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 experiments that have been made 
on the hybridisation 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 Zephyranthes 
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 affecting indi¬ 
viduals of the same species reach their maximum in what are 
called heterostyled, or dimorphic and trimorphic flowers, 
the phenomena presented by which form one of the most 
remarkable of Mr. Darwin’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 
proportions. In one kind the stamens are short, being 
situated about the middle of the tube of the corolla, while the 
