But the
correspondence between systematic affinity and the facility of
crossing is by no means strict. A multitude of cases could be given of
very closely allied species which will not unite, or only with extreme
difficulty; and on the other hand of very distinct species which unite
with the utmost facility. In the same family there may be a genus, as
Dianthus, in which very many species can most readily be crossed; and
another genus, as Silene, in which the most persevering efforts have
failed to produce between extremely close species a single hybrid.
Even within the limits of the same genus, we meet with this same
difference; for instance, the many species of Nicotiana have been more
largely crossed than the species of almost any other genus; but
Gartner found that N. acuminata, which is not a particularly distinct
species, obstinately failed to fertilise, or to be fertilised by, no
less than eight other species of Nicotiana. Very many analogous facts
could be given.
No one has been able to point out what kind, or what amount, of
difference in any recognisable character is sufficient to prevent two
species crossing. It can be shown that plants most widely different in
habit and general appearance, and having strongly marked differences
in every part of the flower, even in the pollen, in the fruit, and in
the cotyledons, can be crossed. Annual and perennial plants, deciduous
and evergreen trees, plants inhabiting different stations and fitted
for extremely different climates, can often be crossed with ease.
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