Haeckel"s Evolutionist Position.
The average type of the Theory of Descent of the older or orthodox school, which still lingers in the background with its Darwinism unshaken, is that set forth by Haeckel, scientifically in his "Generelle Morphologie der Organismen" (1866), and "Systematische Phylogenie" (1896), and popularly in his "Natural History of Creation" and "Riddles of the Universe," with their many editions. We may a.s.sume that it is well known, and need only briefly recall its chief characteristics. The "inestimable value," the "incomparable significance," the "immeasurable importance" of the Theory of Descent lies, according to Haeckel, in the fact that by means of it we can explain the origin of the forms of life "in a mechanical manner." The theory, especially in regard to the descent of man from the apes, is to him not a working hypothesis or tentative mode of representation; it is a result comparable to Newton"s law of gravitation or the Kant-Laplace cosmogony. It is "a certain historical fact." The proofs of it are those already mentioned.
What is especially Haeckelian is the "fundamental biogenetic law,"
"ontogeny resembles phylogeny," that is to say, in development, especially in embryonic development, the individual recapitulates the history of the race. Through "palingenesis," man, for instance, recapitulates his ancestral stages (protist, gastraead, vermine, piscine, and simian). This recapitulation is condensed, disarranged, or obscured in detail by "cenogenesis" or "caenogenesis." The groups and types of organisms exhibit the closest genetic solidarity. The genealogical tree of man in particular runs directly through a whole series. From the realm of the protists it leads to that of the gastraeadae (nowadays represented by the Clentera), thence into the domain of the worms, touches the hypothetical "primitive chordates" (for the necessary existence of which "certain proofs" can be given), the cla.s.s of tunicates, ascends through the fishes, amphibians and reptiles to forms parallel to the modern monotremes, then directly through the marsupials to the placentals, through lemuroids and baboons to the anthropoid apes, from them to the "famous Pithecanthropus" discovered in Java, out of which _h.o.m.o sapiens_ arose. (The easy transition from one group of forms to another is to be noted. For it is against this point that most of the opposition has been directed, whether from "grumbling"
critics, or thoroughgoing opponents of the Theory of Descent.)
Haeckel"s facile method of constructing genealogical trees, which ignores difficulties and discrepant facts, has met with much criticism and ridicule even among Darwinians. The "orator of Berlin," Du Bois-Reymond, declared that if he must read romances he would prefer to read them in some other form than that of genealogical trees. But they have at least the merit that they give a vivid impression of what is most plausible and attractive in the idea of descent, and moreover they have helped towards orientation in the discussion. Nor can we ignore the very marked taxonomic and architectonic talent which their construction displays.
Weismann"s Evolutionist Position.
The most characteristic representative, however, of the modern school of unified and purified Darwinism is not Haeckel, but the Freiburg zoologist, Weismann. Through a long series of writings he has carried on the conflict against heterodox, and especially Lamarckian theories of evolution, and has developed his theories of heredity and the causes of variation, of the non-transmissibility of acquired characters, and the all-sufficiency of natural selection. In his latest great work, in two volumes, "Lectures on the Theory of Descent,"(11) he has definitely summed up and systematised his views. These will interest us when we come to inquire into the problem of the factors operative in evolution. For the moment we are only concerned with his att.i.tude to the Theory of Descent as such. It is precisely the same as Haeckel"s, although he is opposed to Haeckel in regard to the strictly Darwinian standpoint. The Theory of Descent has conquered, and it may be said with a.s.surance, for ever. That is the firm conviction on which the whole work is based, and it is really rather treated as a self-evident axiom than as a statement to be proved. Weismann takes little trouble to prove it. All the well-known, usually very clear proofs from palaeontology, comparative anatomy, &c., which we are accustomed to meet with in evolutionist books are wanting here, the genealogical trees of the Equidae, with the gradually diminishing number of toes and the varying teeth, of _Planorbis multiformis_, of the ammonites, the graduated series of stages exhibited by individual organs, for instance, from the ganglion merely sensitive to light up to the intricate eye, or from the rayed skeleton of the paired fins in fishes up to the five-fingered hands and feet of the higher vertebrates, &c. These are only briefly touched upon in the terse "Introduction," and the whole of the comprehensive work is then directed to showing what factors can have been operative, and to proving that they must have been "Darwinian" (selection in the struggle for existence), and not Lamarckian or any other. This is shown in regard to the coloration of animals, the phenomena of mimicry, the protective arrangements of plants, the development of instinct in animals, and the origin of flowers.
In reality Weismann only adduces _one_ strict proof, and even that is only laying special stress on what is well known in comparative embryology; namely, the possibility of "predicting" on the basis of the theory of descent, as Leverrier "predicted" Neptune. For instance, in the lower vertebrates from amphibians upwards there is an _os centrale_ in the skeleton of wrist, but there is none in man. Now if man be descended from lower vertebrates, and if the fundamental biogenetic law be true (that every form of life recapitulates in its own development, especially in its embryonic development, the evolution of its race, though with abbreviations and condensations), it may be predicted that the _os centrale_ is to be found in the early embryonic stages of man. And Rosenberg found it. In the same way the "gill-clefts" of the fish-like ancestors have long since been discovered in the embryo of the higher vertebrates and of man. Weismann himself "predicted" that the markings of the youngest stage of the caterpillars of the Sphingidae (hawk-moths) would be found to be not oblique but longitudinal stripes, and ten years later a fortunate observation verified the prediction. Because of the abundance of evidential facts Weismann does not go into any detailed proof of evolution. "One can hardly take up any work, large or small, on the finer or more general structural relations, or on the development of any animal, without finding in it proofs for the evolution theory."
But a.s.sured as the doctrine of descent appears,(12) and certain as it is that it has not only maintained its hold since Darwin"s day, but has strengthened it and has gained adherents, this foundation of Darwinism is nevertheless not the unanimous and inevitable conclusion of all scientific men in the sense and to the extent that the utterances of Weismann and others would lead us to suppose. Apart from all apologetic attempts either in religious, ethical, or aesthetic interests, apart, too, from the superior standpoint of the philosophers, who have not, so to speak, taken the theory very seriously, but regard it as a provisional theory, as a more or less necessary and useful method of grouping our ideas in regard to the organic world, there are even among the biologists themselves some who, indifferent towards religious or philosophical or naturalistic dogma, hold strictly to fact, and renounce with nonchalance any pretensions at completeness of knowledge if the data do not admit of it, and on these grounds hold themselves aloof from evolutionist generalisation. From among these come the counsels of "caution," admissions that the theory is a scientific hypothesis and a guide to research, but not knowledge, and confessions that the Theory of Descent as a whole is verifiable rather as a general impression than in detail.
Virchow"s Position.
Warnings of this kind have come occasionally from Du Bois-Reymond, but the true type of this group, and its mode of thought, is Virchow. It will repay us and suffice us to make acquaintance with it through him. His opposition to Darwinism and the theory of descent was directed at its most salient point: the descent of man from the apes. In lectures and treatises, at zoological and anthropological congresses, especially at the meetings of his own Anthropological-Ethnological Society in Berlin, from his "Vortrage uber Menschen-und Affen-Schadel" (Lectures on the Skulls of Man and Apes, 1869), to the disputes over Dubois" _Pithecanthropus erectus_ in the middle of the nineties, he threw the whole weight of his immense learning-ethnological and anthropological, osteological, and above all "craniological"-into the scale against the Theory of Descent and its supporters. Virchow has therefore been reckoned often enough among the anti-Darwinians, and has been quoted by apologists and others as against Darwinism, and he has given reason for this, since he has often taken the field against "the Darwinists" or has scoffed at their "longing for a pro-anthropos."(13) Sometimes even it has been suggested that he was actuated by religious motives, as when he occasionally championed not only freedom for science, but, incidentally, the right of existence for "the churches," leaving, for instance, in his theory of psychical life, gaps in knowledge which faith might occupy in moderation and modesty. But this last proves nothing. With Virchow"s altogether unemotional nature it is unlikely that religious or spiritual motives had any role in the establishment of his convictions, and in Haeckel"s nave bl.u.s.tering at religion, there is, so to speak, more religion than in the cold-blooded connivance with which Virchow leaves a few openings in otherwise frozen ponds for the ducks of faith to swim in! And he has nothing of the pathos of Du Bois-Reymond"s "ignorabimus." He is the neutral, prosaic scientist, who will let nothing "tempt him to a transcendental consideration,"(14) either theological or naturalistic, who holds tenaciously to matters of fact, who, without absolutely rejecting a general theory, will not concern himself about it, except to point out every difficulty in the way of it; in short, he is the representative of a mood that is the ideal of every investigator and the despair of every theoriser.
His lecture of 1869 already indicates his subsequent att.i.tude. "Considered logically and speculatively" the Theory of Descent seems to him "excellent,"(15) indeed a logical moral(!) hypothesis, but unproved in itself, and erroneous in many of its particular propositions. As far back as 1858, before the publication of Darwin"s great work, he stated at the Naturalists" Congress in Carlsruhe, that the origin of one species out of another appeared to him a necessary scientific inference, but--And throughout the whole lecture he alternates between favourable recognition of the theory in general, and emphasis of the difficulties which confront it in detail. The skull, which, according to Goethe"s theory, has evolved from three modified vertebrae, is fundamentally different in man and monkeys, both in regard to its externals, crests, ridges and shape, and especially in regard to the nature of the cavity which it forms for the brain. Specifically distinctive differences in the development and structure of the rest of the body must also be taken into account. The so-called ape-like structures in the skull and the rest of the body, which occasionally occur in man (idiots, microcephaloids, &c.) cannot be regarded as atavisms and therefore as proofs of the Theory of Descent; they are of a pathological nature, entirely facts _sui generis_, and "not to be placed in a series with the normal results of evolution." A man modified by disease "is still thoroughly a man, not a monkey."
Virchow continued to maintain this att.i.tude and persisted in this kind of argument. He energetically rejected all attempts to find "pithecoid"
characters in the prehistoric remains of man. He declared the narrow and less arched forehead, the elliptical form, and the unusually large frontal cavities of the "Neanderthal skull" found in the Wupperthal in 1856, to be simply pathological features, which occur as such in certain examples of _h.o.m.o sapiens_.(16) He explained the abnormal appearance of the jaw from the Moravian cave of Schipka as a result of the retention of teeth,(17) accompanied by directly "antipithecoid" characters.
The proceedings at the meetings of the Ethnological Society in 1895, at which Dubois was present, had an almost dramatic character.(18) In the diverse opinions of Dubois, Virchow, Nehring, Kollmann, Krause and others, we have almost an epitome of the present state of the Darwinian question.
Virchow doubted whether the parts put together by Dubois (the head of a femur, two molar teeth, and the top of a skull) belonged to the same individual at all, disputed the calculations as to the large capacity of the skull, placed against Dubois" very striking and clever drawing of the curves of the skull-outline, which ill.u.s.trated, with the help of the Pithecanthropus, the gradual transition from the skull of a monkey to that of man, his own drawing, according to which the Pithecanthropus curve simply coincides with that of a gibbon (_Hylobates_), and a.s.serted that the remains discovered were those of a species of gibbon, refusing even to admit that they represented a new genus of monkeys. He held fast to his _ceterum censeo_: "As yet no diluvial discovery has been made which can be referred to a man of a pithecoid type." Indeed, his polemic or "caution"
in regard to the Theory of Descent went even further. He not only refused to admit the proof of the descent of man from monkey, he would not even allow that the descent of one race from another has been demonstrated.(19) In spite of all the plausible hypotheses it remains "so far only a _pium desiderium_." The race obstinately maintains its specific distinctness, and resists variation, or gradual transformation into another. The negro remains a negro in America, and the European colonist of Australia remains a European.
Yet all Virchow"s opposition may be summed up in the characteristic words, which might almost be called his motto, "I warn you of the need for caution," and it is not a seriously-meant rejection of the Theory of Descent. In reality he holds the evolution-idea as an axiom, and in the last-named treatise he shows distinctly how he conceives of the process.
He starts with variation (presumably "kaleidoscopic"), which comes about as a "pathological" phenomenon, that is to say, not spontaneously, but as the result of environmental stimulus, as the organism"s reaction to climatic and other conditions of life. The result is an alteration of previous characteristics, and a new stable race is established by an "acquired anomaly."(20)
Other Instances of Dissatisfaction with the Theory of Descent.
What was with Virchow only a suggestion of the 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 h.o.m.ology of individual organs, by explaining that the h.o.m.ology is due to the similarity of function in the different organisms. A similar argument is used in regard to "ontogeny recapitulating phylogeny."
Palaeontology 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, 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 ent.i.tled, "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 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 a.n.a.lyses.
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 ill.u.s.tration 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 (Archaeopteryx, monotremes, &c.) are discredited. So with the "stalking-horse" of evolutionists-the genealogical tree of the Equidae, which is said to be traceable palaeontologically right back, without a break, from the one-toed horses of the present day to 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 palaeontological 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 sh.e.l.ls. Even Haeckel admits that snails of very different bodily structure may form very similar and even hardly distinguishable sh.e.l.ls.
Fleischmann further a.s.serts 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 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 a.s.sociated 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 palaeontological 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 a.s.sumption 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 a.n.a.logous 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 Archaeopteryx 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 cla.s.ses.
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 att.i.tude towards every comprehensive 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 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 pa.s.s 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 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 circ.u.mstantial evidence in favour of it is extremely fragmentary. The main arguments in favour of it appear to him to be the general ones; the h.o.m.ologies and a.n.a.logies revealed by comparative morphology and physiology, the ascending series in the palaeontological record, vestigial organs, parasitic degeneration, the origin of those vital a.s.sociations which we call consortism and symbiosis. These he ill.u.s.trates 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 a.s.sumed 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, a.s.sociating with it a 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 pa.s.ses 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 palaeontological record, and especially its synthetic types, lead Reinke rather to a.s.sume 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 up together or separately in the writings of various evolutionists belonging to the opposition wing. They are usually a.s.sociated 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 Gottingen, 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 a.s.sumption" there are no proofs in embryology, palaeontology, or anatomy. He adopts and expands the arguments and anti-Haeckelian deliverances of His in embryology, of Snell and Heer in palaeontology, of Kolliker 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 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, acc.u.mulations of pigment, eye-spots, lenses, complete eyes, and similarly with the notochord, the excretory organs, and the like. Therefore h.o.m.ology of organs is no proof of their hereditary affiliation.(28) They rather ill.u.s.trate "iterative evolution."
Another favourite argument is the fact of "Paedogenesis." Certain animals, such as _Amphioxus lanceolatus_, _Peripatus_, and certain Medusae, are very frequently brought forward as examples of persistent primitive stages and "transitional connecting links." But considered from the point of view of Paedogenesis, they all a.s.sume quite a different aspect, and seem rather to represent very highly evolved species, and to be, not primitive forms, but conservative and regressive forms. Paedogenesis 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 s.e.xually mature, and produce offspring without having attained their own fully developed form.