Popular Science Monthly/Volume 77/September 1910/Paleontology and the Recapitulation Theory
|PALEONTOLOGY AND THE RECAPITULATION THEORY|
BATHER once said that "If the embryologists had not forestalled them, the paleontologists would have had to invent the theory of recapitulation." This may be considered as a fair sample of the attitude of at least the Hyatt school of paleontologists toward the theory. It is doubtful if any paleontologist could be found who wholly rejects it. In violent contrast with the more or less complete acceptance of the theory by paleontologists, is the attitude of many embryologists and zoologists. Montgomery and Hurst have perhaps put the case against recapitulation more strongly than any one else. The former says, for example,
Hurst is even more emphatic. He says:
It would seem as though two statements could scarcely be more flatly contradictory than those of Bather and Hurst, just quoted. Nevertheless, I venture to make the assertion that both parties to the recapitulation controversy are right, for the simple reason that they are not talking about the same thing. Grabau has called attention to this, by implication, in one of his papers on gastropods. He states that the recapitulation theory has been placed in an evil light by the habit of embryologists of comparing embryonic stages with the adults of existing representatives of primitive types, and that they have commonly neglected to compare the epembryonic stages with the adults of geologically older species. In other words, paleontologists have usually dealt, in their comparisons, with epembryonic stages, and embryologists with embryonic stages.
There arises here a question of definition: does the biogenetic law mean that the ontogeny is a recapitulation of the ph}dogeny, or does it mean that the embryogeny is a recapitulation of the phylogeny? If we take the general consensus of opinion, w T e shall find for the former definition; and if we take the words of Haeckel, whose statement of the law is the one usually quoted, we shall again find for the former definition.
It is certainly true, at any rate, that the epembryonic stages may and do show recapitulation, even when the embryonic stages do not, or when the embryogeny is so obscured by secondary adaptations as to be untrustworthy. There are many reasons why adaptations should occur in infra-uterine or larval life to obscure the ancestral record. These have often been stated and discussed, and I shall pass them with this mere mention. That the record of remote ancestors, contained in the embryogeny, may be lost or obscured, while the record of nearer ancestors, contained in the epembryogeny, is still clear and convincing, is my contention; and I hold that this contention is substantiated by the studies of a host of paleobiologists.
While contrasting the views of biologists and paleobiologists, I do not wish to create the impression that all of the former have turned against the theory of recapitulation. Several recent studies of the development of extant forms seem to afford very satisfactory evidence that the theory is not wholly rejected in the house of its fathers. Of these I may mention the very interesting papers by Griggs on juvenile kelps, Zeleny on the development and regeneration of serpulids, and Eigenmann on the blind vertebrates of North America.
Griggs especially criticizes the views of such critics of recapitulation as His, who holds that the reason why ontogeny seems to recapitulate phylogeny is because the developing organism must from physiological necessity pass from less to more complex stages, more or less resembling ancestral forms; and the views of Morgan, who holds that only embryonic stages of ancestors are repeated. This is the so-called "Repetition Theory." To both of these critics Griggs objects that they confuse physiology and morphology. "The recapitulation theory," he says, "has nothing to do with physiology; it is purely a matter of morphology."
On the first point, that the developmental stages are merely the physiologically necessary steps in the development of the adult organism, the conclusions of Eigenmann and Zeleny are of especial interest. Eigenmann shows that in the blind fish, Amblyopsis, the development of the foundations of the eye is normal, and is phylogenic, while the stages beyond the foundations are direct. Zeleny concludes that the ontogenesis of the opercula of serpulids is phylogenic, and recapitulates ancestral characters; but the regeneratory development of the organ is direct, and may be very different from the ontogenetic development. We may ask, therefore, if development takes a certain course only because that is the physiologically necessary way in which the individual or the organ must develop, why should a condition of perfect blindness, with almost total loss of all the eye structures, be attained only by the round-about method of first developing the foundations of a normal eye? Why, again, if there is any physiologically necessary course of development, should the serpulid be able to regenerate the opercula in a manner entirely different from their ontogenesis?
Hatschek, Hurst, Montgomery and others maintain that, if two individuals differ in the adult, they must also differ in the egg, and consequently must be different at all stages between. From this thesis they draw the conclusion that organisms can not recapitulate adult ancestral characters, because any change in the adult stage of an individual, causing it to be different from its parents, involves a change in the entire ontogeny—"the entire row of cells" from the egg to the adult. That there is some sort of change in the entire row of cells we grant; but that this change necessarily affects the morphology of the individual or of its organs, up to the adult stage, we do not grant. We have here again a confusion of morphology and physiology. The cell energies may indeed be changed; but unless a change in the cell energies inevitably necessitates a change in the morphology of all the cells or of all the organs which they compose, the argument of Montgomery proves nothing.
If inheritance were perfect, the individual would take exactly the same course in development as its ancestors. That it does not do this in all cases is a more remarkable fact than that in so many cases it follows the ancestral mode of development so closely. This loss of inheritance is clue to a progressive condensation of ontogeny, or as it is commonly called, acceleration. Most embryologists misconceive the law of acceleration, limiting it to the omission of characters or stages. With the classic formulation of the law by Hyatt we are all familiar. According to Hyatt, acceleration involves not only omission, but condensation without omission, through the earlier inheritance of characters acquired in the adult or adolescent stages of life. By the unequal acceleration of characters an overlapping, or telescoping, as Grabau calls it, may be introduced. It follows, therefore, that acceleration may be by elimination, by condensation without change in the order of appearance of characters, and by condensation with change in the order of appearance, or telescoping. As conceived by the paleobiologist, the law of acceleration is an explanation of recapitulation, as well as an explanation of the failure to recapitulate.
Another factor in inheritance is retardation, so named by Cope. By the operation of this law, characters that appear late in the ontogeny may disappear in the descendents, because development terminates before the given characters are reached. In this way the ontogeny may be shortened and simplified, and many ancestral characters may be lost entirely. The result of the continued operation of retardation is retrogression, since the loss of the characters of nearer ancestors, with the continued repetition in early ontogeny of the characters of remote ancestors, must eventually cause the species to resemble the remote, rather than the nearer, ancestors.
Of the numerous cases adduced by paleontologists, in which there is clear evidence of recapitulation, I shall mention a few only.
Probably the best known examples of recapitulation are those made known by the researches of Hyatt, Branco, Würtenburger, Buckman, Smith and others among the Cephalopoda. It is shown that Ammonites pass through a goniatite stage, and that, as phrased by the nepionic stage, before the whorls come into contact. It has been carried back in the ontogeny by acceleration. Smith concludes from a study of the development and phylogeny of Placenticeras, an Upper Cretaceous ammonoid, that "the development of Placenticeras shows that it is possible, in spite of dogmatic assertions to the contrary, to decipher the race history of an animal in its individual ontogeny.", "The inner whorls of an ammonite constantly resemble in form, ornament and suture line the adult condition of some previously existing genus or other." The nautilus grows at first straight or orthoceraform, then arched or cyrtoceraform, and finally at the close of the first volution of the shell, becomes close coiled. The impressed zone appears in ancient nautiloidea in the neanic stage, where the whorls first come into contact, and is indeed a result of contact. In modern nautilus, and in Mesozoic and Tertiary nautilus the impressed zone appears in
Among the Gastropoda, Grabau and Burnett Smith have pointed out numerous beautiful cases of recapitulation. In Fusus and its allies, the higher forms quite constantly resemble in their earlier stages the adults of ancestral forms. Even in profoundly modified gerontic types, the young resemble the ancestors. Smith has brought to light in Athleta (Volutilithes) of the Eocene, an almost perfect example of even and regular acceleration, with its correlative, the recapitulation in the young of the Upper Eocene forms of the adult characters of the Lower Eocene forms. The stages passed through by this group of shells are, beginning with the earliest, a smooth, curved rib, cancellated, spiny and sometimes a senile stage. In the ancestral species (A. limopsis) the curved rib stage comes in at the close of the fourth whorl, whereas in the Upper Eocene form (A. petrosa), this stage comes in at the beginning of the third whorl.
Among the Pelecypoda the classic researches of Jackson are familiar to all. He shows that the modern Pecten passes through, in its ontogeny, a series of stages resembling adult Rhombopteria, Pterinopecten and Aviculopecten, and that the geologic order of these genera is the same as the ontogenetic order in Pecten. In such monomyarian genera as Ostrca, the initial shell, or prodissoconch, is dimyarian, and resembles the primitive Nucula. Again, in various more or less widely separated genera, the condition of complete cemented fixation has produced the ostreaform shape. Each one of these genera, however, except where the modification of shape due to fixation appears very early in ontogeny, recapitulates the adult characters of its respective ancestor. The examples of this are Mulleria, a member of the Unionidæ—like Anodon in the yo ung; Einnites, a member of the Pectinacea—like Pecten in the young; Spondylus, another member of the Pectinacea—like Pecten in the young.
Beecher's various studies of the Brachiopoda not only brought out the fact that the initial shell or protegulum of the brachiopod is remarkably similar to the most primitive known Lower Cambrian brachiopods, but have supplied in addition numerous other remarkable examples of recapitulation. One of the most striking of these is the case of the Terebratellidæ. In both the boreal and austral subfamilies a very complete series of genera correspond to the ontogenetic stages of the terminal or highest genera. Another interesting case is that of Orbiculoidea. This discoid shell has at first a straight hinge like Iphidea. It next resembles Obolella, then at a later stage it is like Schizomania, and finally adult growth brings in the characters of Orbiculoidea. Raymond has shown the remarkable similarity of the neanic stage of Spirifer mueronatus to the adult S. crispus of the Niagara. Shinier and Grabau found in the upper Hamilton of Thedford, Ontario, a variety of Spirifer mueronatus that is very mucronate in the young and not at all so in the adult. The derivation of this form from S. mueronatus is beyond question. I have pointed out a precisely similar case in Platystrophia acutilirata var. senex. This variety, which occurs in the upper Whitewater beds of Indiana and Ohio, has a hinge angle of nearly 90° in the adult. In the young, however, the outlines of the shell are exactly like the typical P. acutilirata, from which it is beyond any question descended. Greene has shown that Chonetes granulifer of the Carboniferous is, in the neanic stage, like the Devonian Chonetes, and that the hinge-spines come in at a considerably earlier stage in the Carboniferous than in the Devonian and Silurian forms, showing the acceleration of this character.
In the Bryozoa I have pointed out the fact that the colony behaves as an individual, and like an individual recapitulates in its ontogeny (astogeny) ancestral characters. This is beautifully shown in Fenestella, in which the earlier zooecia are strikingly like the adult zoœcia of the Cyclostomata. The adolescent zocecia of Devonian Fenestella are similar to the adult zocœia of Niagara forms. Lang has brought together numerous cases of recapitulation among Jurassic and Cretaceous Stomatopora and Proboscina. The method of dichotomy in the earlier portions of the colony is constantly more like the normal dichotomy of ancestral species.
In graptolites the remarkable researches of Ruedemann clearly indicate that the graptolite colony recapitulates ancestral characters, the proximal thecæ being similar to ancestral adult thecæ. He says:
The rhabdosomes in toto and their parts, the branches, seem also to pass through stages which suggest phylogenetically preceding forms.
Among the trilobites the studies of Beecher, Walcott and Matthew are classic. Beecher has shown that there is a common larval form, the protaspis, and that in higher genera characters appear in the protaspis that are known only in the adults of more primitive genera. For example, the "main features of the cephalon in the simple protaspis forms of Solenopleura, Liostracus and Ptychoparia are retained to maturity in such genera as Carausia and Acontheus." Larval Sao has characters that occur in the adult of Ctenocephalus. The larval stages of Dalmanites and Proetus have characters that appear only in the adult of ancient genera.
Among the corals Beecher and Girty show that such genera as Favosites have early stages that suggest Aulopora. Lang, in a recent paper, records very interesting cases of recapitulation in the genus Parasmilia of the Cretaceous. Bernard concludes that the coral colony, like the graptolite colony and the bryozoan colony, behaves as an individual.
In the echinoderms the likeness of the stem ossicles and the development of the anal plate of Antedon, to Paleozoic and Mesozoic forms has become one of the stock illustrations of recapitulation. Jackson has found interesting examples of recapitulation in the development of the ambulacral and inter-ambulacral plates of echinoids. Miss Smith has shown that the young Pentremites is exactly similar in form to the adult Codaster. This is an extremely interesting case, for Bather has independently, and from quite different data, come to the conclusion that Pentremites is derived from Codaster.
The idea of recapitulation has been one of the most fertile in the whole realm of biology, and its usefulness to the paleobiologist has been almost incalculable. But while there can be no doubt that recapitulation is a fact, the paleontologist should observe all due care not to assume too much for it. That there are various sorts of adaptations, arising at all stages of life, and that these may greatly obscure the ancestral record, is a fact too well known to require more than mention. There is also always acceleration, sometimes affecting different characters very unequally; and there may be retardation. All of these factors complicate the record of ontogeny. Nevertheless, after all of these have been taken duly into consideration, the parallel between ontogeny and phylogeny remains a powerful aid to investigation for the paleontologist.