Other Instances of Dissatisfaction with the Theory of Descent.
What was with Virchow only a suggestion of the [pg 112] need for caution, or controversial matter to be subsequently allowed for or contradicted, had more serious consequences to others, and led to still greater hesitancy as regards evolutionist generalisations and speculations, and sometimes to sharp antagonism to them.
One of the best known of the earlier examples of this mood is Kerner von Marilaun's large and beautiful work on “Plant Life.”[21] He does, indeed, admit that our species are variations of antecedent forms, but only in a very limited sense. Within the stocks or grades of organisation which have always existed, variations have come about, through “hybridisation,” through the crossing of similar, but relatively different forms; these variations alter the configuration and appearance in detail, but neither affect the general character nor cause any transition from “lower” to “higher.”
Kerner disposes of the chief argument in favour of the theory of descent, the homology of individual organs, by explaining that the homology is due to the similarity of function in the different organisms. A similar argument is used in regard to “ontogeny recapitulating phylogeny.” Palæontology does not disclose in the plant-world any “synthetic types,” which might have been the common primitive stock from which many now divergent branches have sprung, nor does it disclose any “transition links” really intermediate, [pg 113] for instance, between cryptogams and gymnosperms, or between gymnosperms and angiosperms. That the higher races are apparently absent from the earlier strata is not a proof that they have never existed. The peat-bog flora must have involved the existence of a large companion-flora, without which the peat could not have been formed, but all trace of this is absent in the still persistent vestiges of these times.[22] Life, with energy and matter, has existed as a phenomenon of the universe from all eternity, and thus its chief forms and manifestations have not “arisen,” but have always been. If facts such as these contradict the Kant-Laplace theory of the universe, then the latter must be corrected in the light of them, not conversely. The extreme isolation of Kerner and his theory is probably due especially to this corollary of his views.
Among the most recent examples of antagonism to the Evolution-Theory, the most interesting is a book by Fleischmann, professor of zoology in Erlangen, published in 1901, and entitled, “The Theory of Descent.” It consists of “popular lectures on the rise and decline of a scientific hypothesis” (namely, the Theory of Descent), and it is a complete recantation by a quondam Darwinian of the doctrine of his school, even of its fundamental proposition, the concept of evolution itself. For Fleischmann is not guilty, like Weismann, of the inaccuracy of using “Theory of [pg 114] Descent” as equivalent to Darwinism; he is absolutely indifferent to the theory of natural selection. His book keeps strictly to matters of fact, and rejects as speculation everything in the least beyond these; it does not express even an opinion on the question of the origin of species, but merely criticises and analyses.
It does not bring forward any new and overwhelming arguments in refutation of the Theory of Descent, but strongly emphasises difficulties that have always beset it, and discusses these in detail. The old dispute which interested Goethe, Geoffroy St. Hilaire, and Cuvier, as to the unity or the fundamental heterogeneity of the “architectural plan” in nature is revived. Modern zoology recognises not merely the four types of Cuvier, but seventeen different styles, “phyla,” or groups of forms, to derive one of which from another is hopeless. And what is true of the whole is true also of the subdivisions within each phylum; e.g., within the vertebrate phylum with its fishes, amphibians, reptiles, birds and mammals. No bridge leads from one to the other. This is proved particularly by the very instance which is the favourite illustration in support of the Theory of Descent—the fin of fishes and its relation to the five-fingered hand of vertebrates. The so-called transition forms (Archæopteryx, monotremes, &c.) are discredited. So with the “stalking-horse” of evolutionists—the genealogical tree of the Equidæ, which is said to be traceable palæontologically right back, without a break, from the one-toed horses of the present day to [pg 115] the normal five-toed ancestry; and so with another favourite instance of evolution, the history of the pond-snails (Planorbis multiformis), the numerous varieties of which occur with transitions between them in actual contiguity in the Steinheim beds, and thus seem to afford an obvious example of the transformation of species. Against these cases, and against using the palæontological archives as a basis for the construction of genealogical trees in general, the weighty and apparently decisive objection is urged, that nowhere are the soft parts of the earlier forms of life preserved, and that it is impossible to establish relationships with any certainty on the basis of hard parts only, such as bones, teeth and shells. Even Haeckel admits that snails of very different bodily structure may form very similar and even hardly distinguishable shells.
Fleischmann further asserts that Haeckel's “fundamental biogenetic law” has utterly collapsed. “Recapitulation” does not occur. Selenka's figures of ovum-segmentation show that there are specific differences in the individual groups. The origin and development of the blastoderm or germinal disc has nothing to do with recapitulation of the phylogeny. It is not the case that the embryos of higher vertebrates are indistinguishable from one another. Even the egg-cell has a specific character, and is totally different from any unicellular organism at the Protistan level. The much-cited “gill-clefts” of higher vertebrates in the embryonic stage are not persistent reminiscences of [pg 116] earlier lower stages; they are rudiments or primordia shared by all vertebrates, and developing differently at the different levels; (thus in fishes they become breathing organs, and in the higher vertebrates they become in part associated with the organs of hearing, or in part disappear again).
Though Fleischmann's vigorous protest against over-hastiness in construction and over-confidence on the part of the adherents of the doctrine of descent is very interesting, and may often be justified in detail, it is difficult to resist the impression that the wheat has been rejected with the chaff.[23]
Even a layman may raise the following objections: Admitting that the great groups of forms cannot be traced back to one another, the palæontological record still proves, though it may be only in general outline, that within each phylum there has been a gradual succession and ascent of forms. How is the origin of what is new to be accounted for? Without doing violence to our thinking, without a sort of intellectual autonomy, we cannot rest content with the mere fact that new elements occur. So, in spite of all “difficulties,” the assumption of an actual descent quietly forces itself upon us as the only satisfactory clue. And the fact, which Fleischmann does not discuss, that even at present we may observe the establishment of what are at least new breeds, impels us to accept an analogous [pg 117] origin of new species. Even if the biogenetic law really “finds its chief confirmation in its exceptions,” even if we cannot speak of a strict recapitulation of earlier stages of evolution, there are indisputable facts which are most readily interpreted as reminiscences, as due to affiliation (ideal or hereditary), with ancestral forms. (Note, for instance, Weismann's “prediction,” &c.[24]) Even if Archæopteryx and other intermediate forms cannot be regarded as connecting links in the strict sense, i.e., as being stages in the actual pedigree, yet the occurrence of reptilian and avian peculiarities side by side in one organism, goes far to prove the close relationship of the two classes.
Fleischmann's book strengthens the impression gained elsewhere, that a general survey of the domain of life as a whole gives force and convincingness to the Theory of Descent, while a study of details often results in breaking the threads and bringing the difficulties into prominence. But the same holds true of many other theoretical constructions, and yet we do not seriously doubt their validity. (Take, for instance, the Kant-Laplace theory, and theories of ethnology, of the history of religion, of the history of language, and so on.) And it is quite commonly to be observed that those who have an expert and specialist knowledge, who are aware of the refractoriness of detailed facts, often take up a sceptical attitude towards every comprehensive [pg 118] theory, though the ultimate use of detailed investigation is to make the construction of general theories possible. Fleischmann does exactly what, say, an anthropologist would do if, under the impression of the constancy and distinctiveness of the human races, which would become stronger the more deeply he penetrated, he should resignedly renounce all possibility of affiliating them, and should rest content with the facts as he found them. Similarly, those who are most intimately acquainted with the races of domesticated animals often resist most strenuously all attempts, although these seem to others a matter of course, to derive our “tame” forms from “wild” species living in freedom.
But to return. Even where the Theory of Descent is recognised, whether fully or only half-heartedly, the recognition does not always mean the same thing. Even the adherents of the general, but in itself quite vague view that a transformation from lower forms to higher, and from similar to different forms, has taken place, may present so many points of disagreement, and may even stand in such antagonism to one another, that onlookers are apt to receive the impression that they occupy quite different standpoints, and are no longer at one even in the fundamentals of their hypotheses.
The most diverse questions and answers crop up; whether evolution has been brought about “monophyletically” or “polyphyletically,” i.e., through one or many genealogical trees; whether it has taken place in [pg 119] a continuous easy transition from one type to another, or by leaps and bounds; whether through a gradual transformation of all organs, each varying individually, or through correlated “kaleidoscopic” variations of many kinds throughout the whole system; whether it is essentially asymptotic, or whether organisms pass from “labile” phases of vital equilibrium by various halting-places to stable states, which are definitive, and are, so to speak, the blind alleys and terminal points of evolutionary possibilities, e.g., the extinct gigantic saurians, and perhaps also man. And to these problems must be added the various answers to the question, What precedes, or may have preceded, the earliest stages of life of which we know? Whence came the first cell? Whence the first living protoplasm? and How did the living arise from the inorganic? These deeper questions will occupy us in our chapter on the theory of life. Some of the former, in certain of their aspects, will be considered in the sixth chapter, which deals with factors in evolution.
The Theory of Descent itself and the differences that obtain even among its adherents can best be studied by considering for a little the works of Reinke and of Hamann.
Reinke, Professor of Botany in Kiel, has set forth his views in his book, “Die Welt als Tat,”[25] and more recently in his “Einleitung in die theoretische Biologie” (1901). Both books are addressed to a wide [pg 120] circle of readers. Reinke and Hamann both revive some of the arguments and opinions set forth in the early days of Darwinism by Wigand,[26] an author whose works are gradually gaining increased appreciation.
It is Reinke's “unalterable conviction” that organisms have evolved, and that they have done so after the manner of fan-shaped genealogical trees. The Theory of Descent is to him an axiom of modern biology, though as a matter of fact the circumstantial evidence in favour of it is extremely fragmentary. The main arguments in favour of it appear to him to be the general ones; the homologies and analogies revealed by comparative morphology and physiology, the ascending series in the palæontological record, vestigial organs, parasitic degeneration, the origin of those vital associations which we call consortism and symbiosis. These he illustrates mainly by examples from his own special domain and personal observation.
The simplest unicellular forms of life are to be thought of as at the beginning of evolution; and, since mechanical causes cannot explain their ascent, it must be assumed that they have an inherent “phylogenetic potential of development,” which, working epigenetically, results in ascending evolution. He leaves us to choose between monophyletic and polyphyletic evolution, but himself inclines towards the latter, associating with it a [pg 121] rehabilitation of Wigand's theory of the primitive cells. If, in the beginning, primitive forms of life arose (probably as unicellulars) from the not-living, it is not obvious why we need think of only one so arising, and, if many did so, why they should not have inherent differences which would at once result in typically different evolutionary series and groups of forms. But evolution does not go on ad libitum or ad infinitum, for the capacity for differentiation and transformation gradually diminishes. The organisation passes from a labile state of equilibrium to an increasingly stable state, and at many points it may reach a terminus where it comes to a standstill. Man, the dog, the horse, the cereals, and fruit trees appear to Reinke to have reached their goal. The preliminary stages he calls “Phylembryos,” because they bear to the possible outcome of their evolution the same relation that the embryo does to the perfect individual. Thus, Phenacodus may be regarded as the Phylembryo of the modern horse. It is quite conceivable that each of our modern species may have had an independent series of Phylembryos reaching back to the primitive cells. But the palæontological record, and especially its synthetic types, lead Reinke rather to assume that instead of innumerable series, there have been branching genealogical trees, not one, however, but several.
These views, together or separately, which are characterised chiefly by the catch-words “polyphyletic descent,” “labile and stable equilibrium,” and so on, crop [pg 122] up together or separately in the writings of various evolutionists belonging to the opposition wing. They are usually associated with a denial of the theory of natural selection, and with theories of “Orthogenesis,” “Heterogenesis,” and “Epigenesis.”
We shall discuss them later when we are considering the factors in evolution. But we must first take notice of a work in which the theories opposed to Darwinian orthodoxy have been most decisively and aggressively set forth. As far back as 1892 O. Hamann, then a lecturer on zoology in Göttingen, gathered these together and brought them into the field, against Haeckel in particular, in his book “Entwicklungslehre und Darwinismus.”[27]
Hamann's main theme is that Darwinism overlooks the fact that “there cannot have been an origin of higher types from types already finished.” For this “unfortunate and unsupported assumption” there are no proofs in embryology, palæontology, or anatomy. He adopts and expands the arguments and anti-Haeckelian deliverances of His in embryology, of Snell and Heer in palæontology, of Kölliker and von Baer in their special interpretation of evolution, of Snell particularly as regards the descent of man. It is impossible to derive Metazoa from Protozoa in their present finished state of evolution; even the Amoeba is so exactly adapted in organisation and functional activity [pg 123] to the conditions of its existence that it is a “finished” type. It is only by a stretch of fancy that fishes can be derived from worms, or higher vertebrates from fishes. One of his favourite arguments—and it is a weighty one, though neglected by the orthodox Darwinians—is that living substance is capable, under similar stimuli, of developing spontaneously and afresh, at quite different points and in different groups, similar organs, such as spots sensitive to light, accumulations of pigment, eye-spots, lenses, complete eyes, and similarly with the notochord, the excretory organs, and the like. Therefore homology of organs is no proof of their hereditary affiliation.[28] They rather illustrate “iterative evolution.”
Another favourite argument is the fact of “Pædogenesis.” Certain animals, such as Amphioxus lanceolatus, Peripatus, and certain Medusæ, are very frequently brought forward as examples of persistent primitive stages and “transitional connecting links.” But considered from the point of view of Pædogenesis, they all assume quite a different aspect, and seem rather to represent very highly evolved species, and to be, not primitive forms, but conservative and regressive forms. Pædogenesis is the phenomenon exhibited by a number of species, which may stop short at one of the stages of their embryonic or larval development, become sexually mature, and produce [pg 124] offspring without having attained their own fully developed form.
Another argument is the old, suggestive, and really important one urged by Kölliker, that “inorganic nature shows a natural system among minerals (crystals) just as much as animals and plants do, yet in the former there can be no question of any genetic connection in the production of forms.”
Yet another argument is found in the occurrence of “inversions” and anomalies in the palæontological succession of forms, which to some extent upsets the Darwinian-Haeckelian genealogical trees. (Thus there are forms in the Cambrian whose alleged ancestors do not appear till the Silurian. Foraminifera and other Protozoa do not appear till the Silurian.)
From embryology in particular, as elsewhere in general, we read the “fundamental biogenetic law,” that evolution is from the general to the special, from the imperfect to the more perfect, from what is still indefinite and exuberant to the well-defined and precise, but never from the special to the special. According to Hamann's hypothesis we must think of evolution as going on, so to speak, not about the top but about the bottom. The phyla or groups of forms are great trunks bearing many branches and twigs, but not giving rise to one another. Still less do the little side branches of one trunk bear the whole great trunk of another animal or plant phylum. But they all grow from the same roots among the primitive forms of life. Unicellulars these must [pg 125] have been, but not like our “Protists.” They should be thought of as primitive forms having within themselves the potentialities of the most diverse and widely separate evolution-series to which they gave rise, as it were, along diverging fan-like rays.
It would be instructive to follow some naturalist into his own particular domain, for instance a palæontologist into the detailed facts of palæontology, or an embryologist into those of embryology, in order to learn whether these corroborate the assumptions of the Theory of Descent or not. It is just in relation to these detailed facts that criticisms or even denials of the theory have been most frequent. Koken, otherwise a convinced supporter of the theory, inquires in his “Vorwelt,” apropos of the tortoises, what has become of the genealogical trees that were scattered abroad in the world as proved facts in the early days of Darwinism. He asserts, in regard to Archæopteryx, the instance which is always put forward as the intermediate link in the evolution of birds, that it does not show in any of its characters a fundamental difference from any of the birds of to-day, and further, that, through convergent development under similar influences, similar organs and structural relations result, iterative arrangements which come about quite independently of descent. He maintains, too, that the principle of the struggle for existence is rather disproved than corroborated by the palæontological record.
In embryology, so competent an authority as [pg 126] O. Hertwig—himself a former pupil of Haeckel's—has reacted from the “fundamental biogenetic law.” His theory of the matter is very much that of Hamann which we have already discussed; development is not so much a recapitulation of finished ancestral types as the laying down of foundations after the pattern of generalised simple forms, not yet specialised; and from these foundations the special organs rise to different levels and grades of differentiation according to the type.[29] But we must not lose ourselves in details.
Looking back over the whole field once more, we feel that we are justified in maintaining with some confidence that the different pronouncements in regard to the detailed application and particular features of the Theory of Descent, and the different standpoints that are occupied even by evolutionists, are at least sufficient to make it obvious that, even if evolution and descent have actually taken place, they have not run so simple and smooth a course as the over-confident would have us believe; that the Theory of Descent rather emphasises than clears away the riddles and difficulties of the case, and that with the mere corroboration of the theory we shall have gained only something relatively external, a clue to creation, which does not so much solve its problems as restate them. The whole criticism of the “right wing,” from captious objections to actual denials, proves this indisputably. And it seems likely that in the course of time a sharpening of [pg 127] the critical insight and temper will give rise to further reactions from the academic theory as we have come to know it.[30] On the other hand, it may be assumed with even greater certainty that the general evolutionist point of view and the great arguments for descent in some form or other will ultimately be victorious if they are not so already, and that, sooner or later, we shall take the Theory of Descent in its most general form as a matter of course, just as we now do the Kant-Laplace theory.