ich 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.

By a reciprocal cross between two species, I mean the case, for
instance, of a stallion-horse being first crossed with a female-ass, and
then a male-ass with a mare: 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, or of any
recognisable difference in their whole organisation. On the other hand,
these cases clearly show that the capacity for crossing is connected
with constitutional differences imperceptible by us, and confined to the
reproductive system. This difference in the result of reciprocal crosses
between the same two species was long ago observed by Kolreuter. To give
an instance: Mirabilis jalappa 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.
jalappa, 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 forms so closely related (as Matthiola annua
and glabra) that many botanists rank them 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 u