12 THE EVOLUTION OF MAN. 



incidents, which must be explained as kenogenetie pro- 

 cesses ; the formation of the yelk- sac, of the allantois and 

 placenta, of the amnion and chorion, and, generally, of the 

 different egg-membranes and the corresponding systems of 

 blood-vessels; also the transitory separation of the primitive 

 vertebrate plates and the side-plates, the secondary closing 

 of the stomach wall and- the intestinal wall, the formation 

 of the navel, etc. All these, and many other phenomena 

 are evidently not referable to corresponding conditions of 

 an earlier, independent, and fully developed parent form, 

 but must be explained as solely due to adaptation to the 

 peculiar conditions of egg-life or embryo-life (within the 

 egg-membranes). With reference to this fact we may now 

 define our "first principle of Biogeny" more exactly as 

 follows : " The evolution of the germ (Ontogeny) is a com- 

 pressed and shortened reproduction of the evolution of the 

 tribe (Phylogeny) ; and, moreover, this reproduction is more 

 complete, in proportion as, in consequence of constant 

 heredity, the original inherited evolution (Palingenesis) is 

 more closely retained; on the other hand, the repetition 

 is more incomplete, in proportion as the later vitiated 

 evolution (Kenogenesis) is introduced by changing adapta- 

 tion." 10 



The kenogenetie vitiations of the original, palingenetic 

 incidents of evolution depend in great measure on a gradually 

 occurring displacement of the phenomena, which is effected 

 in the course of many thousands of years by adaption to the 

 changed conditions of embryonic existence. This displace- 

 ment may effect either the place or the time of the 

 phenomena. If the former, it is called Heterotopy ; if the 

 latter, Heterochrony. 



