248 GENETICS IN RELATION TO AGRICULTURE 
proportions of the ovules are non-functional. To this category of facts 
must be added the high percentage of seed sterility which is common 
in the genus. If any of this pollen, ovule, and seed sterility is selective, 
then obviously it will be impossible to analyze the progeny successfully, 
unless the exact nature of the non-functional gametes and zygotes may 
be determined. The importance of this point has been indicated in 
the explanation of the frequent occurrence of parental forms among 
the sesqui-hybrids of rye and wheat and of Nicotiana tabacum with N. 
sylvestris, and it has been definitely established for many cases of albinism 
in plants and for peculiar sex ratios and consequent disturbances of 
Mendelian ratios in Drosophila. Until, therefore, a satisfactory account 
can be given of the difficulties which have been enumerated above it 
will be impossible on the one hand to offer a satisfactory Mendelian 
interpretation of the Enothera investigations and illogical on the other 
hand to advance the results of these investigations as evidence of non- 
Mendelian inheritance. 
Moreover, considerable success has attended the efforts to produce 
by species hybridization strains of Ginothera which behave like lamarck- 
zana. It is not without significance that Davis has been able to pro- 
duce forms by crossing O. biennts and O. franctscana so much like 
lamarckiana as to be indistinguishable from it taxonomically. Tower 
also has taken pure species of Leptinotarsa, the Colorado potato beetle, 
and by mating them has produced strains which breed approximately 
true, but which under the stress of unusual conditions may throw off 
small percentages of aberrant forms. In his species crosses in Anti- 
rrhinum, Lotsy has reported the occurrence of races which give small 
proportions of aberrant forms. Since at present we have no certain 
knowledge that lamarckiana is not a form of hybrid origin and that its 
so-called mutants are not really segregants from a race possessing a 
peculiar hybrid constitution, these analogous cases assume considerable 
importance as an indication of the line of attack which may be followed 
for an explanation of the Ginothera phenomena. 
Conclusions.—-If we attempt to outline the present status of our 
knowledge of the phenomena of species hybridization, we see thus far 
no clear evidence of non-conformance to an explanation which is essen- 
tially Mendelian. The strict Mendelian explanation must be modified 
to take into account the peculiar relations which obtain in species hy- 
bridization. For an explanation of such relations the reaction system 
conception has been advanced. According to this conception the total 
set of factors in any species forms a reaction system in which the factors 
display harmonious interrelations with one another. Variety hybridi- 
zation, since it is concerned only with isolated differences in systems 
which are fundamentally identical, usually produces no disturbances in 
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