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 (Enothera which behave like lamarck- 

 iana. It is not without significance that Davis has been able to pro- 

 duce forms by crossing 0. biennis and 0. franciscana 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 (Enothera 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 



