This elaborate piece of evolutionary contortion calls for no comment here. We are only interested in the fact that this wild and weird speculation was originally inspired by the false assumption that the hypophysis was a functionless organ. As a matter of fact, it is the source of two important hormones. The one generated in its anterior lobe is tethelin, a metabolic hormone, which promotes the growth of the body in general and of the bony tissue in particular. Hypertrophy and overfunction of this gland produces giantism, or acromegaly (enlargement of hands, feet, and skull), while atrophy and underfunction of the anterior lobe results in infantilism, acromikria (diminution of extremities, i. e. hands, feet, head), obesity, and genital dystrophy (i. e. suppression of secondary sexual characters). The posterior lobe of the pituitary body constitutes, with the pars intermedia, a second endocrine gland, which generates a stimulatory hormone called pituitrin. This hormone stimulates unstriated muscle to contract, and thereby regulates the discharge of secretions from various glands of the body, e. g. the mammary glands, bladder, etc. Hence the hypophysis, far from being a useless organ, is an indispensable one. Moreover, it is an integral and important part of the cryptorhetic system.
The same story may be repeated of the thyroid glands. These consist of two lobes located on either side of the windpipe, just below the larynx (Adam’s apple), and joined together across the windpipe by a narrow band or isthmus of their own substance. Gaskell homologized them with a gland in scorpions, and Mathew says that, if his surmise is correct, “the thyroid represents an accessory sexual organ of the invertebrate.” (Op. cit., p. 654.) They are, however, endocrine glands, that generate a hormone known as thyroxin, which regulates the body-temperature, growth of the body in general, and of the nervous system in particular, etc., etc. Atrophy or extirpation of these glands causes cretinism in the young and myxoedema in adults. Without a sufficient supply of this hormone, the normal exercise of mental powers in human beings is impossible. The organ, therefore, is far from being a useless vestige of what was formerly useful.
George Howard Parker, the Zoölogist of Harvard, sums up the case against the Darwinian interpretation of the endocrine glands as follows: “The extent to which hormones control the body is only just beginning to be appreciated. For a long time anatomists have recognized in the higher animals, including man, a number of so-called ductless glands, such as the thyroid gland, the pineal gland, the hypophysis, the adrenal bodies, and so forth. These have often been passed over as unimportant functionless organs whose presence was to be explained as an inheritance from some remote ancestor. But such a conception is far from correct. If the thyroids are removed from a dog, death follows in from one to four weeks. If the adrenal bodies are excised, the animal dies in from two to three days. Such results show beyond doubt that at least some of these organs are of vital importance, and more recent studies have demonstrated that most of them produce substances which have all the properties of hormones.” (“Biology and Social Problems,” 1914, pp. 43, 44.)
Even the vermiform appendix of the cæcum, which since Darwin’s time has served as a classic example of a rudimentary organ in man, is, in reality, not a functionless organ. Darwin, however, was of opinion that it was not only useless, but positively harmful. “With respect to the alimentary canal,” he says, “I have met with an account of only a single rudiment, namely, the vermiform appendage of the cæcum. ... Not only is it useless, but it is sometimes the cause of death, of which fact I have lately heard two instances. This is due to small hard bodies, such as seeds, entering the passage and causing inflammation.” (“Descent of Man,” 2nd ed., ch. I, pp. 39, 40.) The idea that seeds cause appendicitis is, of course, an exploded superstition, the hard bodies sometimes found in the appendix being fecal concretions and not seeds—“The old idea,” says Dr. John B. Deaver, “that foreign bodies, such as grape seeds, are the cause of the disease, has been disproved.” (Encycl. Americana, vol. 2, p. 76.) What is more germane to the point at issue, however, is that Darwin erred in denying the utility of the vermiform appendix. For, although this organ does not discharge in man the important function which its homologue discharges in grain-eating birds and also in herbivorous mammals, it subserves the secondary function of lubricating the intestines by means of a secretion from its muciparous glands.
Darwin gives the semilunar fold as another instance of a vestigial organ, claiming that it is a persistent rudiment of a former third eyelid or membrana nictitans, such as we find in birds. “The nictitating membrane, or third eyelid,” he says, “with its accessory muscles and other structures, is especially well developed in birds, and is of much functional importance to them, as it can be rapidly drawn across the whole eyeball. It is found in some reptiles and amphibians, and in certain fishes as in sharks. It is fairly well developed in the two lower divisions of the mammalian series, namely, in the monotremata and marsupials, and in some higher mammals, as in the walrus. But in man, the quadrumana, and most other mammals, it exists, as is admitted by all anatomists, as a mere rudiment, called the semilunar fold.” (Op. cit., ch. I, pp. 35, 36.) Here Darwin is certainly wrong about his facts; for the so-called third eyelid is not well developed in the two lower divisions of the mammalian series (i.e. the monotremes and the marsupials) nor in any other mammalian type. “With but few exceptions,” says Remy Perrier, “the third eyelid is not so complete as among the birds; (in the mammals) it never covers the entire eye. For the rest, it is not really perceptible except in certain types, like the dog, the ruminants, and, still more so, the horse. In the rest (of the mammals) it is less developed.” (“Elements d’anatomie comparée,” Paris, 1893, p. 1137.) Moreover, Darwin’s suggestion leaves us at sea as to the ancestor, from whom our “rudimentary third eyelid” has been inherited. His mention of birds as having a well developed third eyelid is not very helpful, because all evolutionists agree in excluding the birds from our line of descent. The reptiles are more promising candidates for the position of ancestors, but, as no trace of a third eyelid could possibly be left behind in the imperfect record of the fossiliferous rocks (soft parts like this having but slight chance of preservation), we do not really know whether the palæozoic reptiles possessed this particular feature, or not. Nor can we argue from analogy and induction, because not all modern reptiles are equipped with third eyelids. Hence the particular group of palæozoic reptiles, which are supposed to have been our progenitors, may not have possessed any third eyelid to bequeath to us in the reduced and rudimentary form of the plica semilunaris. If it be replied, that they must have had this feature, because otherwise we would have no ancestor from whom we could inherit our semilunar fold, it is obvious that such argumentation assumes the very point which it ought to prove, namely: the actuality of evolution. Rudiments are supposed to be a proof for evolution, and not, vice versa, evolution a proof for rudiments.
Finally, the basic assumption of Darwin that the semilunar fold is destitute of function is incorrect; for this crescent-shaped fold situated in the inner or nasal corner of the eye of man and other mammals serves to regulate the flow of the lubricating lacrimal fluid (which we call tears). True this function is secondary compared with the more important function discharged by the nictitating membrane in birds. In the latter, the third eyelid is a pearly-white (sometimes transparent) membrane placed internal to the real eyelids, on the inner side of the eye, over whose surface it can be drawn like a curtain to shield the organ from excessive light, or irritating dust; nevertheless, the regulation of the flow of lacrimal humor is a real function, and it is therefore entirely false to speak of the semilunar fold as a functionless rudiment.
The coccyx is likewise cited by Darwin as an example of an inherited rudiment in man. “In man,” he says, “the os coccyx, together with certain other vertebræ hereafter to be described, though functionless as a tail, plainly represents this part in other vertebrate animals.” (Op. cit., ch. I, p. 42.) That it serves no purpose as a tail, may be readily admitted, but that it serves no purpose whatever, is quite another matter. As a matter of fact, it serves for the attachment of several small muscles, whose functioning would be impossible in the absence of this bone. Darwin himself concedes this; for he confesses that the four vertebræ of the coccyx “are furnished with some small muscles.” (Ibidem.) We may, therefore, admit the homology between the human coccyx and the tails of other vertebrates, without being forced to regard the latter as a useless vestigial organ. It may be objected that the attachment of these muscles might have been provided for in a manner more in harmony with our ideas of symmetry. To this we reply that Helmholtz criticized the human eye for similar reasons, when he said that he would remand to his workshop for correction an optical instrument so flawed with defects as the human eye. But, after all, it was by the use of these selfsame imperfect eyes that Helmholtz was enabled to detect the flaws of which he complained. When man shall have fully fathomed the difficulties and obstructions with which organic morphogeny has to contend in performing its wonderful work, and shall have arrived at an elementary knowledge of the general laws of morphogenetic mechanics, he will be more inclined to admire than to criticize. It is a mistake to imagine that the finite works of the Creator must be perfect from every viewpoint. It suffices that they are perfect with respect to the particular purpose which they serve, and this purpose must not be narrowly estimated from the standpoint of the created work itself, but from that of its position in the universal scheme of creation. All such partial views as the Helmholtzian one are false views.
Another consideration which Darwin and his partisans have failed to take into account is the possibility of an ontogenetic, as well as a phylogenetic, explanation of rudimentary organs. That is to say, rudimentary organs might, so far as a priori reasons are concerned, be the now useless vestiges of organs formerly developed and functional in the fœtus, and need not necessarily be interpreted as traces of organs that functioned formerly in remote racial ancestors. That there should be such things as special fœtal organs, which atrophy in later adult life, is a possibility that ought not to excite surprise. During its uterine existence, the fœtus is subject to peculiar conditions of life, very different from those which prevail in the case of adult organisms—e.g. respiration and the digestive process are suspended, and there is a totally different kind of circulation. What, then, more natural than that the fœtus should require special organs to adapt it to these special conditions of uterine life? Such organs, while useful and functional in the earlier stages of embryonic development, will, so soon as birth and maturity introduce new conditions of life, become superfluous, and therefore doomed, in the interest of organic economy, to ultimate atrophy and degeneration, until nothing is left of them but vestigial remnants.
The thymus may be cited as a probable instance of such an organ. This organ, which is located in front of the heart and behind the breastbone, in the region between the two lungs, consists, at the period of its greatest development in man, of a two-lobed structure, 5 cm. long and 4 cm. wide, with a thickness of 6 mm. and a maximum weight of 35 grams. It is supplied with numerous lymphoid cells, which are aggregated to form lymphoid follicles (cf. Gray’s “Anatomy,” 20th ed., 1918, pp. 1273, 1274; Burton-Opitz’ “Physiology,” 1920, p. 964). This organ is a transitory one, well developed at birth, but degenerating, according to some authors, after the second year of life (cf. Starling’s “Physiology,” 3rd ed., 1920, p. 1245); according to others, however, not until the period of full maturity, namely, puberty. (Cf. Gray’s “Anatomy,” loc. cit.) W. H. Howell cites both opinions, without venturing to decide the matter (cf. his “Physiology,” 8th ed., 1921, pp. 869, 870). It was at one time classified as a rudimentary or functionless organ. Later on, however, it was thought by certain observers to be an endocrine gland, yielding a secretion important for the growth of young mammals. This took it out of the class of useless vestigial organs, but the recent discovery that it is indispensable to birds as furnishing a secretion necessary for the formation of the tertiary envelopes (egg membrane and shell) of their eggs, has tended to revive the idea of its being a vestigial organ inherited from the lower vertebrates.
Thus Dr. Oscar Riddle, while admitting that the thymus gland in man has some influence on the growth of the bones, contends that the newly-discovered function of this gland in birds is much more important, since without it none of the vertebrates, excepting mammals, could reproduce their young. “It thus becomes clear,” he says, “that though the thymus is almost without use in the human being, it is in fact a sort of ‘mother of the race.’ The higher animals could not have come into existence without it. For even while our ancestors lived in the water, it was the thymus of these ancestors which made possible the production of the egg-envelopes within which the young were cradled and protected until they were ready for an independent life.” (Science, Dec. 28, 1923, Suppl. XIII, XIV.)