Popular Science Monthly/Volume 77/September 1910/Vertebrate Paleontology and the Evidences for Recapitulation
|VERTEBRATE PALEONTOLOGY AND THE EVIDENCES FOR RECAPITULATION|
By L. HUSSAKOF
AMERICAN MUSEUM OF NATURAL HISTORY
AFTER the careful papers of Professors Loomis and Lull in which the doctrine of recapitulation was so fully set forth from the standpoint of vertebrate paleontology, I can perhaps do no better than devote part of the time allotted me to showing how certain leading vertebrate paleontologists have viewed this question. Then I will cite one or two illustrations of this principle drawn from among the lower vertebrates.
Passing over the period of pre-Darwinian paleontology—the paleontology of Cuvier, Owen and Louis Agassiz—we come to the time of Leidy, who, as Professor Osborn has recently shown, was one of the first, if not the first, to bring the fruits of paleontology to the support of evolution. But Leidy, as far as a hasty search through his writings could reveal, nowhere expressly advocated the doctrine of recapitulation. Indeed, he gave but little attention to the philosophical bearings of paleontology, generally partly because of temperament and partly because in those pioneer days material to serve as a basis for generalization was still scanty.
Gaudry, one of the first European paleontologists to champion the cause of evolution, likewise did not specially advocate the doctrine of recapitulation. An examination of his "Philosophic Paleontologique" fails to reveal any definite belief in this doctrine.
Huxley, as far as I can gather from his papers and essays, believed in this doctrine, though with certain implied reservations as to its general applicability. In his presidential address to the Geological Society of London on "Paleontology and the Doctrine of Evolution" delivered in 1870, we find some interesting comment on the significance of the splints of the living horse, which he regards as indicative of the presence of three complete digits in the horse ancestor. But Huxley was never an out-and-out advocate of the biogenetic law.
Cope and Marsh, as we all know, were staunch upholders of evolution; and Cope, at least, was also a staunch upholder of the doctrine of recapitulation. In his "Primary Factors of Organic Evolution," his last contribution to philosophical paleontology, he devotes considerable space to proving this doctrine. He says:
The representatives of each class passed through the stages which are permanent in the classes below them in the series.
And he backs up this proposition with evidence derived from the ontogeny and phylogeny of batrachia, the antlers of deer and the blood trunks of vertebrates generally. For all that, Cope recognized the justice of certain criticisms which had been brought against the doctrine of recapitulation and urged caution in its application.
An example or two of recapitulation may now be cited from the field of the lower vertebrates.
The mode of development of the teeth in Neoceratodus has sometimes been adduced as an illustration of recapitulation. It is well known that the Devonic dipnoans (e. g., Dipterus) had teeth composed of rows of denticles, those in each row being more or lesls fused at their bases. During the history of the dipnoans since the Devonic period, the separate denticles have merged more and more until in Ceratodus and the living Neoceratodus, the rows of denticles are, in the adult, replaced by almost smooth ridges. Now, Semon in his beautiful studies on the development of Neoceratodus has shown that the teeth of this fish at one stage in ontogeny, are represented by rows of denticles even more discrete than the denticles in the Devonic Dipterus; then the denticles gradually merge at their bases, the separate cusps, however, still showing—a stage comparable with the Carboniferous Ctenodus; then they merge still more and assume the ridge-like form seen in the adult Neoceratodus.
Another example: In many sharks the alimentary canal is longer in the embryo than in the adult, the anal opening being situated near the posterior end of the trunk. From such cases one is inclined to believe that in the ancestral sharks this must have been the condition in the adult form; that is to say, the anal opening probably was near the posterior termination of the trunk. We may therefore ask: are there any early fossil sharks which show such a condition? Recently Professor Dean has described a remarkable specimen of Cladoselache from the Upper Devonic of Ohio which seems to indicate such a condition. In this specimen remnants of both kidneys are preserved. They extend in the posterior half of the fish and by their direction indicate that they were drawn together, toward their external opening, not far from the posterior termination of the trunk. This shows that the anal opening in this ancestral shark was very much as in the early shark embryo to-day.
In conclusion perhaps I may venture to make one other point in regard to this question. A vast amount of skepticism concerning the doctrine of recapitulation is to be found in the literature of to-day; and if we study the reasons for this skepticism we find that it is in some measure justified. It is clearly established that among vertebrates as well as among invertebrates there are many examples of structures appearing during embryonic growth which are identical with structures found in the adult of some remote ancestor. But when we reflect on the amount of adaptation which any embryo has undergone in its long evolutional history; when we remember how palingenetic characters are on every hand overlaid by cenogenetic ones; who will say that recapitulation is a principle of general application, or that it is safe to draw conclusions from all embryos concerning their long extinct ancestors? Who will believe that a bony fish which runs through its embryonic development in a few days repeats its ancestral history, when we see at every stage of its ontogeny how it has been adaptively modified for this and for that special need? Only when series of related forms have certain onto genetic stages in common are we justified in inferring that their racial ancestor may have had such characters in the adult state. But it should never be lost sight of that this inference is only a provisional hypothesis which may or may not be verified when the paleontologic record is more complete. It is no surprise that the efforts of some earnest paleontologists have been discredited in some quarters, especially among zoologists. Some of them have invoked recapitulation as a sort of magic spell by which they can conjure up ancestral forms from almost any embryonic series, forgetting the limitations of this doctrine. As far as the attitude of vertebrate paleontologists is concerned, their view has been aptly summarized by Professor Charles Depéret in his book "Les Transformations du Monde Animal" and I can do no better than close with a quotation from him:
If we appeal to paleontology, it must be recognized that this hypothesis [recapitulation] is by no means verified. There do exist here and there certain fossil genera, which all their lives have retained certain youthful characteristics apparent in their living descendants; but when it comes to reconstructing whole series chronologically continuous, grave contradictions are met with, and it is only in the groups of the mammals and perhaps of the reptiles [and, we may add, fishes] that it becomes possible to present a few examples sufficiently demonstrative.
- In his address on "Darwin and Paleontology" printed in "Fifty Years of Darwinism." Centennial addresses in honor of Charles Darwin, New York, 1909, p. 209.
- According to a letter from Darwin to Gaudry dated January 21, 1868. "The Life and Letters of Charles Darwin," edited by his son Francis Darwin, New York, 1899, Vol. II., p. 269.
- "Primary Factors of Organic Evolution," Chicago, 1890, p. 195.
- "Die Zahnentwickelung des Ceratodus forsteri," "Zool. Forsch. in Austral. u. Malay. Archipel.," 1899, pp. 115-135, pis. xviii-xx.
- Mem. Amer. Mus. Nat. Hist., Vol. IX. . p. 232.
- "Les Transformations du Monde Animal," Paris, 11)07, p. 117.