CHAPTER VII 



CAN MENDELIAN TJNIT-CHAEACTERS BE MODIFIED 

 BY SELECTION? 



IF, as suggested in the last chapter, the po- 

 tency of a character in crosses may be 

 modified by selection, why may not the 

 character itself be modified by selection, or are 

 not the two things perhaps identical, viz. modi- 

 fication of the potency of a character and modi- 

 fication of the character itself? Darwin firmly 

 believed that the characters of organisms can 

 be modified by selection, and he made this the 

 foundation stone of his theory of evolution. 

 De Vries and Johannsen, however, have taught 

 us a different doctrine, maintaining that selec- 

 tion is able to affect characters in superficial 

 and transitory ways only, that the slight varia- 

 tions in characters which we see everywhere 

 among organisms have no evolutionary signifi- 

 cance or permanent value ; that they come and 



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