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.

By a reciprocal cross between two species, I mean the case, for
instance, of a female-ass being first crossed by a stallion, and then
a mare by a male-ass: these two species may then be said to have been
reciprocally crossed. There is often the widest possible difference
in the facility of making reciprocal crosses. Such cases are highly
important, for they prove that the capacity in any two species to cross
is often completely independent of their systematic affinity, that is
of any difference in their structure or constitution, excepting in their
reproductive systems. The diversity of the result in reciprocal crosses
between the same two species was long ago observed by Kolreuter. To give
an instance: Mirabilis jalapa can easily be fertilised by the pollen of
M. longiflora, and the hybrids thus produced are sufficiently fertile;
but Kolreuter tried more than two hundred times, during eight following
years, to fertilise reciprocally M. longiflora with the pollen of M.
jalapa, and utterly failed. Several other equally striking cases could
be given. Thuret has observed the same fact with certain sea-weeds
or Fuci. Gartner, moreover, found that this difference of facility in
making reciprocal crosses is extremely common in a lesser degree. He has
observed it even between closely related forms (as Matthiola annua
and glabra) which many botanists rank only as varieties. It is also a
remarkable fact that hybrids raised from reciprocal crosses, though of
course compounded of the very same two species, the one species having
first been used as the father and then as the mother, though they rarely
differ in external characters, yet generally differ in fertility in a
small, and occasionally in a high degree.

Several other singular rules could be given from Gartner: for instance,
some species have a remarkable power of crossing with other species;
other species of the same genus have a remarkable power of impressing
their likeness on their hybrid offspring; but th