EMBRYOLOGY AND EVOLUTION 



to learn the course of past evolution, but owing to the highly 

 speculative character of such studies and to the differences 

 of opinion as to what were original (palingenetic) and what 

 were acquired (coenogenetic) characters, there gradually 

 arose a widespread skepticism concerning the value of 

 embryology for this purpose. Gegenbaur, in 1889, voiced 

 the growing opinion among zoologists in these words: "If 

 we are compelled to admit that coenogenetic characters are 

 intermingled with palingenetic, then we cannot regard ontog- 

 eny as a pure source of evidence regarding phyletic rela- 

 tionships. Ontogeny accordingly becomes a field in which 

 an active imagination may have full scope for its dangerous 

 play, but in which positive results are by no means every- 

 where to be attained. To attain such results the palingenetic 

 and the coenogenetic phenomena must be sifted apart, an 

 operation that requires more than one critical granum salis!' 

 Since the time this was written there have been many other 

 less moderate utterances to the same effect, some even declar- 

 ing that there is no evidence that ontogeny ever recapitulates 

 phylogeny and that Haeckel's "biogenetic law" has no foun- 

 dation in fact. 



But after all, these criticisms of certain details of the 

 recapitulation theory have not destroyed the general and 

 fundamental truth of that theory — namely, that many features 

 of individual development repeat ancestral features. There 

 are many remarkable and undoubted instances in which 

 ontogeny repeats phylogeny and in which the relationships of 

 organisms can be determined only by their embryological 

 history. The most severe critics of Haeckel's "biogenetic 

 law" do not deny this; their criticisms apply to details rather 

 than to foundation principles. 



/It is certainly no mere accident that practically all animals 

 being their individual existence as fertilized eggs ; that before 



[71] 



