) The conception of variety or species has a different value in every small or large department of systematic Zoology and Botany.

But the conception of species has just as little any fixed physiological value. In respect to this we must especially insist that the question of hybrid offspring, the last corner of refuge of all the defenders of the constancy of species, has at present lost all significance as bearing on the conception of species. For we know now, through numerous and reliable experiences and experiments, that two different true varieties can frequently unite and produce fertile hybrids (as the hare and rabbit, lion and tiger, many different kinds of the carp and trout tribes, of willows, brambles, and others); and in the second place, the fact is equally certain that descendants of one and the same species which, according to the dogma of the old schools, could always effect a fertile union under certain circumstances, either cannot effect such a union or produce only barren hybrids (the Porto-Santo rabbit, the different races of horses, dogs, roses, hyacinths, &c.; see “History of Creation,” vol. i., .

For a certain proof that the conception of species rests on a subjective abstraction and has a merely relative value like the conception of genus, family, order, class, &c. no class of animals is of so much importance as that of the Sponges. In it the fluctuating forms vary with such unexampled indefiniteness and variability as to make all distinction of species quite illusory. Oscar Schmidt has already pointed this out in the siliceous sponges and keratose sponges; and I, in my monograph, in three volumes, on the Calcareous Sponges (the result of five years of most accurate investigations of this small animal group), have pointed out that we may at pleasure distinguish 3, or 21, or 111, or 289, or 591 different species. I also believe that I have thus convincingly demonstrated how all these different forms of the calcareous sponges may quite naturally, and without any forcing, be traced to a single common parent-form, the simple and not hypothetical, but existing at this present day the simple Olynthus. Hence I think I have here produced the most positive analytical evidence of the transformation of species, and of the unity of the derivation of all the species of a given group of animals, that is generally possible.

Properly, I might spare myself these disquisitions on the question of species, for Virchow does not go into this main question of the theory of descent but this is very characteristic of his attitude. And just as he nowhere thoroughly discusses the doctrine of transformation, neither does he enter generally on the refutation of any of the other certain proofs of the doctrine of descent which we in fact possess at the present day. Neither the morphological nor the physiological arguments for the theory of descent, neither the rudimentary organs nor the embryonic forms, neither the paleontological nor the chronological argument are anywhere closely examined and tested as to their worth or their worthlessness as “certain proofs.” On the contrary, Virchow takes them quite easily, sets them aside, and declares that “certain proofs” of the doctrine of descent do not exist, but remain to be discovered. To be sure, he does not indicate where they are to be sought, nor can he indicate it. How is this strange conduct to be explained? How is it possible that a distinguished naturalist should resist the most important step forward of modern natural science without in any way specially investigating it, without even practically testing and refuting the most weighty arguments in its favour? To this question there is but one answer. Virchow is not generally intimate with the modern doctrine of evolution, and does not possess that knowledge of natural science which is indispensable for any well-grounded judgment on it.

After collecting and carefully reading all that Virchow, during many years, had written against evolution, I arrived at the conviction that he had not thoroughly read either Darwin’s great work on the Origin of Species, nor any other work on the theory of descent, nor had he thought the matter out with such attention as so serious and intricate a subject absolutely demands. Virchow did with these works as it has been his well-known custom to do with many others he hastily turned over the pages, caught at a few leading words, and without any farther trouble he has discoursed upon them, and, which is worst of all, has perpetuated these discourses through the press.

To excuse this conduct, and to account for Virchow’s enigmatical position in the battle of evolution, we must consider what changes this highly-gifted and meritorious man has gone through in the course of the last thirty years. The most important and fruitful part of his life and labours was indisputably during the eight years when he resided in Wuerzburg, from 1848 to 1856. There Virchow, with all the keenness of his youthful intellect, with a sacred enthusiasm for scientific truth, with indefatigable powers of work and the rarest insight, worked out that glorious reform of scientific medicine which will shine through all time as a star of the first magnitude in the history of medical science. In Wuerzburg, Virchow elaborated that comprehensive application of the cellular theory to pathology which culminates in the conception that the cell is an independent living elementary organism, and that our human organism, like that of all the higher animals, is merely a congeries of cells a highly fertile conception, which Virchow now denies as resolutely as he then supported it. In Wuerzburg, twenty-five years since, I sat devoutly at his feet, and received from him with enthusiasm that clear and simple doctrine of the mechanics of all vital activity a truly monistic doctrine, which Virchow now undoubtedly opposes where formerly he defended it. In Wuerzburg, finally, he wrote those incomparable critical and historical leading articles which are the ornament of the first ten yearly series of his “Archives” of pathological anatomy. All that Virchow effected as the great pioneer of reform in medicine, and by which he won imperishable honour in the scientific treatment of disease, all this was either carried out or preconceived in Wuerzburg; and even the celebrated “Cellular Pathology,” a course of lectures which he delivered during the first year and a half after quitting Wuerzburg for Berlin, consists only of the collected and matured fruits of which the blossoms are due to Wuerzburg.

In the autumn of 1856 Virchow left Wuerzburg to settle in Berlin. The exchange of a narrow sphere of labours for a wider one, of small means and appliances for greater ones, proved unfavourable in this case, as in many similar cases. Since he has been in Berlin, in a “great Institution,” and with luxurious appliances, all the scientific results which Virchow has as yet brought to light are not to be compared, either as to quality or quantity, to the grand and immortal achievements which he himself effected in the little institute of Wuerzburg with the scantiest means a new proof of the maxim enunciated by me, and hitherto never confuted, that “the scientific results of an institute are in inverse proportion to its size.” (See “The Aim and Methods of Modern Evolution.")

Still more grave is the circumstance that, since settling in Berlin, Virchow has more and more exchanged his theoretical scientific activity for practical political life. It is well known how prominent a part he plays there in the Prussian Chamber of Representatives, how he raised himself to be the leader of the party of progress, and, to give this political position a broader basis, took part in the representation of the citizens of the capital; how he has taken a most active interest, as city commissioner, in all the petty anxieties and concerns which the charge of such a city as Berlin entails. I am far from blaming, as many have blamed, the political and civic activity to which Virchow has indefatigably devoted his best powers. If a man feels in himself the inclination and vocation with strength and talent enough, to play a conspicuous political part, by all means let him do so; but verily I do not envy him; for the satisfaction which is derived from the most successful and fruitful political activity is not, to my taste, to be compared with that pure and disinterested satisfaction of the mind which results from absorption in serious and difficult scientific labours. In the turmoil of the political and social struggle, even the most splendid civic crown will be dulled by the stifling dust of practical life, which never reaches the ethereal heights of pure science and never rests on the laurels of the thoughtful investigator. However, as I have said, that is a matter of taste. If Virchow really believes that he is doing a greater service to humanity by his practical political life in Berlin than he formerly did by his theoretical scientific work in Wuerzburg, that is his affair; but for all that, in his former sphere he was incomparable, and cannot be replaced; in the latter this is not the case.

If a distinguished man, be he never so remarkable for uncommon power of work and universal gifts, passes the whole day in the friction of political party-struggles, and throws himself as well into all the petty and wearisome details of daily civic life, it is impossible for him to maintain the requisite feeling for the progress of science particularly when it advances so rapidly and incessantly as is the case in our day. It is therefore quite intelligible that Virchow should soon have lost this feeling, and in the course of the last two decades have become more and more estranged from science. And this estrangement has at last led to so complete a change in his fundamental views, to such a metapsychosis, that the present Virchow of 1878 is hardly in a position to understand the youthful Virchow of 1848.

We have seen a similar mental change occur contemporaneously in our greatest naturalist, Carl Ernst von Baer. This gifted and profound thinker and biologist, whose name marks a new epoch in the history of evolution, had in his later years become wholly incompetent even to understand those most important problems of his youthful labours which opened up new paths of inquiry. While in his early years he laid down principles of the greatest value to our modern doctrine of evolution, and even went very near to adopting this hypothesis into his system, at a later period he utterly denied it, and by his writings on Darwinism proved that he was no longer generally capable of mastering this difficult problem. As I am one of Von Baer’s warmest admirers, and in my “Evolution of Man,” as well as in the “History of Creation,” and in other places, have most emphatically expressed that sincere esteem, I thought I might venture to forbear from calling attention to the discrepancy between the lucid, monistic principles of Von Baer in his youth, and the confused dualistic views of his old age. But as many opponents of Darwinism and among them particularly the Old Catholic philosopher of Munich, Huber, who has written a series of articles in the “Augsburger Zeitung” have made constant capital out of the harmless talk of the feeble old Von Baer, I must in this place explicitly declare that this dualistic prating of the old man is quite incapable of shaking the monistic principles of the young and enterprising pioneers of science, or of giving them the lie.

In his autobiography Von Baer gives us the explanation of this striking contradiction. In 1834 he entirely and for ever abandoned the province of the history of development, at which for twenty years he had laboured incessantly, and where he had earned splendid laurels. To escape from the haunting and importunate ideas of the science which had so wholly absorbed him, he fled from Koenigsberg to Petersburg, and subsequently busied himself in scientific inquiries of a quite different character. Twenty-five long years passed by, and when Darwin’s work appeared in 1859, Von Baer had too long undergone a metapsychosis to be able to understand it. In Von Baer, as in Virchow, the course of this remarkable metapsychosis is highly instructive, and will itself afford to the thoughtful psychologist an interesting evidence of the doctrine of evolution.

However, the lack of comprehension of our modern evolution-hypothesis is easier to explain in Virchow’s case than in Von Baer’s, for this reason: morphological knowledge was greatly lacking to Virchow, while Von Baer possessed it in the highest degree. Now morphology is precisely that very department of inquiry in which our theory of descent has its deepest and strongest roots, and has matured the most glorious fruits of knowledge. The study of organic forms, or morphology, is thus, more than any other science, interested in the doctrine of descent, because through this doctrine it first obtained a practical knowledge of effective causes, and was able to raise itself from the humble rank of a descriptive study of forms to the high position of an analytical science of form. It is true that by the beginning of this century the most comprehensive branch of morphology i.e., comparative anatomy which was founded by Cuvier and splendidly developed by Johannes Mueller, had laid the foundations on which to build a truly philosophical science of form. The enormous mass of various empirical material, which had been accumulated by descriptive systematists and by the dissections of zootomists since the time of Linnaeus and Pallas, had already been abundantly matured and utilised in many ways for philosophic purposes by the synthetic principles of comparative anatomy. But even the most important universal laws of organisation of which the old system of comparative anatomy was one had to take refuge in mystical ideas of a plan of structure and of creative final causes (causae finales); they were incapable of arriving at a true and clear perception of effective mechanical causes (causae efficientes). This last, most difficult, and grandest problem, Charles Darwin was the first to solve in 1859, by setting Lamarck’s theory of descent, which was already fifty years old, on a firm footing by his own theory of selection. By this hypothesis it was first made possible to fit together the rich materials which had been previously amassed, into the splendid edifice of the mechanical science of form. (See my “General Morphology,” vol. i. chap. iv.)

The immeasurable step which Darwin thus made in organic morphology can be adequately appreciated only by those who, like myself, were brought up in the school of the old teleological morphology, and whose eyes were suddenly opened by the theory of selection to a comprehension of that greatest of all biological riddles, the creation of specific forms. The dogma of creation, the mystic and dualistic doctrine of the isolated creation of each separate variety, was annihilated at one blow; the belief in transmutation has now for ever taken its place the mechanistic and monistic doctrine of the metamorphosis of organic forms, of the descent of all the species of one natural class from a common parent-form. How complete a change the science of mechanical morphology has by this means been compelled to undergo, I have endeavoured to point out in my “General Morphology;” and any one who wishes to convince himself clearly of what an enormous revolution has been brought about, particularly in comparative anatomy, may compare the “Outlines of Comparative Anatomy” (Grundzuege der vergleichenden Anatomie), by Carl Gegenbaur, 1870, and the latest edition of his “Elements” (Grundrisses), with the old text-books of that science.

Virchow has no suspicion even of all these immeasurable strides in morphology, for this department always lay out of his ken. His great reforms in pathology were founded in the province of physiology, and more especially in cellular physiology. But within the last twenty years these two main branches of biological inquiry have grown more and more apart. The great Johannes Mueller was the last biologist who was able to keep these departments of organic inquiry together, and who won equally immortal honours in both divisions of the subject. After Mueller’s death in 1858 they fell asunder. Physiology, as the science especially of the functions or living activity of the organism, addressed itself more and more to exact and experimental methods: morphology, on the contrary, as the science of the forms and structure of animals and plants, could naturally make but very small use of this method; it must take refuge more and more in the history of evolution, and so constitute an historical natural science. It was on this very historical and genetic method of morphology, in contradistinction to the exact and experimental method of physiology, that I based my Munich address; and if Virchow in his answer had really and thoroughly refuted this position, instead of fighting with mere phrases and denunciations, this radical opposition would have been well worthy of the fullest discussion. At the same time I have no wish to reproach Virchow for being wholly fettered by the one-sided views of the modern school-physiology, nor because morphology lies so far out of his ken that he has not been able to form an independent judgment of its aims and methods; but when, in spite of all this, he on every occasion lets fall a disparaging judgment of it, we must dispute his competence. It is true that in his Munich address he emphasises the statement, “That which graces me best is that I know my ignorance,” by printing it in italics. I only regret that I am forced to deny his possession of this very grace. Virchow does not know how ignorant he is of morphology, else he would never have uttered his annihilating verdict on it, else he would not continually designate the study of the theory of descent as dilettanteism and vain dreaming, as “a fanciful private speculation which is now making its way in several departments of natural science.” In truth, Virchow does me greatly too much honour when he designates as my “personal crotchet” an idea which for the last ten years has been the most precious common possession of all morphological science. If Virchow were not so unfamiliar with the literature of morphology, he must have known that it is penetrated throughout by this principle of descent, that every morphological inquiry which conscientiously pursues a well-considered problem now assumes the doctrine of descent as granted and indisputable. Of all this he is ignorant, and so it is intelligible that he should continue to demand “certain proofs” of this hypothesis, although those proofs have long since been produced.