SPECIES HYBRIDIZATION 235 
Sterility in crosses between apparently good species may be at. times 
almost completely lacking. Thus the crosses between Antirrhinum 
majus and A. molle and some other crosses made by Baur within the 
genus Antirrhinum proved fully fertile. The same condition has been 
found in other species crosses. Thus Nicotiana alata grandiflora and 
N. langsdorffiz, although they differ strikingly in their characters, give 
hybrids which are about as fertile as the parents. Certain orchid 
crosses are also reputed to display a high degree of fertility, but on the 
whole crosses between good species very rarely show even an approxi- 
mation to the full degree of fertility, and this is true of both plant and 
animal hybrids. 
The sterility displayed by species hybrids may not always be equiva- 
lent in both sexes. Thus one of Baur’s Antirrhinum crosses, that of A. 
majus X A. siculum proved completely sterile as far as the production 
of good ovules is concerned, but some good pollen grains are produced 
which can be used in back crosses to the parents. In the case of Cavia 
porcellus X C. rufescens, we have already noted that the males are 
sterile and the females fertile. Detlefsen attempted to follow out the 
inheritance of fertility in this case, and attacked the problem from 
many angles. The fertility of the females appears to be complete, 
since the Ff’; females produce litters of approximately the average number 
of young of those of the two parent species. The offspring of the 
hybrid females when crossed back to the tame guinea-pig again produce 
fertile females and sterile males. With each successive back cross to 
the tame guinea-pig the percentage of fertile males rises in a fairly 
regular fashion as is shownin Table XX XVII. Detlefsen points out for 
this case that the assumption that the wild species carries eight disturbing 
dominant factors gives a very close agreement with the observed re- 
Taste XXXVII.—PeErRcenTaAGES OF Hyprip OrrspriInG with Many MosiLe SPERM 
In Matinas or Femaue Hysrips with Tame GUINEA-PIGS, AND FEMALE 
Hyprips with Fertine Mate Hyprips (After Detlefsen) 
Offspring of female hybrids 
Offspring of female hybrids L 
i i and fertile male hybrids 
and guinea-pigs 
Generation ee | (Calculatedtor 
of females . eight factors 
INTABEE | arg tisetle sperm | IMimber | eaacy sacle “oor 
I evra cxnn es ooo 1 00.0 1 00.0 00.0 
| Oi: essere Ayn eae 8 00.0 2 00.0 00.4 
dD es tate renee ea 49 14.3 a 14.3 10.0 
| Ur bases ta epee 99 33.3 17 58.8 34.4 
| iss eo aU ea ee 150 60.7 11 3.6 | 59.7 
JBN 6 etcee aapatcees Os 49 CORTE eee hiked 
Ledeen tacts 15 73.3 1 100.0 88 .2 
Digitized by Microsoft® 
