82 University of California Publications in Agricultural Sciences [Vol. 4 



If we use the term factor mutation,- (Babcock, 1918) where the 

 eytological change occurs within a locus, transforming a factor into a 

 different factor, two analogous terms will apply where the eytological 

 change is external to the locus. When the eytological change consists 

 of a loss, reduplication, or transposition of one or more loci it may 

 be called a locus mutation, and when the change consists in such 

 phenomena affecting a whole chromosome it may be called a chromo- 

 some mutation. If the term mutation is applied to the eytological 

 change itself, the last two types of nmtati(m may be grouped together 

 as extralocus mutations, while the first type consists of intralocus 

 mutations. Examples of factor mutation are white eye in Brosopliila, 

 and probably the ruhrinervis type in Oenothera; an example of locus 

 mutation is (possibly) "deficiency" in Brosopliila; and examples of 

 chromosome mutation are Oenothera gigas and 0. lata. 



It is now evident that the immediate problem with Oenothera relates 

 to the mechanism of heredity in the genus. There are two sharply 

 opposed views. One is that recently emphasized by Atkinson (1917, 

 p. 254), when he says, "The evidence from Oenothera cultures points 

 more and more to the conclusion of Shull that 'a hereditary mechanism 

 must exist in Oenothera fundamentally different from that which dis- 

 tributes the Mendelian unit-characters.' " The opposing view is 

 represented by Muller's (1918) strictly Mendelian explanation for 

 Oenothera, based on "an Oenothera-\\kQ case in Drosophila'^ ; he says, 

 "The striking parallel between the above behavior and that exhibited 

 in Oenothera makes it practically certain that this, too, is a complicated 

 case of balanced lethal factors." 



A notable feature of the extensive genetic study of Oenothera is 

 the lack of progress toward any definitely supported explanation of 

 its hereditary mechanism which is not Mendelian. The only definite 

 non-Mendelian hypothesis of chromosome behavior so far proposed, 

 aside from "merogony" and other hypotheses (Goldschmidt, 1916) 

 apparently possible but not proved for Oenothera, which as.sume loss 

 of chromatin after fertilization, seems to be Swingle's (1911) 

 "zygotaxis," proposed for the apparently parallel case of Citrus. 

 This .suggestion that Fj hybrids may differ, apart from non-uniformity 

 of the Pi gametes, because of the establishment of permanently differ- 

 ent arrangements of the chromosomes in the fertilized egg, still seems 

 to be purely speculative. 



With more or less " Oenothera-like' ^ cases in other genera, the only 

 definite progress in analysis seems to have resulted from the assump- 



2 Muller (imS) lias recently used point mutation in the same sense. 



