88 MUTATIONS, VARIATIONS, AND RELATIONSHIPS OF THE OENOTHERAS. 



tancc, since it offers the suggestion that we may become able, more or less at 

 will, to modify existing streams of heredity and cause new organisms to arise. 

 The point not to be lost sight of is that the action of external agents as 

 described and deemed possible is not through the somatic tissues to the egg or 

 germ-plasm, but is exerted directly upon the reproductive elements themselves. 

 To assume that such action is stimulative, as has been done by Tower in dis- 

 cussing the earlier announcement of these results (Tower, 1906), is to adhere to 

 one of a number of allowable suppositions — one, however, which seems most 

 applicable to the results secured by him with beetles, but one to which we are 

 by no means confined in the consideration of the induction of the atypic forms 

 in plants. The determination of the character of this action constitutes one of 

 the most difficult problems in connection with the entire matter. 



