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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.


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