ow sterile these latter generally are. When, on the other
hand, a hybrid is crossed with either pure parent-species, the sterility is
usually much lessened: and so it is when an illegitimate plant is
fertilised by {183} a legitimate plant. In the same manner as the sterility
of hybrids does not always run parallel with the difficulty of making the
first cross between the two parent species, so the sterility of certain
illegitimate plants was unusually great, whilst the sterility of the union
from which they were derived was by no means great. With hybrids raised
from the same seed-capsule the degree of sterility is innately variable, so
it is in a marked manner with illegitimate plants. Lastly, many hybrids are
profuse and persistent flowerers, whilst other and more sterile hybrids
produce few flowers, and are weak, miserable dwarfs; exactly similar cases
occur with the illegitimate offspring of various dimorphic and trimorphic
plants.

Altogether there is the closest identity in character and behaviour between
illegitimate plants and hybrids. It is hardly an exaggeration to maintain
that the former are hybrids, but produced within the limits of the same
species by the improper union of certain forms, whilst ordinary hybrids are
produced from an improper union between so-called distinct species. We have
already seen that there is the closest similarity in all respects between
first illegitimate unions, and first crosses between distinct species. This
will perhaps be made more fully apparent by an illustration: we may suppose
that a botanist found two well-marked varieties (and such occur) of the
long-styled form of the trimorphic _Lythrum salicaria_, and that he
determined to try by crossing whether they were specifically distinct. He
would find that they yielded only about one-fifth of the proper number of
seed, and that they behaved in all the other above-specified respects as if
they had been two distinct species. But to make the case sure, he would
raise plants from his supposed hybridised seed, and he would find that the
seedlings were miserably dwarfed and utterly sterile, and that they behaved
in all other respects like ordinary hybrids. He might then maintain that he
had actually proved, in accordance with the common view, that his two
varieties were as good and as distinct species as any in the world; but he
would be completely mistaken.

The facts now given on dimorphic and trimorphic plants are important,
bec