Popular Science Monthly/Volume 77/September 1910/Paleontology and Ontogeny
|PALEONTOLOGY AND ONTOGENY|
By A. W. GRABAU
ONTOGENY, or the life history of the individual, is commonly interpreted by zoologists as its embryology, the later stages of development, from infancy to old age, being deemed of little or no importance. This was the case fifty years ago; this is largely the case to-day. From the days when Agassiz first called the attention of zoologists to their one-sided attack of the problem of ontogeny, and urged them to pay attention to the important post-embryonic stages, down to our own time, students of recent animals have for the most part been content to follow the beaten path. They have left to the paleozoologist the study of the later stages in the life history of the individual, and the latter's endeavors in this direction have developed the science of zoontogeny as to-day understood. There was, perhaps, a natural cause for this separation, in the fact that the student of soft tissues finds few changes which he deems worthy of attention, between the embryo and the adult; whereas the student of hard structures generally sees an abundance of such changes. This is especially true of invertebrates, more particularly of such as build external hard structures in which successive additions are marked by the lines of growth. Vertebrates, and invertebrates without permanent hard parts, such as the Crustacea, require series of individuals showing the successive steps in development. But mollusks, brachiopods and corals show, by their incremental lines, the steps in the life history during the post-embryonic period, so that one perfect individual suffices to present these later stages in development.
It is not infrequently urged that the hard parts of invertebrates, especially the shells of mollusks, are not reliable indices of ontogenetic development, since they represent only the integument, which is subject to ready modification under the influence of the environment. Such an argument is based on a total ignorance of the relation of the shell or other hard structure to the soft parts of the animal. The paleontologist is convinced that the hard parts of animals are the best indices of its development, since they record in a permanent form all the minute modifications which are not even recognizable in the soft parts. More than this, I believe that shells, those of mollusks at any rate, furnish us with a record of changes wholly independent of the environment, and referable entirely to an inherited impulse towards progressive modification, along definitely determinable lines. I am well aware that I am not expressing the opinion of all paleontologists in this statement, and that this view, moreover, is strongly opposed by some of our ablest European conchologists. But here again I contend that this difference of opinion is due to a difference of method. "When the student of shells directs his attention chiefly to adult characters, this definitely directed variation, independent of environment, is not recognized by him. But no one can study the details of shell ontogeny, especially in the earlier stages, without quickly realizing that ontogenetic development is orthogenetic, and that the inherited impulse towards determinate modifications is the most powerful controlling factor of the animal's life history.
So far as invertebrates are concerned, the study of post-embryonic development was first seriously undertaken by the immortal Hyatt, in his work on the ammonites. To be sure, others before him—notably d'Orbigny—noticed that a distinct series of changes was recognizable in the shell of ammonites, but no one before Hyatt actually employed this method. He himself once told me that when, in the early sixties, he first realized the importance of this method of study when actually applied to shelled organisms, and its value as a guide in phylogeny, it seemed so marvelously simple that he felt sure that the method and its application must be fully understood by all working naturalists. "But," he added, "I soon found that I practically stood alone, and I have spent my life since in the endeavor to convert them to my point of view."
This misunderstanding, on the part of many zoologists, of the ontogenetic method has given rise to their false attitude towards the doctrine of the recapitulation of ancestral characters. This subject will be adequately treated by some of my successors, but I can not forbear to anticipate them to the extent of pointing out this fact: When the embryologist seeks for proof or disproof of this concept in the enormously condensed record of the stages between the ovum and birth, he is bound to be grievously disappointed; for this record, necessarily modified by eliminations, can only furnish general resemblances of the embryo to earlier types, and can not be said to actually recapitulate the life history of the entire race. When, however, the student of postembryonic ontogeny compares the youthful stages of an individual with the adult of immediately preceding species of the same genetic series, the fact of recapitulation becomes at once apparent.
The post-embryonic life history of an individual falls readily into stages, of which four major ones have been recognized and named, chiefly by Hyatt. These are: (1) the infant or nepionic stage; (2) the adolescent or neanic stage; (3) the adult or ephebic stage, and (4) the senile or gerontic stage, followed by death. These onto-stages, as they may be called, are further divided into substages, designated by the prefixes ana, meta and para, and they may be observed in the ontogeny of all individuals. Moreover, in closely related members of one genetic group, the duration of these stages and substages is approximately uniform. Change in form, however, may vary greatly, and have no necessary relation to the onto-stages, even if they coincide with them. We have thus a second group of stages, which we may designate form stages, or morphic stages, and there will be required distinct designations in each case. The best method of naming these stages is to refer them to the adult ancestral type which they represent.
Thus, in all species of the gastropod shell Fusus, the earliest morphic stages are a close recapitulation of the adult of Fusus porrectus of the Eocenic. These stages may therefore be called the F. porrectus stage. It may be continued for a considerable period of the early life history, covering several onto-stages, or it may be condensed into a short portion of one stage or substage, in accelerated individuals.
It is of considerable importance that onto-stages and morphic stages should be discriminated, so I will introduce another illustration.
In the Miocenic of the Atlantic coast we have the gastropod genus Fulgur well represented. Fulgur fusiformis is normally characterized, in the adult, by the possession of a pronounced flat shoulder, which is separated from the body of the shell by an angulation carrying rounded tubercles. Some of the more specialized individuals lose the angulation and tubercles in the last whorl and become rounded. Thus, while normally the species is tuberculated in the ephebic onto-stages, specialized individuals acquire a new morphic stage through the loss of ornamentation. This morphic stage is prophetic of the normal adult of Fulgur maximum, and hence may be called the F. maximum stage. F. maximum itself has in its nepionic onto-stage the characters of adult F. fusiformis; hence it may be designated the F. fusiformis stage. Some individuals acquire a new stage, namely, a spinous stage, characteristic of the adult of F. carica. In the type designated as F. tritonis, the nepionic stage is characterized by a fusiformis morphic stage, the neanic largely by the maximum stage, though some of the later neanic stages may actually acquire the carica stage. In less specialized individuals the maximum stage may continue into the early ephebic in more specialized ones it ceases early in the neanic, the carica stage taking its place. Finally, Fulgur carica is characterized by the elimination of the maximum morphic stage, so that the neanic as well as the ephebic onto-stages are characterized by the spines of the carica stage, which may even begin in the late nepionic.
In the foregoing, the different morphic stages are shown to be telescoped with the onto-stages, appearing either earlier and earlier in the ontogeny of successive individuals, through the operation of the law of acceleration or tachygenesis; or later and later, through the operation of the complementary law of retardation or bradygenesis. These laws are, of course, only applicable to an orthogenetic series, but in such a series they are competent to produce, by interaction, all conceivable combinations of characters.
The paleontologist, more than any other naturalist, is concerned with the product of these interactions, and to him, oftener than to others, has come the question, Are these results species? and, if so, what are the criteria for the separation of species? The student of hard structures appreciates the difficulty of drawing sharp lines, and one of his most trying tasks is to satisfy the idiosyncrasies of his colleagues in the making of species, subspecies, varieties, etc. The student of hard parts finds transitional forms the rule, and he dare not grind them to powder under his heel with the remark credited to Stimpson, that "that is the proper way to dispose of those damned transitional forms."
The philosophic paleontologist recognizes more readily than any one else the truth of the dictum that nature knows only individuals, and that species are special creations, called into being by the fiat of the naturalist. He is concerned not so much with the origin of species as with the origin of individuals; and while he makes use of the artificial divisions called species, and sometimes finds his chief joy in multiplying and subdividing them, he still recognizes their non-existence, and turns to individuals. He may, perhaps, prefer to speak of mutations, meaning individuals, nevertheless.
But individuals are complex entities, and the paleontologist can not investigate their genesis before he has thoroughly investigated the origin of the parts composing it. As Professor Osborn has said, the paleozoologist is concerned primarily with the origin of structures. He alone is able to trace their development, for he is present at their birth, he follows their whole history, and will be present also at their extinction, for the paleontologist alone is immortal.