MUTATION IN MATTHIOLA 225 



tion of Mendelian segregation. With Oenothera itself, the trend of 

 the evidence tends to favor this form of explanation. 



This fact is strikingly illustrated by two papers of de Vries (1918, 

 1919) which have appeared since the present paper was written, 

 especially as Muller's (1918) complete report on the beaded- wing 

 case in Drosophila (see especially pp. 471-474, 489, and 498-499) 

 indicates that de Vries had hardly yet realized the full possibilities of 

 the balanced- factor hypothesis. In the light of Muller's masterly 

 demonstration of these possibilities, we may be confident that "mass 

 mutation" is merely ordinary segregation, and that the "unisexual" 

 crosses of Oenothera are really "Mendelian" in their essential phe- 

 nomena. Some difference of usage respecting the inclusiveness of 

 the term Mendelian may be involved here, it is true, since apparently 

 de Vries would apply it only to cases where strictly homologous factors 

 are opposed in homologous chromosomes. Since, however (Muller, 

 1918), there are good reasons for expecting the occurrence of grada- 

 tions of similarity and of synaptic attraction between opposed loci, and 

 hence of gradations of linkage, the criterion of Mendelian behavior 

 should obviously be the occurrence of segregation between homologous 

 chromosomes, whatever their degree of similarity or amount of cross- 

 ing over. We have no reason to assume that an "unpaired" factor 

 in a parent would so divide as to be included in all gametes; on the 

 other hand, we have learned of a mechanism capable of insuring, in 

 certain particular cases, the inclusion of a certain factor or group of 

 factors either in every functional gamete or in every viable zygote. 



No doubt, as Davis (1917) says, "A great forward step will be 

 taken in Oenothera genetics when types of proven purity have been 

 established . . . ." Meanwhile, cases of "Oenothera-like" heredity in 

 species known to possess the Mendelian mechanism deserve most 

 thorough investigation. Special interest consequently attaches to the 

 peculiar inheritance of certain apparent mutations of the ten-weeks 

 stock (Matthiola anmia Sweet), a species in which various character- 

 istics are typically Mendelian. A remarkable series of aberrant forms 

 in this species 3 has been briefly discussed in two preliminary com- 

 munications (Frost, 1912 and 1916), and the present paper gives a 

 fuller account of the same phenomena. 4 



sin the variety "Snowflake, " a glabrous, double-producing form with white 

 flowers. 



* While this paper was in press Blakeslee and Avery (1919), have reported 

 the occurrence of apparent mutations in Datura, which seem to be similar in 

 almost every respect to those here discussed. 



F83] 



