STERILITY OF HYBRIDS 243 



and the hare have ever interbred. Many of the wild species of 

 Bos have been crossed and recrossed both with each other and 

 with many domesticated races, but I understand that no cross 

 with the Indian buffalo (Bos bubalus) has yet been successful 

 even in producing a live calf. 8 In the genus Primula many 

 hybrids are known and several of them occur in nature, but 

 hitherto no certain hybrid between P. sinensis and any other 

 species has been made, in spite of repeated attempts. 



In Nicotiana many doubtless all the various forms of N. 

 tabacum can be crossed together without diminution of fertility, 

 though some are very distinct in appearance, but crosses between 

 tabacum and sylvestris are highly sterile (in my experience totally 

 sterile 9 ) , though the distinctions between them are not to outward 

 observation nearly so great as those which can be found between 

 the various races of Primula sinensis. 



Recently some remarkable experiments bearing closely on 

 these questions have been published by F. Rosen. 10 They con- 

 cern the forms of Erophila (Draba) verna, celebrated in the history 

 of evolutionary theory as the plants especially chosen by Alexis 

 Jordan for the exposition of his views on these subjects. 



The " species " contains a profusion of forms dissimilar in 

 many structural characters, such as the size and shape of leaves, 

 flowers, fruits, etc. Of these forms many grow in association. 

 Jordan found, on experiment, that each, to the number of some 

 two hundred, bred true, and that therefore, the conventional 

 assumption that polymorphism of this kind must mean great 

 contemporary variability had no foundation in fact. So far 



8 This is a case of a somewhat different order and I mention it partly for that 

 reason as an illustration of the complexity which such negative instances may 

 present. The difficulty is that though the buffalo and the zebu can breed together, 

 the foetus is too large to be born alive. (See Ackermann Ber. d. Ver. f. Naturk., 

 Kassel, 1898, p. 69. Prof. S. Nathusius, of Halle, who has great experience in 

 crossing Bovidae, tells me that he has always failed to cross the buffalo with 

 other species.) 



9 In a paper to be published in the Report of the Genetic Conference, Paris, 

 1911, Bellair states that he obtained some partially fertile hybrids in the cross 

 N. sylvestris X tabacum. As to the various degrees of sterility in hybrids between 

 Nicotiana species see Lock, R. H., Ann. Roy. Bot. Gardens. Peradeniya, IV, 1909, 



P- 195- 



10 Beitrage zur Biol. der Pflanzen., X, 1911, p. 379. 



