514 



THE AMERICAN NATURALIST [Vol.L 



plant gives rise to mutations of two main types, those in 

 which the chromosome number differs from that of the 

 parent form, and those in which it does not. The most 

 striking ones belong to the former category ; among them 

 we need mention particularly only (E. gigas and (E. lata, 

 which have, respectively, 28 and 15 chromosomes, instead 

 of 14, the normal number in the species. Gates has espe- 

 cially emphasized the fact that in the mutating (Enotheras 

 the pairing of the chromosomes, previous to the reduc- 

 tion division, is very loose, a condition that would favor 

 irregularities in the distribution of the chromosomes to 

 the gametes. By irregular reduction divisions, gametes 

 with a greater or less number of chromosomes than 7 

 might easily be formed. From these irregular gametes 

 there would be derived, in turn, zygotes with irregular 

 chromosome numbers. Mutationists are now pretty well 

 agreed that the characters of certain mutations are corre- 

 lated with an unusual complement of chromosomes. If 

 one believes at all that the chromosomes provide the 

 mechanism of Mendelian inheritance, it is hard to escape 

 the conclusion that the cytological studies of Lutz, Gates 

 and others give a firm basis for removing at least part of 

 the mutation phenomena from the domain of Mendelian 

 segregation. 



Even Davis, who has been one of the chief opponents of 

 the mutation theory, has admitted that some of the vari- 

 ants from (Enothera Lamarckiana and other species are 

 probably due to irregularities in chromosome distribu- 

 tion. He makes the point, to be sure, that the unpaired 

 condition of the chromosomes previous to reduction in 

 (E. Lamarckiana is itself exceptional, and presumably the 

 result of a hybrid constitution. Davis himself has shown 

 that in at least one of the numerous strains of (E. grandi- 

 flora which occur at the type locality of that species in 

 Alabama, the chromosomes, in preparation for the reduc- 

 tion division, become associated definitely and closely into 

 ring-shaped pairs. Moreover, this same strain shows a 

 negligibly low pollen and seed sterility. Davis views it as 

 essentially a genetically pure strain, whether judged by 



