312 CENOTHERA AND THE MUTATION THEORY 



in all that show this peculiar behavior. It may be 

 supposed that the eggs that contain the chromo- 

 some carrying the dominant genes distinguishing 

 these species from each other all die, and that the 

 pollen containing the chromosome that carries the 

 recessive genes also dies. That is, we are dealing 

 with a case of gametic lethals, the result of which is 



Fig. 73.— Twin hybrids; a, Oenothera laeta, and b, Oenothera 

 velutina. (After de Vries.) 



to make the race in question breed constant though 

 heterozygous. 



When 0. biennis, 0. syrticola, or certain . other 

 species are fertilized by Lamarckiana or by certain 

 of its "mutants" (e.g., nanella, rubrinervis, or ob- 

 longa), the first generation hybrids are of two 'dis- 

 tinct types, called by de Vries "laeta" and "velutina." 

 (Fig. 73.) This phenomenon was shown by deVries 

 to be due to a peculiarity of Lamarckiana and 



