THE PALEONTOLOGIC RECORD 51 



ONTOGENY: A STUDY OF THE VALUE OF YOUNG FEA- 

 TUEES IN DETERMINING PHYLOGENY 



BY PROFESSOR F. B. LOOMIS 



AMHERST COLLEGE 



IN this paper I want to study what value is to be given to the prin- 

 ciple 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. 



