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ported and protected by its parents, it is withdrawn from 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 
