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. langsdorffii, 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 FI 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 shown in Table XXXVII. 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- 



TABLE XXXVII. PERCENTAGES OP HYBRID OFFSPRING WITH MANY MOBILE SPERM 



IN MATINGS OF FEMALE HYBRIDS WITH TAME GUINEA-PIGS, AND FEMALE 



HYBRIDS WITH FERTILE MALE HYBRIDS (After Detlefsen) 



