218 Inheritance of Acquired Characters 



old only after the completion of the development of these 

 latter, it is sufficient to suppose that the agent of trans- 

 mission of an acquired character becomes active in onto- 

 geny, only when the young organism finds itself in the 

 same conditions in which the parent organism was when 

 it acquired this character. 



As soon as one admits this condition for the mech- 

 anism of inheritance the law of the repetition of phylo- 

 geny by ontogeny appears to be merely the immediate 

 consequence of the inheritance of acquired characters. 



For so long as the embryo is developing in the egg or 

 in the maternal body and so long as it is nourished, sup- 

 ported and protected by its parents, it is withdrawn from 

 the changing influences of the environment. It is only 

 when the individual is left to himself that he finds him- 

 self driven perhaps to new functional adaptations. In 

 other words it is only in the adult state, after it has com- 

 pleted or almost completed its specific development, that 

 the organism in general can find itself in conditions neces- 

 sary for the acquisition of new characters. 



But another fact also can explain why new phylo- 

 genetic characters are acquired only when all the old ones 

 are already quite developed. We have indeed already 

 seen that the organism undergoing development is much 

 more elastic but much less plastic than the adult, so that 

 the modifications which arise in it from the action of an 

 external force, even when it acts for a long time, have 

 the tendency to disappear without leaving any trace be- 

 hind so long as the organism has not yet completed its 

 development, whereas this tendency is no longer inherent 

 in the adult organism. 



We have already mentioned the experiment of Roux, 

 in which he distorted a few frog embryos within their 



