232 PROCEEDINGS OF THE ACADEMY OF [April, 



isms which are isolated at the present time, whose more immediate 

 relatives have become extinct, it is particularly necessary to know 

 the organization at all its periods. 



In conclusion, we have to bear in mind that of first importance 

 in classification is the terminal, perfected stage of the organism, 

 since this ontogenetic stage is also an actual phylogenetic stage. 

 The further removed from this stage the further from actual 

 phyletic conditions, and therefore the greater chance of error in 

 interpretation. The correspondences between the ontogeny and the 

 l)hylogeny are not repetitions but parallels, and in many cases 

 parallels that are so imperfect that it is always necessary to decide 

 what is palingenetic and what cenogenetic. The decision in regard 

 to this point is necessarily more arbitrary and subjective than a 

 decision upon similarities of adult structure ; for the embryological 

 method, in addition to assuming in common with comparative 

 anatomy that essential similarity of structure and position denotes 

 homology, has also to assume as correct that the course of the onto- 

 geny is more or less similar to that of the phylogeny. Comparative 

 anatomy is not obliged to make this assumption, and so has one 

 premise less. In this conclusion we do not diminish the value of 

 comparative embryology, but only restrict the application of its 

 facts. Comparative embryology has been fully as productive as 

 comparative anatomy in establishing working hypotheses, the value 

 of which for research is indisputable. But it is, in the main, from 

 comparative anatomy that the phylogeny is to be determined ; and 

 from comparative embryology an understanding of the growth- 

 energies, and therefore a more complete comprehension of the adult 

 structures. And it is the logical method to investigate all the 

 series of forms of the organism, so as to reach the most comprehen- 

 sive concept of the organization as a whole. 



