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 concern 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 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 presumably 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 undiminished, and the various degrees of sterility found in the other crosses ranged up to the extreme infertility of the hybrids between _E. stricta_ × _elata_. 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 F_{2} 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]