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 used as the father and then as the mother, 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 these
two powers do not at all necessarily go together. There are certain
hybrids which instead of having, as is usual, an intermediate character
between their two parents, always closely resemble one of them; and
such hybrids, though externally so like one of their pure
parent-species, are with rare exceptions extremely sterile. So again
amongst hybrids which are usually intermediate in structure between
their parents, exceptional and abnormal individuals sometimes are born,
which closely resemble one of their pure parents; and these hybrids are
almost always utterly sterile, even when the other hybrids raised from
seed from the same capsule have a considerable degree of fertility.
These facts show how completely fertility in the hybrid is independent
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