22 Plant Life and Evolution 



dividual characters, and these characters must be 

 transmitted through cell division to all of the de- 

 scendants of the germ cell. The ultimate structure 

 of the germ cells of two species being different, it 

 follows that their responses to the various stimuli 

 to which they are subjected during the develop- 

 ment of the embryo will also differ, and moreover 

 the conditions to which the developing embryos of 

 any two species are exposed also may be supposed 

 to differ to a greater or less degree. 



In a general way we may say that the degree 

 of difference between two organisms is deter- 

 mined, first, by structural differences of the germ 

 cells, and secondly by the different conditions to 

 which the germ cell is exposed in the course of 

 its normal development. Given two identical 

 germ cells, subject to identical conditions through- 

 out their development, and the result must be 

 two identical organisms. Pfeffer very properly 

 lays stress upon the importance of the physi- 

 ological factors in heredity as distinguished from 

 purely structural ones, and this view has also 

 been expounded by Peirce, Farmer, and other 

 physiologists. The chemical and physical stimuli 

 to which the protoplasmic units are constantly sub- 

 jected are quite as potent in determining the char- 

 acter of the resulting structure as is the mere chem- 

 ical composition of the different protoplasmic units 

 of which the germ cell is composed. It does not, 

 however, follow that the offspring must be an exact 



