6 THE DEVELOPMENT OF THE CHICK 



(1821). v. Baer opposed MeckePs view that higher organisms 

 pass through the definitive stages of the lower organisms, and 

 formulated his conclusions on the subject in 1828 in the following 

 theses : 



1. "The more general features of a large division of animals 

 arise in the embryo earlier than the more special features." 



2. " From the most general features of structure arise those that 

 are less general, and so on until the most specific features arise." 



3. "The embryo of any definite species tends away from the 

 specific forms of other species instead of passing through them." 



4. "Fundamentally, therefore, the embryo of any higher 

 species is never like a lower species, but only like its embryo." 



Some embryologists profess to prefer the laws of v. Baer to 

 the recapitulation theory as a formulation of the actual facts. 

 But it is obvious that the only possible explanation of the facts 

 is found in the theory of descent, and that therefore they must 

 be formulated in terms of this theory. The method of formula- 

 tion will depend on the conception of the nature of the factors 

 of organic evolution. Haeckel stated his theory in Lamarckian 

 terms, which renders it inacceptable in many places to those 

 who cannot accept the Lamarckian point of view. But as the 

 basis of any theory of descent is heredity, and it must be recog- 

 nized that ontogenies are inherited, the resemblance between the 

 individual history and the phylogenetic history necessarily fol- 

 lows. If one holds, as does the present writer, that phylogenetic 

 variations are germinal in their character, then one must admit 

 that every phase of development of every part has two aspects, 

 viz.: the modern, specific, or coenogenetic, and the ancestral or 

 palingenetic aspect. The latter aspect may be more or less com- 

 pletely obscured in the course of evolution, but it can never 

 entirely vanish because it is the original germ of the specific 

 form acquired. It is not correct from this point of view to classify 

 some features of development as coenogenetic and others as palin- 

 genetic, though it is obvious that some characters may exhibit 

 the ancestral conditions in more apparent and others in less 

 apparent form. 



III. THE PHYSIOLOGY OF DEVELOPMENT 



To explain how a germ possessed the potency of forming an 

 adult, the preformationists of the eighteenth century assumed 



