220 Inheritance of Acquired Characters 



overmastering power, it will not leave behind any trace 

 in the individual and certainly none in the species. 



Thus the inheritance of acquired characters, thanks 

 to these two facts that the embryo is usually withdrawn 

 from the influence of the environment, and that or- 

 ganisms undergoing development are elastic and not 

 plastic, shows itself to be completely capable of account- 

 ing for the fundamental biogenetic law. Whatever in itj 

 could seem marvelous and enigmatic finds its natural so- 

 ultion and the law itself becomes an immediate and neces- 

 sary consequence of this inheritance. 



What on the contrary is the explanation which Weis- 

 mann is able to give for this law? He thinks to explain 

 it merely by the following laconic words : 



"The biogenetic law rests upon this, that phylogenetic 

 development is accomplished partly by the addition of 

 new ontogenetic stages at the end of ontogeny. In order 

 that this latter may be attained, the preceding terminal 

 stages must each time be run through again." 167 



But in this Weismann leaves just the most important 

 part of the question out of consideration. Why can 

 phylogenetic development take place only by the addition 

 of new ontogenetic stages at the end of ontogeny? 



According to Weismann's theory, there is no reason 

 whatever why one should believe the determinants cor- 

 responding to the last ontogenetic stage to be the only 

 ones to undergo modifications, for one cannot forget that 

 according to this theory each cell of each ontogenetic 

 stage must have its own determinants. 168 The same 

 causes of differences in the nutrition or any other thing, 

 which are capable of modifying the determinants cor- 



167 Weismann : Das Keimplasma. P. no. 

 1<58 Weismann : Ibid., e. g. P. 97, 100, 232233, 596. 



