ATKINSON— TWIN HYBRIDS. 141 



give twin hybrids in the first generation, with sphtting of one of the 

 twins in the second generation (de Vries, 1913, p. 131). The re- 

 sults of reciprocal crosses of CE. franciscana wath Q^. pycnocarpa are 

 a close parallel, and indicate that the genetic constitutions of (E. 

 franciscana and CE. hookeri are very similar. 



Summary and Conclusion. 



The history of the progeny of the crosses of CEnothera lamarcki- 

 ana and CE. franciscana with CE. pycnocarpa belongs in the series of 

 some of the most interesting phenomena of hybridization known in 

 the CEnotheras, and discovered by de Vries (1907, 1908, 1909, 

 1913). These phenomena are, first, the production of twin hybrids 

 in the first generation of a cross, the two hybrids being fixed in the 

 Fi generation and continuing to reproduce themselves in the F^ and 

 succeeding generation; and, second, the production of twin hybrids 

 in the F^ with one-sided splitting in the Fo, and succeeding genera- 

 tions. In the second case one of the twin hybrids of the first gen- 

 eration is fixed in the F,, the other splits in the Fo, into two types, 

 like the twnns of the Fj, one of which is fixed while the other splits 

 in the Fo and so on. In the crosses of CEnothera lamarckiana and 

 CE. franciscana with CE. pycnocarpa, the cultures have not been 

 carried beyond the second generation. But the existence of this 

 peculiar phenomenon of Inheritance in certain crosses among the 

 evening primroses has been so thoroughly demonstrated by de Vries 

 for several succeeding generations that there can be no reasonable 

 doubt that it applies also to the behavior of the crosses here de- 

 scribed, for succeeding generations. 



In the reciprocal crosses of CE. lamarckiana with CE. pycnocarpa 

 the twin hybrids are fixed in the first generation, and " breed true " 

 in the second, and in all probability, in succeeding generations. 

 Each of the twin hybrids is very uniform, at least in the rosette 

 stage, and shows a minimum of fluctuating variations. The pycno- 

 carpa twin type is a physiological homozygote but its fundamental 

 heterozygotic constitution is now and then manifested bv the salta- 

 tory production of lamarckiana forms, and also of forms approach- 

 ing pycnocarpa, or the lamarckiana twin type. The reaction system 



