Page:Popular Science Monthly Volume 77.djvu/298

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ONTOGENY: A STUDY OF THE VALUE OF YOUNG FEATURES IN DETERMINING PHYLOGENY

N 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.