(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.
§ 3. Rudimentary Organs
Darwin attached great importance to the existence in man of so-called rudimentary organs, which he regarded as convincing evidence of man’s descent from the lower forms of animal life. Nineteenth century science, being ignorant of the functional purpose served by many organs, arbitrarily pronounced them to be useless organs, and chose, in consequence, to regard them all as the atrophied and (wholly or partially) functionless remnants of organs that were formerly developed and fully functional in remote ancestors of the race. Darwin borrowed this argument from Lamarck. It may be stated thus: Undeveloped and functionless organs are atrophied organs. But atrophy is the result of disuse. Now disuse presupposes former use. Consequently, rudimentary organs were at one time developed and functioning, viz. in the remote ancestors of the race. Since, therefore, these selfsame organs are developed and functional in the lower forms of life, it follows that the higher forms, in which these organs are reduced and functionless, are descended from forms similar to those in which said organs are developed and fully functional.
This argument, however, fairly bristles with assumptions that are not only wholly unwarranted, but utterly at variance with actual facts. In the first place, it wrongly assumes that all reduced organs are functionless, and, conversely, that all functionless organs are atrophied or reduced. Facts, however, prove the contrary; for we find frequent instances of reduced organs which function, and, vice versa, of well-developed organs which are functionless. The tail, for example, in cats, dogs, and certain Catarrhine monkeys, though it discharges neither the prehensile function that makes it useful in the Platyrrhine monkey, nor the protective function that makes it useful to horses and cattle in warding off flies, is, nevertheless, despite its inutility or absence of function, a quite fully developed organ. Conversely, the reduced or undeveloped fin-like wings of the penguin are by no means functionless, since they enable this bird to swim through the water with great facility.