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.
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