INHERITANCE OF PARENTAL MODIFICATIONS 8 1 



blood, might so affect the germ cells as to cause the offspring 

 which arise from them to diverge somewhat from the usual 

 character. It is hard to see how this somewhat indefinite 

 effect of soma upon germ could be avoided. We have, how- 

 ever, no evidence that the substances given off by the several 

 sorts of soma cells into the blood affect the germ cells in 

 such a way that when they give rise to new organisms these 

 will repeat in their own bodies those peculiar modified 

 somatic activities of their parents which gave into the blood 

 the substances which caused the modification of the germ 

 cells. So, while we recognize the probability that germ cells 

 are constantly affected by changes in the blood due to the 

 activity of soma cells, and while recognizing also that we may 

 have here a real cause of variation, we still have no evidence 

 that these somatic influences upon the germ are of such a 

 nature as to cause the offspring to inherit the adventitious, 

 accidental, or secondarily acquired somatic characters of the 

 parent. We have here a probable cause of variation, but 

 not a means for securing the inheritance in kind of modifica- 

 tions of the parental soma. 



Again observe that w r hen a spermatozoon unites with an 

 egg in the process of fertilization, there are mingled germ 

 cells from two different ancestors, each with its own he- 

 reditary potentialities. The organism resulting from the 

 development of this compound cell will naturally be dif- 

 ferent from either of its parents, the hereditary tendencies 

 received from one parent being modified by those from the 

 other parent. For a proper understanding of the possi- 

 bilities of variation which are involved in this fact of the 

 union of two germ cells in the process of fertilization one 

 needs to be familiar with some of the most intricate 



