454 BOTANICAL GAZETTE [decembee 



Several mutations of each class have been studied by the writer 

 in more or less detail, and the results wiU soon be pubhshed. As 

 already announced," the interesting mut. nummularia belongs to 

 class I, as do also all of the mutations characteristic of Lexington E. 

 Mut. latifolia is a typical member of class II. There are mutations, 

 of course, which show neither type of behavior, but they need not 

 be involved in the present discussion. 



Heribert-Nilsson's hypothesis demands the recessiveness of 

 mutations of class I, regardless of which way they are crossed with 

 the parent. This condition is not fulfilled. It demands that the 

 female gametes of the mutations of class II should be of one kind, 

 and the pollen of two kinds. Neither is this condition fulfilled. 

 His hypothesis makes no provision for the appearance of mutations 

 in excess of one-third of the progeny. In this respect it is quite 

 inadequate. On MendeUan grounds it is as difficult to account 

 for too many mutations as for too few. His assumption is that after 

 a homozygous and recessive condition has been attained in O. 

 Lamarckiana, except for one of the plural factors which produce 

 the Lamarckiana phaenotype, monohybrid spHtting will take place. 

 The one-fourth of dominant homozygotes will be eliminated, and 

 therefore the progeny will consist of heterozygotes and recessives 

 (mutations) in a 2:1 ratio. He has not attempted to explain how 

 more than one-third of a progeny can consist of mutations, although 

 he states in a vague and general way that the discovery of highly 

 mutable strains is an argument in favor of his thesis. Nothing, he 

 says, has made the mutation phenomena appear so exceptional as 

 the low frequency of mutations. In his opinion, the high mutability 

 of 0. Reynoldsii has rendered the mutation fiction an absurdity. 



Further comment on this opinion is rendered unnecessary by 

 the serious discrepancies between Heribert-Nilsson's hypothesis 

 and the facts. It can do no harm to point out, however, that even 

 if mutations appeared through the operation of Mendehan segre- 

 gation, as no one denies may sometimes be the case, it is still neces- 

 sary to account for the origin of heterozygosis in the parent strain. 

 The writer beUeves that mutations may often appear as a result 

 of segregation, but that the antecedent heterozygosis has its origin 

 in a mutative change. To attempt to account for the hetero- 



" Amer. Jour. Bot. 2:146. 1915. 



