312 GNOTHERA 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 O. biennis, O. 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 deVries “‘leta’’ and “velutina.”’ 
(Fig. 73.) This phenomenon was shown by deVries 
to be due to a peculiarity of Lamarckiana and 
