HYBRIDISATION. 917 



containing embryos, but these embryos having no power of germination. Further 

 steps are indicated by the number of embryos which have the power of germination 

 that are produced in the ovary ^ 



5. When pollen from different species is applied simultaneously to the same 

 stigma, only one kind is potent, viz. that from the species which has the greatest 

 sexual affinity to the one that is pollinated. And since, as a general law, pollen is 

 most efficacious on a different flower of the same species — in other words, the highest 

 degree of sexual affinity occurs between different individuals of the same species — 

 when a stigma is pollinated at the same time with pollen of the same and of another 

 species, the first only is potent. But since, on the other hand, hybrids are sometimes 

 more easily produced between varieties than between individuals of the same variety, 

 in this case the foreign pollen may be prepotent over that of the same kind. When 

 the pollen of different species reaches the stigma at the same time, and if that which 

 reaches it later has a greater sexual affinity, it can only be potent when the first is 

 not potent or acts injuriously. In Nicotiana the production of hybrids can no longer 

 be prevented by its own pollen after two hours, in Malva and Hibiscus after three 

 hours, in Dianthus after five or six hours. 



6. The hybrid is possessed of external characters intermediate between those of 

 its parent-forms, usually nearly half way between ; less often it resembles one of the 

 parent-forms more nearly than the other, and this is more often the case with variety- 

 hybrids than with species-hybrids. It follows that in reciprocal hybrids from the 

 species A and B, the hybrid A B is generally similar externally to the hybrid B A, 

 though the two forms may differ somewhat internally. Thus, according to Gartner, 

 the hybrid Nicotiana paniculato-rustica is more fertile than the reciprocal hybrid 

 Nicotiana rustico-paniculata'^. An internal difference between reciprocal hybrids is 

 also shown by the fact that one is more variable than the other ; thus, according to 

 Gartner, the progeny of Digitalis purpureo-lutea is more variable than that of D. luteo- 

 purpurea, the progeny of Dianthus pulchello-arenarius more variable than that of 

 D. arenario-pulchellus. 



When two species A and B hybridise, and the one species A exercises a 

 greater influence on the form and properties of the hybrid than the other species 

 B, the hybrid or its descendants, if fertilised by A, will revert more quickly to the 

 parent-form A than it will to the parent-form B if fertilised by it. Thus Gartner 

 states that the hybrid of Dianthus chinensis and D. Caryophyllus reverts to the 

 latter form after three or four generations if repeatedly fertilised by it, while it 

 requires fertilisation for five or six generations by D. chinensis in order to revert to 

 that form. 



7. The characteristics of the parent-forms are as a rule so transmitted to the 

 hybrid that the influence of both is manifested in all its characters, producing a 

 fusion of the different peculiarities. This is more evident in the species- than in 

 the variety-hybrids ; in the latter some of the non-essential characters of the parents 

 sometimes present themselves in the offspring uncombined side by side ; e.g. various 



^ See Hildebrand, Bastardirungsversuche an Orchideen, Bot. Zeit. 1865, No. 31. 



^ In this mode of designating hybrids, the name of the male parent-plant stands first; thus 

 Nicotiana rustico-paiiiciilala is the product of the fertilisation of N. paniculata by the pollen of 

 N. riisiica. 



