156 DARWINISM chap. 



together, though both are known to be perfectly fertile with 

 other males and females. Cases of this kind have occurred 

 among horses, cattle, pigs, dogs, and pigeons ; and the 

 experiment has been tried so frequently that there can be no 

 doubt of the fact. Professor G. J. Romanes states that he 

 has a number of additional cases of this individual incom- 

 patibility, or of absolute sterility^ between two individuals, 

 each of which is perfectly fertile with other individuals. 



During the numerous exj^eriments that have been made 

 on the h}'1);idisation of plants similar peculiarities have been 

 noticed, some individuals being capable, others incapable, of 

 being crossed with a distinct species. The same individual 

 peculiarities are found in varieties, species, and genera. 

 Kolreuter crossed five varieties of the common tobacco 

 (Nicotiana tabacum) with a distinct species, Nicotiana 

 glutinosa, and they all yielded very sterile hybrids ; but 

 those raised from one variety were less sterile, in all the 

 experiments, than the hybrids from the four other varieties. 

 Again, most of the species of the genus Nicotiana have been 

 crossed, and freely produce hybrids ; but one species, N. 

 acuminata, not particularly distinct from the others, could 

 neither fertilise, nor be fertilised by, any of the eight other 

 species experimented on. Among genera we find some — 

 such as Hippeastrum, Crinum, Calceolaria, Dianthus — almost 

 all the species of which will fertilise other species and produce 

 hybrid offspring ; while other allied genera, as Zejjhyranthes 

 and Silene, notwithstanding the most persevering efforts, have 

 not produced a single hybrid even between the most closely 

 allied species. 



Dimorphism and Trimorphism. 



Peculiarities in the reproductive system aff'ecting indi- 

 viduals of the same species reach their maximum in what are 

 called heterostyled, or dimorphic and trimorphic floAvers, 

 the i)henoniena presented by which form one of the most 

 lemarkable of Mr. Dar^vin's many discoveries. Our common 

 cowslip and primrose, as well as many other species of the 

 genus Primula, have two kinds of flowers in about equal 

 I)roportions. In one kind the stamens are short, being 

 situated about the middle of the tube of the corolla, while the 



