Xo. 597] 



THE MUTATION THEORY 



521 



do not contain normal embryos. Moreover, when crossed 

 with certain species, the first hybrid generation consists 

 of two types, the twin hybrids of de Vries. Eenner as- 

 sumes that (E. Lamarckiana is heterozygous and that it 

 produces two types of functional gametes. Its progeny 

 under ordinary circumstances would therefore be ex- 

 pected to consist of recessive homozygotes, heterozygotes, 

 and dominant homozygotes in the familiar 1:2:1 ratio. 

 He further assumes, however, that the homozygotes are 

 incapable of developing beyond a young embryonic stage, 

 and that the species is therefore maintained in a hetero- 

 zygous condition from generation to generation. This 

 simple hypothesis obviously does not account for the 

 mutability of (E. Lamarckiana. It has been amplified 

 with this end in view by lleribert-Xilsson, whose highly 

 involved explanation of mutability from the standpoint 

 of the plural factor hypothesis must receive a brief con- 

 sideration. For several years this worker has busied 

 himself in an attempt to demonstrate Mendelian inheri- 

 tance in (E. Lamarckiana. In one case he thought he had 

 found simple monohybrid segregation in crosses between 

 red- and white-nerved races, and announced that the 

 nerve color acted as a simple Mendelian character. It 

 developed later, however, that his ratios were aberrant, 

 and that the progenies entirely lacked a class of plants 

 homozygous with regard to the supposed dominant char- 

 acter. According to Heribert-Nilsson's interpretation, 

 the progenies consisted only of heterozygotes and reces- 



dominaS for by assuming that 



in certain cases an incompatibility, or, as he puts it, a 

 prohibition, exists between like gametes. His whole hy- 

 pothesis is based upon this idea of prohibition. He as- 

 sumes that the assemblage of characters which we recog- 

 nize in Oenothera Lamarckiana may be brought about by 

 many combinations of plural factors. Any one of these 

 plural factors in the heterozygous condition gives a plant 

 the Lamarckiana habit, and prohibition prevents the pres- 

 ence of any of them in the homozygous condition. Segre- 



