6 THE DEVELOPMENT OF THE CHICK 
(1821). vy. Baer opposed Meckel’s 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. 
II. THe Puysrotocy or DEVELOPMENT 
To explain how a germ possessed the potency of forming an 
adult, the preformationists of the eighteenth century assumed 
