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of the same capsule and exposed to
exactly
....
the same conditions.
By the term systematic affinity is meant, the
general resemblance
resemblance
between species in structure and
in
in
constitution.
constitution,
more
more
especially
especially
in
in
the
the
structure
structure
of
of
parts
parts
which
which
are
are
of
of
high
high
physiological
physiological
importance
importance
and
and
which
which
differ
differ
little
little
in
in
the
the
allied
allied
species.
species.
Now the fertility of first
crosses,
crosses
between species,
between species,
and of the hybrids produced from them, is largely governed by their systematic affinity. This is clearly shown by hybrids never having been raised between species ranked by systematists in distinct families; and on the other hand, by very closely allied species generally uniting with facility. 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 Gärtner found that N. acuminata, which is not a particularly distinct species, obstinately failed to fertilise, or to be fertilised
by
by,
no less than eight other species of Nicotiana.
Very
....
many
Many
analogous facts could be given.
No one has been able to point out what
kind
kind,
or what
amount
amount,
of
difference,
difference
in any recognisable
character,
character
is sufficient to prevent two species crossing. It can be shown that plants most widely different in habit and