GENETICS: B. M. DAVIS 
707 
progeny in the first generation fall into two groups sharply separated 
from one another and these De Vries termed 'twin hybrids.' Since the 
twin hybrids are produced in crosses of Lamarckiana with several species 
some of which when crossed among themselves give uniform progeny in 
the first generation the evidence indicates that Lamarckiana must supply 
the two different types of gametes which m^ake possible this splitting in 
the first generation. De Vries holds that the cause of twin hybrids lies 
in the state within the gametes of certain factors called pangens whether 
active, inactive or labile and this appears to be an admission that La- 
marckiana does not form equivalent gametes. 
Long experience of plant and animal breeders has led them to suspect 
that pronounced sterility in an organism indicates hybrid constitution 
and critics of the purity of Oenothera Lamarckiana have pressed the 
point that in this plant approximately one-half of the pollen grains and 
ovules abort and that the proportions of fertile seed are low, being from 
about 30 to 40 %. 
Extensive studies of Geerts followed by observations of other workers 
have shown these conditions to be generally characteristic of species of 
Oenothera and allied genera. These facts indicate the necessity of 
detailed studies on the cytology of gametogenesis, fertilization and 
embryo formation. Thus if it could be shown that in every group of 
four pollen grains, tetrad, formed as the result of the reduction mitoses 
only two grains are perfect the conclusion would be justified that the 
pollen sterility was the result of this segregation division. Unfortu- 
nately the abortion of pollen grains takes place after the members of 
the tetrad have separated and the relation of sterile pollen grains to one 
another and to the perfect grains is not evident, but it is a fact that 
shriveled, sterile pollen is distributed among the perfect grains so evenly 
as to suggest an origin through the reduction division rather than from 
some physiological cause such as malnutrition, which under certain 
conditions is known to produce high degrees of sterility . 
The facts of gametic and zygotic or seed sterility have, however, 
suggested certain working hypotheses that must be considered in present 
and future research on Oenotheras. Thus it is possible to conceive of 
impure or heterozygous species capable of reproducing their lines (breed- 
ing true) and showing little or none of the phenomenon of hybrid split- 
ting provided only such gametes and seeds are fertile as will reproduce 
the hybrid type. Renner has applied this line of reasoning to Lamarck- 
I , iana by assuming that the small proportion of fertile seeds of this 
plant are those formed by the union of the two different types of gametes 
