Professor Fleischmann, zoologist in Erlangen, recently published a book bearing the title, “Die Descendenztheorie,” in which he opposes every theory of Descent. The book is made up of lectures delivered by the author before general audiences of professional students, hence is popular in form and of very special apologetic value. Numerous excellent illustrations aid the reader in understanding the text.

One statement in the Introduction characterizes the decided position assumed by the author. He says: “After long and careful investigation I have come to the conclusion that the doctrine of Descent has not been substantiated. I go even farther and maintain that the discussion of the question does not belong to the field of the exact sciences of zoology and botany.” At the outset, Fleischmann establishes the fact that in the animal kingdom there are rigidly separated types, which cannot be derived from each other, whereas the doctrine of Descent postulates “one single common model of body-structure” from which all types have been developed. Cuvier in his day, set up four such types of essentially different structure; when Darwin’s work appeared two more had been added; R. Hertwig postulates even seven, Boas nine (both 1900); J. Kennel (1893) seventeen, and Fleischmann himself sixteen. In consequence the doctrine of Descent has become more complicated since it now embraces sixteen or seventeen different problems, each of which in turn gives rise to many subordinate problems.

The discussion which the author inaugurates regarding the domain to which the question of Descent belongs, is very well-timed. He forcibly and definitely discountenances the method which transfers it to the domain of religion. The question must be decided by the naturalists themselves according to the strict inductive method; that is, the solution must be based on well ascertained facts, without resorting to conclusions deduced from general principles. “Exact research must show that living organisms actually have overstepped the bounds defining their species, and not merely that they conceivably may have done so.” Hence it is absolutely necessary to procure the intermediary forms. This is the foundation on which Fleischmann builds and against which no opponent can prevail. Fleischmann first discusses the differences between the classes of vertebrates; the mammals, birds, reptiles, amphibians and fish. For if the differences of their bodily structure could be shown to be one of degree and not radical, it could be supposed that the lines of demarcation which now delimitate the larger types might some day vanish. A single illustration suffices for Fleischmann’s purpose, viz., the plan of structure of the limbs of the different classes of vertebrates. The four higher classes are characterized by a common underlying plan of limb structure, whilst fish have one peculiar to themselves. On the other hand it is an inevitable postulate of the doctrine of Descent that fish are the original progenitors of all other vertebrates. Hence the five-joint limbs of the latter must have developed from the fins of fish. This derivation was actually attempted but without success, as Fleischmann points out at considerable length. By means of citations taken from the writings of Darwinian adherents, he illustrates the confusion which even now reigns among them on this matter. The evolution of the remaining vertebrates from the fish is therefore a wholly gratuitous assumption devoid of any foundation in fact.

Fleischmann further discusses the “parade-horse” of the theory of Descent. It has been the common belief, especially fostered by Haeckel, that the history of the Descent of our present horse lies before us in its complete integrity as pictured in the drawings of Marsh. Here Fleischmann again proves at great length the insufficiency of actually available materials. Of special importance is his repeated demand that not only individual parts of the animals but the whole organism as well should be derived from the earlier forms. If, for instance, it be possible to arrange horses and their tertiary kindred in an unbroken line of descent according to the formation of their feet, whilst the other characteristics (teeth, skull-structure, etc.,) do not admit of arrangement in a corresponding series, the first line must be surrendered.

Very similar to this is the case of the “family history of birds,” which as all know, has been traced back to reptiles. It is in this matter that the famous Archaeopteryx plays an important part. Unfortunately, however, grave difficulties are again encountered in this connection. This primitive form is a real bird according to Zittel; and according to the same investigator as also according to Marsh, Dames, Vetter, Parker, Tuerbringen, Parlow and Mehnert, it is inadmissible to connect birds with a definite class of reptiles. Haeckel finds his way out of the difficulty by supplying hypothetical forms which no one has ever seen, but which his imagination has admirably depicted as transitional forms. In so doing, however, he abandons the inductive method of natural science.

It is impossible for us to treat at such length all the remaining sections of this important book. We may mention in passing that Fleischmann examines the “roots of the mammal stock,” and enters upon a detailed discussion of “the origin of lung-breathing vertebrates,” the “real phylo-genetic problem of the mollusks,” and “the origin of the echinodermata.” It is evident that he boldly takes up the most important problems connected with the theory of Descent, and does not confine himself to a one-sided discussion of individual points. As he did not fear to examine thoroughly the famous, and as it hitherto appeared, invulnerable, “parade-horse,” so neither does he hesitate to demolish the other reputed proof for the doctrine of Descent, e.g., the fresh-water snail of Steinheim, the remains of which Hilzendorf and Neumayr examined and were said to have arranged in lines of descent that “would actually stagger one.” It is important to call especial attention to this because the adversaries of the book ignore it. He next shows up the so-called “fundamental principle of biogenesis” according to which organisms are supposed to repeat during their individual development the forms of their progenitors (enunciated by Fritz Mueller and Haeckel). Fleischmann points out the exceptions which Haeckel attributes to “Cenogenesis,” (that is to falsification) and shows the disagreement among contemporary naturalists regarding this fundamental principle. Even Haeckel’s friend and pupil, O. Hertwig sounds the retreat.

The 15th chapter deals with the “Collapse of Haeckel’s Doctrine,” which is revealed in the fact that “the practical possibility of ascertaining anything regarding the primitive history of the animal kingdom is completely exhausted and the hope of so doing forever frustrated.” “Instead of scientists having been able from year to year to produce an increasing abundance of proof for the correctness of the doctrine of Descent, the lack of proofs and the impossibility of procuring evidence is to-day notorious.” In the last chapter Fleischmann finally attempts to prove on logical principles the untenableness of the evolutionary idea.

He starts from the fact that philosophers use the word development to designate a definite sequence of ideas, i.e., in a logical order. “Metamorphosis, says Hegel, belongs to the Idea as such since its variation alone is development. Rational speculation must get rid of such nebulous concepts as the evolution of the more highly developed animal organisms from the less developed, etc.”

Naturalists use the word in a different sense. Instead of a sequence of grades of being they posit a sequence of transformations; instead of a logical sequence of ideas they posit a transforming and progressive development. Zoology constructs a system of specific and generic concepts, “an animal kingdom with logical relations.” Our concepts are derived from natural objects, but in reality do not perfectly correspond to them. The phylogenetic school commits the capital mistake of presenting a transformation which can be realized only in logical concepts, as an actually occurring process, and of confounding an abstract operation with concrete fact. “The logical transformation of the concept ape into the concept man is no genealogical process.” The mathematician may logically ‘develop’ the concept of a circle from that of a polygon, but it by no means follows that the circle is phylo-genetically derived from the polygon.

Because the concept of species is variable, the species themselves, according to Darwin, should be subject to a continual flux; whereas the real cause of the variability which he observed lies in the discrepancy between objective facts and their logical tabulation, in the narrowness of our concepts and in the lack of adequate means of expression. He thus makes natural objects responsible for our logical limitations.

With regard to organisms the Descent-school confounded the purely logical signification of the word “related” with that of blood or family affinity. But surely when they speak of the relation of forms in the crystal systems, they do not refer to genetic connection. To-day this interchange of concepts is so general that one needs to exercise great care if one would avoid it.

The theory which postulates the blood-relationship of individuals of the same species may be correct, but it is utterly incapable of proof, and the same is true in a greater degree when there is question of individuals of the same class but of different species. Since a direct proof is impossible, an attempt was made to construct an indirect proof by a comparison of bodily-organs. But in so doing the Descent theorizers had to relinquish scientific analysis altogether.

In conclusion Fleischmann states that he does not mean to discard every hypothesis of Descent. He simply gives warning against an over-estimation of the theory. In opposition to those who esteem it as the highest achievement of science, he looks upon it as a necessary evil. Its proper sphere is the laboratory of the man of science, and not the thronging market-place.

“The Descent hypothesis will meet the same fate (be cast aside), since its incompatibility with facts of ordinary observation is manifesting itself. At the time of its appearance in a new form, forty years ago, it exercised a beneficial influence on scientific progress and induced a great number of capable minds to devote themselves to the study of anatomical, palaeontological and evolutionary problems. Meanwhile, however, viewed in the light of a constantly increasing wealth of actual materials, the hypothesis has become antiquated and the labors of its industrious advocates makes it obvious to unbiased critics, that it is time to relegate it ad acta.”

My own views agree with those of Fleischmann as presented above, except in regard to his last chapter. I must, of course, admit that his criticism has discredited the doctrine of Descent as a scientifically established theory. Hence, as I have always asserted, it must be excluded from the realm of exact science. No doubt people will come gradually to see that the theory involves a creed and therefore belongs to the domain of cosmic philosophy. All this I readily admit.

Not so, however, as regards the concept of “development.” It seems to me to be incorrect to regard this as a logical concept only, even with reference to organisms. True, the whole zoological system is in reality nothing more than a logical abstraction. And in view of this fact one must be on one’s guard against confusing a logical transformation of concepts with a genealogical development.

We must, however, not forget that we possess the wonderful analogy of ontogeny (individual development) and above all, the fact of mutation and of metagenesis. And even if we wish to avoid the error of Haeckel and others who find a necessary connection between ontogeny and phylogeny, nevertheless the analogy will still entitle us to picture to ourselves the development of the whole range of living organisms. Such a representation will, of course, have only a subjective value.

No doubt, it is logically unjustifiable to argue from the variable concept to the variability of the species. Still there is something real in plants and animals which corresponds to our specific concepts. In some cases the corresponding reality may be so well defined that it is not difficult to form the concept accurately; whereas in other cases where the task is more difficult, the difficulty must be due to the object. Under these circumstances we may safely conclude from the lack of definiteness in our concepts to a certain lack of rigid delimitation in the organic forms.

This blending of certain forms suggests the idea of transformation, but does not furnish definite proof of it. Such proof can be had only by the direct observation of a transformation. And no doubt in certain cases a transformation may occur. As regards animals, I may call attention, for instance, to the experiments made with butterflies by Standfuss, and as regards plants, to the experiments of Haberlandt, of which I treated in Chapter III. The limits within which these transformations take place are indeed very narrow as are also the limits of those indisputable varieties which naturally arise within an otherwise rigidly defined species. I am aware that the transformation of one species into another has not yet been effected, but the above-mentioned attempts at transformation have nevertheless demonstrated that certain organic forms when subjected to changed conditions of life, display certain mutations which clearly show that variability is to be attributed, not, certainly, to the specific concepts, but to the corresponding reality. This observation and reflexion, joined with the fact that organisms form a progressive series from the simple to the more complex, and with the observed phenomena of individual development, lead me to regard the concept of Descent as admissible, and in a certain sense, even probable. But I agree with Fleischmann in saying that this is a mere belief, and that all attempts to give it a higher scientific value by inductive proof have signally failed.

My standpoint, moreover, requires me to admit the validity of the hypothesis of Descent as an heuristic maxim of natural science. I believe that we shall be justified in the future, as we were forty years ago, in directing our investigation in the direction of Descent, and I do not consider such investigation so utterly hopeless as Fleischmann represents it. However, I entirely concur with him in the opinion that we are here concerned (and shall be for a long time to come) with a mere hypothesis which belongs not in the market-place, nor among the world views of the multitude, but in the study of the man of science.

Above all it must not be mixed up with religious questions. Whether the hypothesis will ever emerge from the study of the man of science as a well-attested law, is still an open question, incapable of immediate solution.

It is of interest for us to inquire what reception Fleischmann’s protest against the theory of Descent has been accorded by his associates.

Fleischmann was formerly an advocate of the theory of Descent. He was a pupil and assistant of Selenka, who was then at Erlangen (died in Muenster 1902). He had previously written a number of scientific works from the standpoint of the Descent theory. In the year 1891, investigations regarding rodents led him to oppose that theory. During the winter term of 1891-92 he gave evidence of this change in a public lecture. Not until 1895 was there question of his appointment to the chair of zoology in Erlangen. In 1898 he published a Manual of Zoology based on principles radically opposed to the doctrine of Descent. This manual irritated Haeckel so much that he issued one of his well-known articles, Ascending and Descending Zoology, in which, after his usual manner, he casts suspicion on Fleischmann of having received his appointment to the chair at Erlangen by becoming an anti-Darwinian in accordance with a desire expressed at the diet of Bavaria. I am not aware that Haeckel has paid any attention to the work of Fleischmann which we have just reviewed.

By its publication, however, the author disturbed a hornet’s nest. Dispassionate, but still entirely adverse is Professor Plate’s review in the “Biologisches Zentralblatt,” while the “Umschau” publishes two criticisms, one by Professor von Wagner, the other by Dr. Reh, which for want of sense could not well be equalled. It was the former who furnished material for our sixth chapter and who there displayed such utter confusion of thought regarding the inductive method. The same confusion is apparent in his recent utterance in which he observes that Fleischmann’s whole aim is to accumulate observational data, meanwhile avoiding speculation as far as possible. His criticism is replete with bitter personal epithets, e.g., “reactionary,” “mental incompetency,” “dishonest mask of hypercritical exactness,” which manifest the writer’s inability to enter upon an objective discussion of the question.

A still more reprehensible position is assumed by Dr. Reh, who censures Fleischmann for introducing to the general public the question of Descent which belongs properly to the forum of science. He claims that Fleischmann, by so doing, forfeited his right to an unbiased hearing. Dr. Reh forgets that but a short time ago he had no word of censure for Haeckel’s Weltraetsel which was intended for a far wider circle of readers. He next appropriates Haeckel’s suspicion regarding Fleischmann which we noticed above, and then adds the entirely untrue assertion that the first half of Fleischmann’s Manual, written before he took possession of the chair in Erlangen, is written in the spirit of Darwin, whereas the second half which appeared at a later date is written in the contrary spirit. He then takes individual points of Fleischmann’s treatise out of their context in order to execute a cheap and nonsensical criticism of them. Haeckel has evidently been giving instructions on the best manner of dealing with adversaries. And very docile disciples they are who imitate his method even to the extent of defaming and abusing their scientific opponents.

But is not this another plain indication of the decay of Darwinism? Of course Haeckel recognized at the very beginning of his career that it was necessary to support the theory by means of personal bitterness, forgeries and misrepresentations. But if the last surviving advocates of Darwinism must needs have recourse to the same disreputable means, to what a low estate, indeed, has it fallen!

Let us hope that these last wild convulsions are really the signs of approaching dissolution.