In directing attention to the fact that a permanent and inheritable reduction of organs to the vestigial state can result from mutation, we do not, of course, intend to exclude the possible occurrence of somatic atrophy due to lack of exercise rather than to germinal change. Thus the blind species of animals in caves may, in some instances, be persistently blind, because of the persistent darkness of the environment in which they live, and not by reason of any inherited factor for blindness. Darwin gives one such instance, namely, that of the cave rat Neotoma. To test such cases, the blind animals would have to be bred in an illuminated environment. If, under this condition, they failed to develop normal eyes, the blindness would be due to a germinal factor, and would be inherited in an illumined, no less than a dark, environment.
In any case, a mutation which suppresses a character is not, as we have seen, a specific change, but merely one of the varietal order, which does not result in the production of a genuine new species. The factorial mutant with a vestigial wing or eye belongs to the same species as its wild or normal parent stock. Moreover, neither disuse nor natural selection has the slightest power to induce mutations of this kind. If mutation be the cause of the blindness of cave animals, then their presence in such caves must be accounted for by supposing that they migrated thither because they found in the cave a most suitable environment for safety, foraging, etc. Darkness alone, however, could never induce germinal, but, at most, merely somatic blindness. The Lamarckian factor of disuse and the Darwinian factor of selection have been definitely discredited as agents which could bring about hereditarily-transmissible modifications.
§ 4. Fossil Links
All efforts, then, to establish, by means of anatomical and embryological homologies, the lineal descent of man from any known type of monkey or ape have ended in ignominious failure. Comparative anatomy and embryology can, at most, only furnish grounds for extremely vague and indefinite speculations regarding the descent of man, but they can never become a basis for specific conclusions with respect to the phylogeny of Homo sapiens. Every known form of ape, whether extant or extinct, is, as we have seen, far too specialized in its adaptation to arboreal life to pass muster as a feasible ancestor. The only conceivable manner in which the human body could be related to simian stock is by way of collateral descent, and the only means of proving such descent is to adduce a series of intermediate fossil types connecting modern men and modern apes with this alleged common ancestor of both. “The ascent (sic) of man as one of the Primates,” says Henry Fairfield Osborn, “was parallel with that of the families of apes. Man has a long line of ancestry of his own, perhaps two million or more years in length. He is not descended from any known form of ape either living or fossil.” (The Ill. London News, Jan. 8, 1921, p. 40.)
This theory of a hypothetical primate ancestor of man, which is supposed to have inhabited the earth during the earlier part of the Tertiary period, and to have presented a more man-like appearance than any known type of ape, was first propounded by Karl Snell in 1863. It was popularized at the beginning of the present century by Klaatsch, who saw in it a means of escape from the absurdities and perplexities of the theory of lineal descent—“the less,” says the latter, “an ape has changed from its original form, just so much the more human it appears.” This saying is revamped by Kohlbrugge to read: “Man comes from an original form much more like himself than any existing ape.” Kohlbrugge’s comment is as follows: “The line of descent of man thus receives on the side of the primates a quite different form from its previous-one. Such new hypotheses as those of Hubrecht and Klaatsch seem, therefore, fortunate for nature-philosophers, because evolution always failed us when we compared known forms in their details, and led us only to confusion. But if one works with such distant hypothetical ancestors, one escapes much disillusioning.” (Quoted by Dwight, op. cit., p. 195.)
One thing, at any rate, is certain, namely: that we do not possess any fossils of this primitive “large brained, erectly walking primate,” who is alleged to have roamed the earth during the eocene or oligocene epoch. The Foxhall Man, whose culture Osborn ascribes to the Upper Pliocene, is far too recent, and, what is worse, far too intelligent, to be this Tertiary Ancestor. The Pithecanthropus erectus, likewise, is excluded for reasons which we shall presently consider. Meanwhile, let it be noted, that we have Osborn’s assurance for the fact that we are descended from a brainy and upright oligocene ancestor, as yet, however, undiscovered.
But the situation is more hopeful, if we hark back to a still more remote period, whose remains are so scarce and fragmentary, as to eliminate the possibility of embarrassment arising from intractable details. “Back of this,” says Osborn, “ ... was a prehuman arboreal stage.” (Loc. cit.) Here, then, we are back again in the same old rut of tree-climbing simian ancestry, whence we thought to have escaped by abandoning the theory of lineal descent; and, before we have time to speculate upon how we got there, Prof. Wm. Gregory of the American Museum is summoned by Osborn to present us with specimens of this prehuman arboreal stage. This expert, it would seem, favored up till the year 1923 the fossil jaw of the Propliopithecus as representing the common root, whence the human race diverged, on one side, and the races of anthropoid apes, on the other. (Cf. Osborn’s Museum-leaflet of 1923 on “The Hall of the Age of Man,” p. 29.) On April 14, 1923, however, Gregory announced the deposition of Propliopithecus and the enthronement of the jaw of Dryopithecus. This sudden accession of Dryopithecus to the post of common ancestor of apes and men was due to the discovery by Dr. Barnum Brown of three fossil jaws of Dryopithecus in the Miocene deposits of the Siwalik beds in northern India. By some rapturous coincidence, the three jaws in question happen to come from three successive “horizons,” and to be representative of just three different stages in the evolution of Dryopithecus. Doctor Gregory finds, moreover, that the patterns of the minute cracks and furrows on the surviving molar teeth correspond to those on the surface of the enamels of modern ape and human teeth. Hence, with that ephemeral infallibility, which is characteristic of authorities like Doctor Gregory, and which is proof against all discouragement by reason of past blunders, the one who told us but a year ago that the cusps of all the teeth of Propliopithecus “are exactly such as would be expected in the common starting point for the divergent lines leading to the gibbons, to the higher apes, and to man” (loc. cit.), now tells us that both we and the apes have inherited our teeth from Dryopithecus, who had heretofore remained neglected on the side-lines. In 1923, apparently, Dr. Gregory was unimpressed with the crown patterns of Dryopithecus, whose jaw he then excluded from the direct human line. (Cf. Museum-leaflet, p. 5.) Now, however, that the new discoveries have brought Dryopithecus into the limelight, and, particularly because these jaws were found in Miocene deposits, Gregory has shifted his favor from Propliopithecus to Dryopithecus. (Cf. Science, April 25, 1924, suppl. XIII.)
When palæontologists are obliged to do a volte face of this sort, one ought not to scoff. One ought to be an optimist, and eschew above all the spirit of the English statesman, who, on hearing a learned lecture by Pearson on the question of whether Man was descended from hylobatic, or troglodytic stock, was guilty of the following piece of impatience: “I am not particularly interested in the descent of man ... this scientific pursuit of the dead bones of the past does not seem to me a very useful way of spending life. I am accustomed to this mode of study; learned volumes have been written in Sanscrit to explain the conjunction of the two vowels ‘a’ and ‘u’. It is very learned, very ingenious, but not very helpful.... I am not concerned with my genealogy so much as with my future. Our intellects can be more advantageously employed than in finding our diversity from the ape.... There may be no spirit, no soul; there is no proof of their existence. If that is so, let us do away with shams and live like animals. If, on the other hand, there is a soul to be looked after, let us all strain our nerves to the task; there is no use in digging into the sands of time for the skeletons of the past; build your man for the future.” (Smithson. Inst. Rpt. for 1921, pp. 432, 433.) It is to be hoped, however, that this reactionary spirit is confined to the few, and that the accession of this new primitive ancestor will be hailed with general satisfaction. At any rate, we can wish him well, and trust that the fossilized jaw of Dryopithecus will not lose caste so speedily as that of Propliopithecus.
Propliopithecus, or Dryopithecus? Hylobatic, or troglodytic affinities? Such questions are scarcely the pivots on which the world is turned! Nevertheless, we rejoice that Doctor Gregory has again settled the former problem (provisorily, at least) to his own satisfaction. More important, however, than that of the dentition of Dryopithecus, is the crucial question of whether or not Palæontology is able to furnish evidence of man’s genetic continuity with this primitive pithecoid root. Certainly, no effort has been spared to procure the much desired proofs of our reputed bestial ancestry. The Tertiary deposits of Europe, Asia, Africa, America, and the oceanic islands have been diligently ransacked for fossil facts that would be susceptible to an evolutionary interpretation. The aprioristic criterion that all large-brained men are recent, and all small-brained men with recessive chins are necessarily ancient, has always been employed in evaluating the fossil evidence. Notwithstanding all endeavors, however, to bring about the consummation so devoutly desired, the facts discovered not only fail to support the theory of collateral descent, but actually militate against it. For assuming that man and the anthropoid apes constitute two distinct lines of evolution branching out from common Tertiary or pre-Tertiary stock, palæontology should be able to show numerous intermediate fossil forms, not alone for the lateral branch of the apes, but also, and especially, for the lateral line connecting modern men with the common root of the primate tree. But it is precisely in this latter respect that the fossil evidence for collateral descent fails most egregiously. Palæontology knows of many fossil genera and species of apes and lemurs, that might conceivably represent links in a genetic chain connecting modern monkeys with Tertiary stock, but it has yet to discover so much as a single fossil species, much less a fossil genus, intermediate between man, as we know him, and this alleged Tertiary ancestor common to apes and men.
Not even catastrophism can be invoked to save this irremediable situation; for any catastrophe that would have swept away the human links would likewise have swept away the ape links. The presence of many genera and species of fossil apes, in contrast to the absence of any fossil genus or species of man distinct from Homo sapiens, is irreconcilable with the theory of collateral descent. Such is the dilemma proposed to the upholders of this theory by Wasmann, in the 10th chapter of his “Die Moderne Biologie” (3rd edition, 1906), a dilemma, from which, as we shall see, their every attempt to extricate themselves has failed most signally.