60 THE THEORY OF EVOLUTION 



which have been secondarily introduced in re- 

 sponse to the changing needs of embryonic or larval 

 life. These secondary features are termed cen- 

 ogenetic. 



"If we are compelled to admit that cenogenetic 

 characters are intermingled with palingenetic, then 

 we cannot regard ontogeny as a pure source of evi- 

 dence regarding phyletic relationships. Ontogeny 

 accordingly becomes a field in which an active imag- 

 ination has full scope for its dangerous play, but in 

 which positive results are by no means everywhere 

 to be obtained. To attain such results, the palin- 

 genetic and cenogenetic phenomena must be sifted 

 apart, an operation which requires more than one 

 critical grain of salt. On what grounds shall this 

 critique be based? assuredly not by way of a vicious 

 circle on the ontogeny again; for if cenogenetic 

 characters are present in one case, who will guarantee 

 that a second case, used for a comparison with the 

 first, does not likewise appear in cenogenetic dis- 

 guise? If it once be admitted that not everything in 

 development is palingenetic, that not every onto- 

 genetic fact can be accepted at its face value, so to 

 speak, it follows that nothing in ontogeny is imme- 

 diately available for the critique of embryonic 

 development. The necessary critique must be drawn 

 from another source." l 



These remarks of Gegenbaur's were called forth 

 by the state of wild speculation into which embryo- 



1 C. Gegenbaur, Morphologisches Jahrbuch, Vol. XV, p. 5. 



