This conclusion, however, is far too hasty. For, even if we disregard as negligible the minor function, that Riddle assigns to the thymus in man, there remains another possibility, which H. H. Wilder takes into account, namely, that the thymus may, in certain cases, be a temporary substitute for the lymphatic vessels. Having called attention to certain determinate channels found in some of the lower vertebrates, he tells us that these “can well be utilized as adjuncts of the lymphatic system until their function can be supplied by definite lymphatic vessels.” He then resumes his discussion of the lymph nodules in mammals as follows: “Aside from the solitary and aggregated nodules, both of which appear to be centers of origin of lymphocytes, there are numerous other places in which the cellular constituents of the blood are developed. Many of these, as in the case of the aggregated nodules of the intestines, are developed within the wall of the alimentary canal and are therefore endodermic in origin. These include the tonsils, the thymus, and thyroid glands, the associated epithelial bodies, and, perhaps, the spleen.... In their function as formative nidi for the cellular elements of the blood these organs form physiologically important auxiliaries to the vascular system as a whole, but belong elsewhere in their anatomical developmental affinities.” (“History of the Human Body,” 2nd ed., 1923, p. 395—italics mine.)

This being the case, it is much more reasonable to interpret the thymus as an ontogenetic (embryonic), rather than a phylogenetic (racial) rudiment. It has been observed that, in the case of reptiles which lack definite lymphatic glands (which function in man as formative centers of lymphocytes or white blood corpuscles), the thymus is extraordinarily developed and abounds in lymphoid cells. It has also been observed that the formation of lymphocytes in the lymphatic glands is regulated by the digestive process; for, after digestion, the activity of these glands increases and the formation of leucocytes is accelerated. Since, then, the lymphatic glands appear to require the stimulus of the digestive process to incite them to action, it is clear that in the fœtus, which lacks the digestive process, the lymphatic glands will not be stimulated to action, and that the task of furnishing lymphocytes will devolve upon the thymus. After birth, the digestive process commences and the lymphatic glands become active in response to this stimulus. As the function of forming lymphocytes is transferred from the thymus to the lymphatic glands, the former is gradually deprived of its importance, and, in the interest of organic economy, it begins to atrophy, until, at the end of the child’s second year, or, at latest, when the child has reached sexual maturity, nothing but a reduced vestige remains of this once functional organ. “The thymus,” says Starling, “forms two large masses in the anterior mediastinum which in man grow up to the second year of life and then rapidly diminish, so that only traces are to be found at puberty. It contains a large amount of lymphatic tissue and is therefore often associated with the lymphatic glands as the seat of the formation of lymph corpuscles.... In certain cases of arrested development or of general weakness in young people, the thymus has been found to be persistent.” (“Physiology,” 3rd ed., 1920, p. 1245.)

In the light of these facts, it is utterly unreasonable to regard the thymus as a practically useless rudiment inherited from the lower vertebrates. “That they have an important function in the young animal,” says Albert Mathews, “can hardly be doubted.” (“Physiological Chemistry,” 1916, p. 675.) In fact, the peculiar nature of their development in the young and their atrophy in the adult forces such a conclusion upon us. The thymus, therefore, is, in all probability, an ontogenetic, and not a phylogenetic, rudiment. It might conceivably be exploited as a biogenetic recapitulation of a reptilian stage in man, just as the so-called fish-kidney of the human embryo is exploited for evolutionary interpretation. The principles by which such a view may be refuted have been given previously. But, in any case, it is folly to interpret the thymus as a rudiment in the racial, rather than embryonic sense. Moreover, the possibility of an ontogenetic interpretation of rudiments must not be restricted to the thymus, but must be accepted as a general and legitimate alternative for the phylogenetic interpretation.

In the last place, it remains for us to consider the Darwinian argument, based upon so-called rudimentary organs, from the standpoint of the science of genetics. Darwin, as we have remarked elsewhere, was ignorant of the non-inheritability of those inconstant individual variations now known as fluctuations. He was somewhat perplexed, when Professor L. Meyer pointed out the extreme variability in position of the “projecting point” on the margin of the human ear, but he still clung to his original contention that this “blunt point” was a surviving vestige of the apex of the pointed ears found in donkeys and horses, etc. “Nevertheless,” he says, “in some cases my original view, that the points are vestiges of the tips of formerly erect and pointed ears, still seems to be probable.” (“Descent of Man,” 2nd ed., ch. I, p. 34.) Darwin, as Ranke points out, was mistaken in homologizing his famous “tubercule” with the apex of bestial ears. “The acute extremity of the pointed animal ear,” says this author, “does not correspond to this prominence designated by Darwin, but to the vertex of the helix.” (“Der Mensch,” II, p. 39.) The feature in question is, moreover, a mere fluctuation due to the degree of development attained by the cartilage: hence its variability in different human beings. In very extreme cases, fluctuations of this sort, may be important enough to constitute an anomaly, and, as anomalies are often interpreted as atavisms and reversions to a primitive type, it may be well to advert to this subject here.

Dwight has an excellent chapter on anatomical variations and anomalies. (Cf. “Thoughts of a Catholic Anatomist,” 1911, ch. IX.) He tells us that “a thigh bone a little more bent, an ear a little more pointed, a nose a little more projecting ... a little more or a little less of anything you please—this is variation.” “An anatomical anomaly,” he says, “is some peculiarity of any part of the body which cannot be expressed in terms of more or less, but is distinctly new.” He divides the latter into two classes, namely: those which consist in the repetition of one or more elements in a series, e.g. the occurrence of supernumerary legs in an insect, and those which consist in the suppression of one or more elements in a series, e.g. the occurrence of eleven pairs of ribs in a man. Variations and anomalies are fluctuational or mutational, according as they are based on changes in the soma alone, or on changes in the germ plasm. Variations, however, are more likely to be non-inheritable fluctuations, and anomalies to be inheritable mutations. We shall speak of the latter presently. In the meantime we may note that the main trouble with interpreting these anatomical irregularities as “reversive” or “atavistic” is that they would connect man with all sorts of quite impossible lines of descent. “In my early days of anatomy,” says Dwight, “I thought that I must be very ignorant, because I could not understand how the occasional appearance in man of a peculiarity of some animal outside of any conceivable line of descent could be called a reversion, as it soon became the custom to call it.... It was only later that I grasped the fact that the reason I could not understand these things was that there was nothing to understand. It was sham science from beginning to end.” (Op. cit., p. 209.) By way of anomaly, almost any human peculiarity can occur in animals, and, conversely, any bestial peculiarity in man, but the resemblance to man of an animal outside of the alleged line of human descent represents a grave difficulty for the theory of evolution, and not an argument in its favor.

The human body is certainly not a mosaic of heterogenetic organs, i.e. a complex of structures inherited from any and every sort of animal, whether extant or extinct; for such a vast number and variety of ancestors could not possibly have coöperated to produce man. Prof. D. Carazzi, in his Address of Inauguration in the Chair of Zoölogy and Comparative Anatomy at the University of Padua, Jan. 20, 1906, excoriated with scathing irony the sham Darwinian science, of which Dwight complains. “But even in the serious works of pure science,” says the Italian zoölogist, “we read, for example, that the over-development of the postauricular muscles sometimes observed in man is an atavistic reminiscence of the muscles of the helix of the ear of the horse and the ass. And so far so good, because it gives evidence of great modesty in recognizing as our ancestors those well-deserving and long-eared quadrupeds. But this is not all; there appear at times in a woman one or more anomalous mammary glands below the pectoral ones; and here, too, they insist on explaining the anomaly as a reversion to type, that is, as an atavistic reminiscence of the numerous mammary glands possessed by different lower mammals; the bitch, for example....

“But the supernumerary mammary glands are not a reversion to type; anomalous mammary glands may appear upon the median line, upon the deltoid, and even upon the knee, regions far-distant from the ‘milk-line.’ So with regard to the postauricular muscles we must say that according to the laws of Darwinism the cases of anomalous development are not interpretable as reversions to type. All these features are not phylogenetic reminiscences, but anomalies of development, of such a nature that, if we should wish to make use of them for establishing the line of human descent, we would have to say that man descends from the swine, from the solipeds and even from the cetaceans, returning, namely, to the old conception of lineal descent, that is, to Buffon’s idea of the concatenation of creatures.” (“Teorie e critiche nella moderna biologia,” 1906.)

Darwin’s doctrine, however, on the origin and significance of rudimentary organs has been damaged by genetic analysis in a yet more serious fashion. In fact, with the discovery that anomalous suppression and anomalous duplication of organs may result from factorial mutation, this Darwinian conception received what is tantamount to its deathblow. Darwin, it will be remembered, was convinced that the regression of organs was brought about by “increased disuse controlled by natural selection.” (Cf. “Origin of Species,” 6th ed., ch. V.) Such phenomena, he thought, as the suppression of wings in the Apteryx and the reduction of wings in running birds, arose from their “inhabiting ocean islands,” where they “have not been exposed to the attacks of beasts, and consequently lost the power of using their wings for flight.” (“Descent of Man,” 6th ed., ch. I, p. 32.) In some cases, he believed that disuse and natural selection had coöperated ex aequo to produce results of this nature, e.g. the reduction of the eyes in the mole and in Ctenomys; for this reduction, he claims, has some selection-value, inasmuch as reduction of the eyes, adhesion of the lids, and covering with hair tends to protect the unused and useless eye against inflammation. In other cases, however, he is inclined to discount the idea that suppression of organs is an “effect of long-continued disuse,” and to regard the phenomenon as “wholly, or mainly, due to natural selection,” e.g. in the case of the wingless beetles of the island of Madeira. “For during successive generations,” he reasons, “each individual beetle which flew least, either from its wings having been ever so little less developed or from indolent habit, will have had the best chance of surviving from not being blown out to sea; and, on the other hand, those beetles which most readily took to flight would oftenest have been blown to sea, and thus destroyed.” In a third class of instances, however, he assigns the principal rôle to disuse, e.g. in the case of the blind animals “which inhabit the caves of Carniola and Kentucky, because,” as he tells us, “it is difficult to imagine that eyes, though useless, could be injurious to animals living in darkness.” Hence he concludes that, as the obliteration of eyes has no selection-value, under the circumstances prevailing in dark caves, “their loss may be attributed to disuse.” (Cf. “Origin of Species,” 6th ed., ch. V, pp. 128-133.)

Morgan’s comment on these elaborate speculations of Darwin is very caustic and concise. Referring to factorial mutations, which give rise to races of flies having supernumerary and vestigial organs, he says: “In contrast to the last case, where a character is doubled, is the next one in which the eyes are lost. This change took place at a single step. All the flies of this stock, however, cannot be said to be eyeless, since many of them show pieces of eye—indeed the variation is so wide that the eye may even appear like a normal eye unless carefully examined. Formerly we were taught that eyeless animals arose in caves. This case shows that they may also arise suddenly in glass milk bottles, by a change in a single factor.

“I may recall in this connection that wingless flies also arose in our cultures by a single mutation. We used to be told that wingless insects occurred on desert islands because those insects that had the best developed wings were blown out to sea. Whether this is true or not, I will not pretend to say, but at any rate wingless insects may also arise, not through a slow process of elimination, but at a single step.” (“A Critique of the Theory of Evolution,” 1916, pp. 66, 67.)