GENETICS: B. M. DAVIS 
709 
mediate characters such as frequently appear in the first generation. 
Second generations in some cases breed fairly true, in others show ex- 
tensive and characteristic splitting usual to hybrids. Back crosses in 
some combinations reproduce almost exactly one of the parent types. 
Shull in a remarkable series of crosses has obtained in the first genera- 
tion polymorphic progenies of much greater complexity than the twin 
hybrids of De Vries. Atkinson has described quadruple hybrids in the 
first generation from crosses between two wild American species. Bart- 
lett has found that selfed lines of certain American wild species may 
throw 'mutants' in proportions as high as 50, 80 or even 100 per cent 
of the cultures. 
Explanations of these extraordinary types of behavior are not yet 
clear. Atkinson, insisting on the purity of the species with which he 
worked, proposes a view that multiple progenies in the first generation 
are determined by the selection or differentiation of factors in the fer- 
tilized egg or zygote, an hypothesis which will require cytological evi- 
dence to be convincing. Bartlett holds the view that different classes 
of gametes are formed, an attitude in accord with much evidence from 
various studies on animals and plants. This seems to the writer to 
be an admission of impurity of germinal constitution unless it be shown 
that gametes may mutate in immense proportions, a view which has 
no support from genetical and cytological studies in general. None 
of these investigators seem inclined to admit the possibility or proba- 
bility that the complexities of Oenothera genetics may be the result of 
germinal impurity widespread among the species as the result of exten- 
sive hybridization. 
Yet the high degrees of gametic and zygotic sterility now known for 
certain forms of Oenothera makes it possible to conceive these species 
as impure, maintaining themselves because for the most part only those 
gametic combinations are fertile which will reproduce the heterozygous 
condition. Variants in selfed lines from such stock would most nat- 
urally be interpreted not as mutations but as the result of other gametic 
combinations which prove to be fertile. Crosses between impure spe- 
cies are, of course, crosses between hybrids and the behavior of their 
progeny, especially when high degrees of sterility are present, would 
naturally be expected to prove unusual and irregular. Only recently 
has the importance of sterility and delayed seed germination received 
serious consideration in problems of Oenothera genetics and no investi- 
gations have as yet been published which give the matters full 
consideration. 
