1902.] NATURAL SCIENCES OF PHILADELPHIA. 225 



from essential similarily of adult structure, or of development ? As 

 to which of these methods is the more correct, has been and prob- 

 ably will continue to be a question of dispute. The comparative 

 anatomists maintain one side, the embryologists another; and 

 probably because the former are less conversant with the facts of 

 embryology, and the latter with the facts of adult structure. Those 

 who are at the same time comparative anatomists and embryolo- 

 gists have in general taken more of a middle course. 



The argimient for the embryological method is mainly based 

 upon the hypothesis that the ontogeny is a more or less true parallel 

 to or repetition of the phylogeny. The strength of the embryo- 

 logical method depends mainly upon the strength of this hypothesis, 

 so that we have first to examine its correctness. This is the basis 

 of the embryological method in the study of phylogeny, and there- 

 fore in the search for homologies, using the term ' * homology ' ' in 

 the sense of " homophyly." Certain writers have been more 

 cautious, considering homology to depend upon corresponding onto- 

 genetic mode of development, whether the ontogenetic correspond to 

 the phylogenetic or not. But in assuming that the ontogenetic 

 development may not correspond to the phylogenetic, they have no 

 basis for assuming that essential ontogenetic agreement denotes ho- 

 mology, if homology be used in the sense of homophyly. 



(a) Hypothesis of the Correspondence of Ontogeny and Phylogeny. 



It is not my purpose to give a historical sketch of this hypothe- 

 sis and of the many opinions which have been expressed with 

 regard to it, but merely to state briefly the tenets, and then to 

 examine them critically for our present purpose. 



This hypothesis is an old one, going back to the eighteenth cen- 

 tury, if not indeed to the seventeenth, and as von Baer expresses 

 it, is " mehr eine Entwickelungsstufe der Wissenschaft als das 

 Eigenthum eines eiuzelnen Maunes." It was held in the early 

 statement of it, that a higher form in its development passes succes- 

 sively through lower animal forms — i. e., that the ontogeny is a 

 relatively exact repetition of the phylogeny. The first logical criti- 

 cism of the hypothesis thus stated was made by von Baer,'' who 

 postulated the following laws of the development of the individ. 



*' Entwickslungsgeschichte der Thiere, 1828. 



15 



