Popular Science Monthly/Volume 77/September 1910/The Paleontologic Record IV: Ontogeny: A Study of the Value of Young Features in Determining Phylogeny
|THE PALEONTOLOGIC RECORD|
By Professor F. B. LOOMIS
IN this paper I want to study what value is to be given to the principle that ontogeny is a brief recapitulation of phylogeny, when it comes to the concrete determination of the ancestry of a given genus. For the purpose three types have been studied carefully and several more for confirmations, the principal study being between the young and adult of the pig, cat and man, the differences being noted to see if they suggested the forms considered ancestral.
First let us consider the skull of a six weeks' pig in comparison with that of the adult, the two having been drawn to the same length. The first and most marked variation is in the brain case, that of the young being relatively vastly larger. The same is especially true of the sense capsules of the ear and eye. The later growth is much greater in those parts of the skull designated as facial, or having to do with the jaws and their supports. Then there is a change in the axis of the skull, this being due to the growth of the maxilla region, and lastly where there is any cellular bone or bone spaces they are developed in later life. This factor is especially well shown in the development of the elephant skull and in ruminants. It is coincident with high crests and marked protuberances.
While most of the features have been indicated in the pig, the same comparison in the cat reveals the same excessive development of the brain case and sense organs, the same weakness of the jaws and change in the axial relations, and this may be further confirmed in looking at the contrast between a three-year-old child's skull and that of an adult.
The conclusions then to be drawn from this hasty comparison of the two skulls are, first, that the shape of the skull in the young shows the excessive development of the brain and sense capsules, so that the appearance is not that of a primitive animal, but exactly the contrary, the appearance which the genus would assume were its mental or nervous development carried to a much higher degree than is the case. The embryonic development of the brain and sense organs is pushed far toward the beginning, and is matured, as far as size is concerned, the earliest of any of the systems. The skull is first an envelope for the brain and sense organs and is therefore profoundly modified by this embryonic peculiarity, and the younger the individual the less like the adult or ancestor the skull is shaped.
Secondly, the change of axis is not in the ancestral direction, the excessive weak condition of the jaws being again an embryonic adaptation and not an ancestral one.
Lastly, in the development of cancellous tissue is a condition which more nearly approximates the phylogenetic development, but here even the use of young features is deceptive, for it is seldom that this cellular bone is developed in the immediate ancestor but is rather found in several genera back, being usually an accompaniment of the development of the heavy facial portion of the skull. So much for form.
Turning to the dentition. The milk set of the pig and those of the adult are drawn side by side, and it is seen that while the front teeth of the young approximate those of the adult, the comparison is between the complicated premolar and molar sets. Briefly, of the four premolars, if all present, in the young (and often but three are developed) the two in front resemble the premolars to succeed them in the permanent set, while the two rear milk premolars resemble the permanent molars, the last milk premolar being especially like the last molar. This granted, the interest centers around whether the pattern of the milk teeth is such as to indicate the ancestry. A glance at the pig and its young will show that while the detail is not exactly the same in young and old, yet they are so alike that no one would identify a single milk molar as Hyotherium or any other suine genus, but would have to put it in the genus Sus. Taking other cases among the Ungulata, the history of the naming of the Miocene genera of horses gives a good example. There are, according to Gidley, four genera, Hyohippus, Parahippus, Merychippus and Protohippus; of these, three were founded on young teeth, i. e., the first three named. When it was recognized that they were young teeth, they were by Cope assigned to Protohippus, but when the adult teeth were found it was clear that the distinctive features of these young teeth were the distinctive features of the adult. For the genus Merychippus there is a difference in that the young teeth are not cemented, while the adult are. That is ancestral. In analyzing the descriptions of several genera of horses usually some feature can be found in the milk tooth which is ancestral.
In the Carnivora there is the carnassial tooth which is specialized; in the upper jaw it is the third milk premolar and the fourth in the adult; in the lower jaw it is the fourth milk premolar, and the first molar of the adult. Thus it is clear that it is a different dental follicle which forms the young and the adult carnassial. In the case of the dog the permanent and milk carnassials are approximately alike, but in the case of the cat the inner lobe or protocone occupies a very different place in the young from that of the adult, a position characteristic of none of the Felidæ and suggests some of the apparently unrelated Creodonts.
In the matter of the succession of teeth the follicles which form the last two—the milk premolars—form teeth in the first set of a totally different and usually more advanced character than the teeth to he formed from the same follicles in the permanent set. As a general thing then the conclusion would be that the milk teeth tend to have the same characters as mark the permanent set, but when they vary they often retain characters of the phylogenetically ancestral form. Weber adds that the later the succession the less the difference between the milk and permanent sets.
Turning to the limbs, there are again several distinctly ontogenetic characters, which are by no means ancestral. First, the formation of epiphyses, so that a bone ossifies from three or more centers. This is purely an ontogenetic adaptation and has no phylogenetic significance. Then the articular ends of all the limb bones are greatly enlarged as compared with adults. This again is not phylogenetic but an adaptation, the joints and their ligaments being early approximated to their permanent conditions. Then the length of limbs seems to be effected as an embryonic adaptation. First take the case of man born with disproportionately short arms and legs. The legs have been interpreted as representing a phylogenetic condition, but the same rule does not apply to the arms which were ancestrally long. This feature of short limbs is also characteristic of carnivora and I feel that it is an embryonic adaptation; certainly the ancestral limb can not be deduced from the young condition. Quite the reverse of conditions obtains among the Ungulata where the young at birth have disproportionately long limbs, which with equal certainty does not represent any ancestral condition recapitulated, for the ancestral limb in ancestral forms is shorter. Again, I believe the anomalous legs are adaptations to either the necessity for speed on the part of the young, or for height to reach the teats, suckling being while the parent is standing.
In the cases of the reduction of digits, greater portions of the reduced digits are usually found in the young animals than in the adults, but in the case of the entire loss of a digit it is also lacking in the young and embryo.
The general conclusion of the whole matter would then be that the young give us very little which is not deceptive in reconstructing ancestral forms. In certain cases, namely in the teeth and in reduction of digits, confirmatory points may be obtained, but these must be used with care, the valuable constructive evidence being rather found in adult skeletons, and in morphological comparisons. While allowing that many stages are recapitulated in the development of an individual, the vast number of adaptations impressed on the young to be used after birth, make their skeletons specialized even from birth, and such differences as exist are seldom reminiscent.