§ 2. Embryological Resemblances
Much has been made of the so-called biogenetic law as an argument for the bestial origin of mankind. This theory of the embryological recapitulation of racial history was first formulated by Fritz Müller. Haeckel, however, was the one who exploited it most extensively, and who exalted it to the status of “the fundamental law of biogenesis.”[16] The latter’s statement of the principle is as follows: “Die Ontogenesis ist die Palingenesis der Phylogenesis.”—Ontogeny (the development of the individual) is a recapitulation of phylogeny (the development of the race). For a long time this law was received with uncritical credulity by the scientific world, but enthusiasm diminished when more careful studies made it clear that the line of descent suggested by embryology did not agree with what was inferred from comparative anatomy and the sequence of fossil forms. Besides, it was manifest that certain organs in embryos were distinctively embryonic and could never have functioned in adult forms, e.g. the yolk sac and the amnion. “It was recognized,” says T. H. Morgan, “that many embryonic stages could not possibly represent ancestral animals. A young fish with a huge yolk sac attached could scarcely ever have led a happy, free life as an adult individual. Such stages were interpreted, however, as embryonic additions to the original ancestral type. The embryo had done something on its own account. In some animals the young have structures that attach them to the mother, as does the placenta of mammals. In other cases the young develop membranes about themselves—like the amnion of the chick and the mammal—that would have shut off an adult animal from all intercourse with the outside world. Hundreds of such embryonic structures are known to embryologists. These were explained as adaptations and as falsifications of the ancestral records.” (“Critique of the Theory of Evolution,” pp. 16, 17.)
The result has been that this so-called law has fallen into general disrepute among scientists, especially as a means of reconstructing the phylogeny of modern organisms. It is recognized, of course, that comparative embryology can furnish embryological homologies analogous to the homologies of comparative anatomy, but it is now generally acknowledged that the view, which regards the embryological process as an abridged repetition of the various states through which the species has passed in its evolutionary career must be definitively abandoned, and that, as a general law of organic development, the biogenetic principle has been thoroughly discredited. “This law,” says Karl Vogt of Geneva, “which I long held as well-founded, is absolutely and radically false. Attentive study of embryology shows us, in fact, that embryos have their own conditions suitable to themselves, and very different from those of adults.” (Quoted by Quatrefages De Breau, in his “Les Emules de Darwin,” vol. II, p. 13.) “There can no longer be question,” says Prof. M. Caullery of the Sorbonne, “of systematically regarding individual development as a repetition of the history of the stock. This conclusion results from the very progress made under the inspiration received from this imaginary law, the law of biogenesis.” (Smithson. Inst. Rpt. for 1916, p. 325.)
This collapse of the biogenetic law has tumbled into ruins the elaborate superstructure of genealogy which Haeckel had reared upon it. His series of thirty stages extending from the fictitious “cytodes” up to man, inclusively, is even more worthless today than it was when Du Bois-Reymond made his ironic comment: “Man’s pedigree, as drawn up by Haeckel, is worth about as much as is that of Homer’s heroes for critical historians.” (Revue Scientifique, 1877, I, p. 1101.) Haeckel tried in vain to save his discredited law by means of the expedient of cænogenesis, that is, “the falsification of the ancestral record (palingenesis).” That Nature should be guilty of “falsification” is an hypothesis not to be lightly entertained, and it is more credible, as Wasmann remarks, to assume that Haeckel, and not Nature, is the real falsifier, inasmuch as he has misrepresented Nature in his “fundamental biogenetic law.” Cænogenesis is a very convenient device. One can alternate at will between cænogenesis and palingenesis, just as, in comparative anatomy, one can alternate capriciously between convergence and homology, on the general understanding of its being a case of: “Heads, I win; tails, you lose”—certainly, there is no objective consideration to restrain us in such procedure. “Such weapons as Cænogenesis and Convergence,” says Kohlbrugge (in his “Die Morphologische Abstammung des Menschen,” 1908) “are unfortunately so shaped that anyone can use them when they suit him, or throw them aside when they do not. They show, therefore, in the prettiest way the uncertainty even now of the construction of the theory of descent. As soon as we go into details it leaves us in the lurch; it was only while our knowledge was small that everything seemed to fit together in most beautiful order.” (Quoted by Dwight in “Thoughts of a Catholic Anatomist,” p. 187.)
It is undeniable, indeed, that in many cases the young of higher animals pass through stages in which they bear at least a superficial resemblance to adult stages in inferior and less complex organisms. Obviously, however, there cannot be any direct derivation of the embryonic features of one organism from the adult characters of another organism. This preposterous implication of the Müller-Haeckel Law must, as Morgan points out, be entirely eliminated, before it can merit serious consideration. Referring to the spiral cleavage exhibited by annelid, planarian and molluscan eggs, Morgan says: “It has been found that the cleavage pattern has the same general arrangement in the early stages of flat worms, annelids and molluscs. Obviously these stages have never been adult ancestors, and obviously if their resemblance has any meaning at all, it is that each group has retained the same general plan of cleavage possessed by their common ancestor.... Perhaps someone will say, ‘Well! is not this all that we have contended for! Have you not reached the old conclusion in a roundabout way?’ I think not. To my mind there is a wide difference between the old statement that the higher animals living today have the original adult stages telescoped into their embryos, and the statement that the resemblance between certain characters in the embryos of higher animals and corresponding stages in the embryos of lower animals is most plausibly explained by the assumption that they have descended from the same ancestors, and that their common structures are embryonic survivals.” (Op. cit., pp. 22, 23.)
After this admission, however, nothing remains of the law of “recapitulation” except simple embryological homology comparable, in every sense, to adult homology, and adding nothing essentially new to the latter argument for evolution. It is, therefore, ridiculous for evolutionists to speak of branchial (gill) arches and clefts in man. The visceral or pharyngeal arches and grooves appearing in the human embryo are unquestionably homologous with the genuine branchial arches and clefts in a fish embryo. In the latter, however, the grooves become real clefts through perforation, while the arches become the lamellæ of the permanent gills, thus adapting the animal to aquatic respiration. It is, accordingly, perfectly legitimate to refer to these embryonic structures in the young fish as gill arches and gill clefts. In man, however, the corresponding embryonic structures develop into the oral cavity, auditory meatus, ossicles of the ear, the mandible, the lower lip, the tongue, the cheek, the hyoid bone, the styloid process, the thymus, the thyroid and tracheal cartilages, etc. There is no perforation of the grooves, and the arches develop into something quite different than branchial lamellæ. Hence the correct name for these structures in the human embryo is pharyngeal (visceral) arches and grooves, their superficial resemblance to the embryonic structures in the fish embryo being no justification for calling them branchial. In short, the mere fact that certain embryonic structures in the young fish (homologous to the pharyngeal arches and grooves in the human embryo) develop into the permanent gills of the adult fish, is no more significant than the association of homology with divergent preadaptations, which is of quite general occurrence among adult vertebrate types. In all such cases, we have instances of fundamentally identical structures, diverted, as it were, to entirely different purposes or functions (e.g. the arm of a man and the flipper of a whale). Hence the argument drawn from embryological homology is no more cogent than the argument drawn from the homologies of comparative anatomy, which we have already discussed in a previous chapter. The misuse of the term branchial, to prejudge matters in their own favor, is in keeping with the customary policy of evolutionists. It is intended, naturally, to convey the impression that man, in the course of his evolution, has passed through a fish-like stage. At bottom, however, it is nothing more than a verbal subterfuge, that need not detain us further.
The theory of embryological recapitulation is often applied to man, with a view to establishing the doctrine of his bestial ancestry. We have seen one instance of this application, and we shall consider one other, for the purpose of illustrating more fully the principles involved. The claim is made by evolutionists, that man must have passed through a fish or amphibian stage, because, in common with all other mammals, he exhibits, during his embryological development, a typical fish (or, if you prefer, amphibian) kidney, which subsequently atrophies, only to be replaced by the characteristic mammalian kidney. The human embryo, therefore, repeats the history of our race, which must have passed through a fish-like stage in the remote past. In consequence of this phenomenon, therefore, it is inferred that man must have had fish-like ancestors. Let us pause, however, to analyze the facts upon which this inference is based.
In annelids, like the earthworm, the nephridia or excretory tubules are arranged segmentally, one pair to each somite. In vertebrates, however, the nephridial tubules, instead of developing in regular sequence from before backwards, develop in three batches, one behind the other, the anterior batch being called the pronephros, the middle one, the mesonephros and the posterior one, the metanephros. This, according to J. Graham Kerr, holds true not only of the amniotic vertebrates (reptiles, birds, and mammals) but also, with a certain reservation, of the anamniotic vertebrates (fishes and amphibians). “In many of the lower Vertebrates,” says this author, “there is no separation between the mesonephros and metanephros, the two forming one continuous structure which acts as the functional kidney. Such a type of renal organ consisting of the series of tubules corresponding to mesonephros together with metanephros may conveniently be termed the opisthonephros.” (“Textbook of Embryology,” II—Vertebrata, p. 221.) If we accept this view, it is not quite accurate to regard the mesonephros in man as a homologue of the opisthonephros of a fish, seeing that the latter is composed not only of mesonephridia (mesonephric tubules), but also of metanephridia (metanephric tubules). A brief description of the three nephridial systems of vertebrate embryos will serve to further clarify their interrelationship.
(1) The pronephric system: This consists of a collection of tubules called the pronephros, and a pronephric duct leading to the cloaca, or terminal portion of the alimentary canal. The pronephros is a functional organ in the frog tadpole and other larval amphibia. It is also found in a few teleosts, where it is said to persist as a functional organ in the adult. In other fishes, however, and in all higher forms the pronephros atrophies and becomes reduced to a few rudiments.[17]
(2) The mesonephric system: This consists of a collection of nephridial tubules called the mesonephros (Wolffian body). The tubules of the mesonephros do not develop any duct of their own, but utilize the posterior portion of the pronephric duct, the said tubules becoming secondarily connected with this duct in a region posterior to the pronephridia (tubules of the pronephros). The pronephric tubules together with the anterior portion of the pronephric duct then atrophy, while the persisting posterior portion of this duct receives the name of mesonephric or Wolffian duct. The duct in question still terminates in the cloaca, and serves, in the male, the combined function of a urinary and spermatic duct; but, in the female, a special oviduct (the Müllerian duct) is superadded because of the large size of the eggs to be transmitted, the Wolffian or mesonephric duct subserving only the urinary function. The mesonephros is functional in mammalian embryos, but atrophies and disappears coincidently with the development of the permanent kidney. The same is true of amniotic vertebrates generally, except that in the case of reptiles the mesonephros persists for a few months after hatching in the adult, the definitive kidney of the adult being reinforced during that interval by the still functional mesonephros. In anamniotic vertebrates, however, no separation exists between the mesonephros and the metanephros, the two forming one continuous structure, the opisthonephros, which acts as the functional kidney of the adult.
(3) The metanephric system: In the amniotic vertebrates the mesonephros and metanephros are distinct, the former being functional in embryos and in adult reptiles (for a few months after hatching), while the metanephros becomes the definitive kidney of the adult. The metanephros is a collection of nephridial tubules provided with a special urinary duct called the ureter, which empties into the bladder (not the cloaca). The Wolffian or mesonephric duct is retained as a sperm duct in the male (of amniotic vertebrates), but becomes vestigial in the female. Only a certain number of the nephridial tubules of the embryonic metanephros are taken over to form part of the permanent or adult kidney (in mammals, birds, and reptiles).
If, then, as we have previously observed, we follow Kerr in regarding the fish kidney, not as a simple mesonephros, but as an opisthonephros (i.e. a combination of mesonephros and metanephros), there is no warrant for interpreting the embryonic mesonephros of man and mammals generally as the fish-kidney stage. But waiving this consideration, and assuming, for the sake of argument, that the fish kidney is a perfect homologue of the human mesonephros, the mere fact of the adoption by the human embryo of a temporary solution of its excretory problem similar to the permanent solution of that problem adopted by the fish, would not, of itself, imply the common ancestry of men and fishes. Such a coincidence would be fully explicable as a case of convergent adaptation occurring in the interest of embryonic economy.
It is, indeed, a well-known fact that larval and embryonic organisms are often obliged to defer temporarily the construction of the more complex structures of adult life, and to improvise simpler substitutes for use until such a time as they have accumulated a sufficient reserve of energy and materials to complete the work of their more elaborate adult organization. The young starfish, for example, arising as it does from an egg but scantily supplied with yolk, is forced, from the very outset, to shift for itself, in coping with the food-getting problem. Under stress of this necessity, it economizes its slender resources by constructing the extremely simple digestive and motor apparatus characteristic of the larva in its bilaterally-symmetrical Bipinnaria stage, and postponing the development of the radially-symmetrical structure characteristic of the adult stage, until it has stored up the wherewithal to complete its metamorphosis.
From this viewpoint, there is no difficulty in understanding why temporary solutions of the excretory problem should precede the definitive solution of this problem in mammalian embryos. The problem of excretion is urgent from the outset, and its demands increase with the growth of the embryo. It is only natural, then, that a series of improvised structures should be resorted to, in a case of this kind; and, since these temporary solutions of the excretory problem must, of necessity, be as simple as possible, it should not be in the least surprising to find them coinciding with the permanent solutions adopted by inferior organisms less complexly organized than the mammals. Hence the bare fact of resemblance between the transitory embryonic kidney of a mammal and the permanent adult kidney of a fish would have no atavistic significance. We know of innumerable cases in which an identical adaptation occurs in genetically unrelated organisms. The cephalopod mollusc Nautilus, for example, solves the problem of light-perception in the identical manner in which it is solved by the vertebrates. This mollusc has the perfect vertebrate type of eye, including the lens and all other parts down to the minutest detail. The fact, however, that the mollusc solves its problem by using the stereotyped solution found in vertebrates rather than by developing a compound eye analogous to the type found among arthropods, is wholly destitute of genetic significance. In fact, the genetic interpretation is positively rejected by the evolutionists, who interpret the occurrence of similar eyes in molluscs and vertebrates as an instance of “accidental convergence.” Even assuming, then, what Kerr denies, namely, a perfect parallelism between the mesonephros of the human embryo and the permanent kidney of an adult fish, the alleged fact that the human embryo temporarily adopts the same type of solution for its excretory problem as the one permanently employed by the fish would not in itself be a proof of our descent from a fish-like ancestor.
In fact, not only is embryological homology of no greater value than adult homology as an argument for evolution, but it is, on the contrary, considerably inferior to the latter, as regards cogency. Differentiation pertains to the final or adult stage of organisms. Embryonic structures, inasmuch as they are undeveloped and undifferentiated, present for that very reason an appearance of crude and superficial similarity. “Most of what is generally ascribed to the action of the so-called biogenetic law,” says T. Garbowski, “is erroneously ascribed to it, since all things that are undeveloped and incomplete must be more or less alike.” (“Morphogenetische Studien,” Jena, 1903.) When we consider the fact that the metazoa have all a similar unicellular origin, are subject to uniform morphogenetic laws, and are frequently exposed to analogous environmental conditions demanding similar adaptations, it is not at all surprising that they should present many points of resemblance (both in their embryonic and their adult morphology) which are not referable to any particular line of descent. At all events, these resemblances are far too general in their extension to enable us to specify the type of ancestor responsible therefor. More especially is this true of embryological homologies, which are practically valueless as basis for reconstructing the phylogeny of any type. “That certain phenomena,” says Oskar Hertwig, “recur with great regularity and uniformity in the development of different species of animals, is due chiefly to the fact that under all circumstances they supply the necessary condition under which alone the next higher stage in ontogeny (embryological development) can be produced.” (“Allgemeine Biologie,” 1906, p. 595.) The same author, therefore, proposes to revamp Haeckel’s “biogenetisches Grundgesetz” as follows: “We must leave out the words ‘recapitulation of forms of extinct ancestors’ and substitute for them ‘repetition of forms regularly occurring in organic development, and advancing from the simple to the more complex.’” (Op. cit., p. 593.)
Finally, when applied to the problem of man’s alleged genetic connection with the ape, the biogenetic principle proves the exact reverse of what the Darwinians desire; for as a matter of fact the young apes resemble man much more closely in the shape of the skull and facial features than do the adult animals. Inasmuch, therefore, as the ape, in its earlier development, reveals a more marked resemblance to man than is present in its later stages, it follows, according to the “biogenetic law,” that man is the ancestor of the ape. This, however, is inadmissible, seeing that the ape is by no means a more recent type than man. Consequently, as applied to man, the Haeckelian principle leads to a preposterous conclusion, and thereby manifests its worthlessness as a clue to phylogeny. Julius Kollmann, it is true, gives serious attention to this likeness between young apes and men, and makes it the basis of his scheme of human evolution. “Kollmann,” says Dwight, “starts from the fact that the head of a young ape is very much more like that of a child than the head of an old ape is like that of a man. He holds that the likeness of the skull of a very young ape is so great that there must be a family relationship. He believes that some differentiation, some favorable variation, must occur in the body of the mother and so a somewhat higher skull is transmitted to the offspring and is perpetuated. Concerning which Kohlbrugge remarks that ‘thus the first men were developed, not from the adult, but from the embryonic forms of the anthropoids whose more favorable form of skull they managed to preserve in further growth.’ ... Schwalbe makes the telling criticism of these views of Kollmann that much the same thing might be said of the heads of embryonic animals in general that is said of those of apes, and that thus mammals might be said to have come from a more man-like ancestor.” (Op. cit., pp. 186, 187.) All of which goes to show that the “biogenetic law” is more misleading than helpful in settling the question of human phylogeny.