This
will be best illustrated by a few of the examples furnished us by Mr.
Darwin. The two distinct species of plants, Mirabilis jalapa and M.
longiflora, can be easily crossed, and will produce healthy and fertile
hybrids when the pollen of the latter is applied to the stigma of the
former plant. But the same experimenter, Koelreuter, tried in vain, more
than two hundred times during eight years, to cross them by applying the
pollen of M. jalapa to the stigma of M. longiflora. In other cases two
plants are so closely allied that some botanists class them as varieties
(as with Matthiola annua and M. glabra), and yet there is the same great
difference in the result when they are reciprocally crossed.

_Individual Differences in respect to Cross-Fertilisation._

A still more remarkable illustration of the delicate balance of
organisation needful for reproduction, is afforded by the individual
differences of animals and plants, as regards both their power of
intercrossing with other individuals or other species, and the fertility
of the offspring thus produced. Among domestic animals, Darwin states
that it is by no means rare to find certain males and females which will
not breed 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 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 fe