(ENOTHERA AND THE MUTATION THEORY 313 



its derivatives ratlier than to the other species. 

 An explanation was suggested in 1914 by Renner, 

 who found that about half of the seeds of Lamarck- 

 iana fail to germinate. He supposed that the 

 species was a permanent heterozygote for the laeta- 

 velutina pair of characters, which he treated as 

 allelomorphs. The homozygous types were both 

 assumed to be inviable, and to be represented by 

 the seeds that fail to germinate. This view was at 

 first opposed by deVries, but has since been adopted 

 by him, and has been substantiated by so much 

 evidence that it can now scarcely be doubted. 



By 1917, then, the situation had been analyzed 

 far enough to indicate that CEnothera Lamarckiana 

 is a permanent heterozygote in at least two respects, 

 and that its peculiar behavior in crosses is at least 

 in part due to this heterozygosis. But in two 

 respects the results were still unsatisfactorv^ No 

 analogous case was kno^^'n in any other organism, 

 so that the hypotheses seemed curiously artificial 

 and improbable; and there was no obvious relation 

 between the heterozygous nature of O. Lamarckiana 

 and the frequency with which it produces new types. 



Both these objecticms were eliminated by the 

 work of 3.1uller on certain peculiar races of Dro- 

 sophila. Muller examined th(^ }:)ead.ed race, which 

 had long been a puzzle to those studying Dro- 

 sophila. The beaded race, when first studied, did 

 not breed true, but constantly produced normal 

 flies. After a long period of selection, however, it 

 began to breed almost true. In crosses the character 



