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.”