the size of the roots, stems,
&c., of his hybrids, as the result of a sort of compensation due to
their sterility, in the same way as many emasculated animals are larger
than the perfect males. This view seems at first sight extremely
probable, and has been accepted by various authors;[299] but
Gaertner[300] has well remarked that there is much difficulty in fully
admitting it; for with many hybrids there is no parallelism between the
degree of their sterility and their increased size and vigour. The most
striking instances of luxuriant growth have been observed with hybrids
which were not sterile in any extreme degree. In the genus Mirabilis,
certain hybrids are unusually fertile, and their extraordinary
luxuriance of growth, together with their enormous roots,[301] have
been transmitted to their progeny. The increased size of the hybrids
produced between the fowl and pheasant, and between the distinct
species of pheasants, has been already noticed. The result in all cases
is probably in part due to the saving of nutriment and vital force
through the sexual organs not acting, or acting imperfectly, but more
especially to the general law of good being derived from a cross. For
it deserves especial attention that mongrel animals and plants, which
are so far from being sterile that their fertility is often actually
augmented, have, as previously shown, their size, hardiness, and
constitutional vigour generally increased. It is not a little
remarkable that an accession of vigour and size should thus arise under
the opposite contingencies of increased and diminished fertility.

It is a perfectly well ascertained fact[302] that hybrids will
invariably breed more readily with either pure parent, and not rarely
with a distinct species, than with each other. Herbert is inclined to
explain even this fact by the advantage derived from a cross; but
Gaertner more justly accounts for it by the pollen of the hybrid, and
probably its ovules, being in some degree vitiated, whereas the pollen
and ovules of both pure parents and of any third species are sound.
Nevertheless there are some well-ascertained and remarkable facts,
which, as we shall immediately see, show that the act of crossing in
itself undoubtedly tends to increase or re-establish the fertility of
hybrids.

_On certain Hermaphrodite Plant