rtilised by, any of the eight other species
experimented on. Among genera we find some--such as Hippeastrum, Crinum,
Calceolaria, Dianthus--almost all the species of which will fertilise
other species and produce hybrid offspring; while other allied genera,
as Zephyranthes and Silene, notwithstanding the most persevering
efforts, have not produced a single hybrid even between the most closely
allied species.

_Dimorphism and Trimorphism._

Peculiarities in the reproductive system affecting individuals of the
same species reach their maximum in what are called heterostyled, or
dimorphic and trimorphic flowers, the phenomena presented by which form
one of the most remarkable of Mr. Darwin's many discoveries. Our common
cowslip and primrose, as well as many other species of the genus
Primula, have two kinds of flowers in about equal proportions. In one
kind the stamens are short, being situated about the middle of the tube
of the corolla, while the style is long, the globular stigma appearing
just in the centre of the open flower. In the other kind the stamens are
long, appearing in the centre or throat of the flower, while the style
is short, the stigma being situated halfway down the tube at the same
level as the stamens in the other form. These two forms have long been
known to florists as the "pin-eyed" and the "thrum-eyed," but they are
called by Darwin the long-styled and short-styled forms (see woodcut).

[Illustration: FIG. 17.--Primula veris (Cowslip).]

The meaning and use of these different forms was quite unknown till
Darwin discovered, first, that cowslips and primroses are absolutely
barren if insects are prevented from visiting them, and then, what is
still more extraordinary, that each form is almost sterile when
fertilised by its own pollen, and comparatively infertile when crossed
with any other plant of its own form, but is perfectly fertile when the
pollen of a long-styled is carried to the stigma of a short-styled
plant, or _vice versa_. It will be seen, by the figures, that the
arrangement is such that a bee visiting the flowers will carry the
pollen from the long anthers of the short-styled form to the stigma of
the long-styled form, while it would never reach the stigma of another
plant of the short-styled form. But an insect visiting, first, a
long-styled plant, would deposit the pollen on the stigma of another
plant of the same kind if it were next visited; and this is probably the
reason wh