244 PROBLEMS OF GENETICS 



indeed is the evidence from favouring the belief that such forms 

 are in any way transitional or indeterminate, that, as is well 

 known, Jordan used it with every plausibility to support the 

 doctrine of the fixity of species. To certain aspects of Jordan's 

 work we will return later in this chapter, but the matter is in 

 the present connection of especial interest for the reason that 

 Rosen has lately found by experiment that some of these pre- 

 sumably very closely allied forms, crossed together, gave hybrids 

 more or less sterile. In the case of the offspring of one pair of 

 forms only (E. cochleata and stricta) was the fertility undi- 

 minished, and the various degrees of sterility found in the 

 other crosses ranged up to the extreme infertility of the hybrids 

 between E. stricta X data. From this cross ten plants were 

 bred. Of these the four strongest were chosen to breed from, 

 but two of the four proved totally sterile ; one had only bad seeds ; 

 and from the fourth a single seedling was raised which in its 

 turn proved to be sterile. From the less sterile hybrids F2 

 families were raised, with the usual experience that in this and 

 subsequent generations the sterility diminished among extracted 

 forms, new and true-breeding types with complete fertility being 

 thus derived from the original cross. 11 



The production of sterility as a consequence of crossing 

 plants so nearly approaching each other as these Erophila 

 " species " do is not a little interesting, and the fact well ex- 

 emplifies the futility of the various attempts to frame general 

 expressions as to specific properties or behaviour. Commenting 

 on his results Rosen argues that the polymorphic group commonly 

 called by systematists Erophila (Draba) verna may now be 

 regarded as having arisen by crossing, as did his own types men- 

 tioned above. The question, however, what species were the 

 original progenitors of the group cannot be answered. Rosen 

 considers that no form which he knows satisfies the requirements, 



11 One very peculiar feature was observed, namely, that all the new forms in Fa 

 which were bred from came true. As I understand, this statement applied to five 

 such new types, and they were represented by 76 individuals in F3, but further 

 details on this point are desirable. Another curious fact was observed, namely 

 that one of the Fi forms (cochleata X radiata) when fertilised by cochleata gave a 

 highly polymorphic family, but fertilised by radiata the resulting offspring were 

 almost uniform. 



