120 Shull. 



individual g:enes have not been identified? To avoid confusion from 

 tliis inability to decide whether in any case we are dealing with poly- 

 mery or not, I will distinguish in what follows, between "duplicate" 

 determiners and "plural"' determiners f(ir any given character. These 

 two terms must still be understood as making absolutely no assnm|)tion 

 regarding the nature or identity of the genes themselves. By "dupli- 

 cate" determiners I understand those wliich, when separated from each 

 other, produce charactei'S so like that they can not be distinguished 

 fi-oiii one another; c. g., if P and E are duplicate determiners and XX 

 the residual genotypie "nucleus", then XXPPrr will be indistinguishable 

 from XXppRR and from XXPpRr. By "pluial" determiners I shall 

 indicate two or more genes which independently produce a given 

 character, or which modify it in any way whatever, which does not de- 

 stroy its identity. "Plural" determiners thus also include "duplicate" 

 determiners, of course. In this sense the above mentioned h^\i)othetical 

 genes, N and L, for the internode-number and internode-length, respec- 

 tively, are "plural" genes for plant-height, though by no means 

 "duplicate" genes. This distinction has not been clearly made by 

 writers who have tliscussed the Mendelian inheritance of quantitative 

 (characters, and as such discussions have invariably taken as their point 

 of departure, cases in which duplicate determiners have" been demon- 

 strated, there has always been a more or less obvious implication, if 

 not a direct statement, that in the inheritance of these various quanti- 

 tative characters, duplicate determiners are involved. 



The consequences which result from the existence of duplicate 

 determiners for single characters, have been so well discussed by 

 NilSSON-EhLE (1908, 1909, 1911), EAST (1910, 1912), LaNG (1910, 

 1911), lilMERSON and East (1913) and others, that it may suffice to 

 indicate here the characters for which duplicate determiners, or at least 

 plural determiners, have been demonstrated (in full-faced type), and 

 those incompletely analyzed characters for which, as a sequel to the 

 discovery of duplicate determiners, a plurality of Mendelian genes has 

 been assumed to exist, 'i'ln' following list is believed to be fairly com- 

 plete for the cases in wliitli relevant data are given, or definitely re- 

 ferred to. In some of the more enthusiastic statements regarding the 

 importance of plural Mendcslian genes, suggestions of their applicability 

 to other cases have been made, as, e. g., in East's interpretation 

 (1912) of hybridization phenomena in Oenothera, but such cases are 

 not here included. 



