e
reproductive system. This difference in the result of reciprocal crosses
between the same two species was long ago observed by Koelreuter. 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
Koelreuter 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. Gaertner,
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 {259} 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 Gaertner: 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 of its external resemblance to either pure parent.

Considering the several rules now given, which govern the fertility of
first crosses and of hybrids, we see that when forms, which must be
considered as good and distinct species, are united, their fertility
graduates from zero to perfect fertility,