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 incompatibility, 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.

Koelreuter 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.