terostyled plants. The discoveries which Darwin
made in the course of his investigations of these plants belong to the
most brilliant in biological science.

The case of Primula is now well known. C.K. Sprengel and others were
familiar with the remarkable fact that different individuals of the
European species of Primula bear differently constructed flowers; some
plants possess flowers in which the styles project beyond the stamens
attached to the corolla-tube (long-styled form), while in others the
stamens are inserted above the stigma which is borne on a short style
(short-styled form). It has been shown by Breitenbach that both forms of
flower may occur on the same plant, though this happens very rarely. An
analogous case is occasionally met with in hybrids, which bear flowers
of different colour on the same plant (e.g. Dianthus caryophyllus).
Darwin showed that the external differences are correlated with others
in the structure of the stigma and in the nature of the pollen.
The long-styled flowers have a spherical stigma provided with large
stigmatic papillae; the pollen grains are oblong and smaller than those
of the short-styled flowers. The number of the seeds produced is smaller
and the ovules larger, probably also fewer in number. The short-styled
flowers have a smooth compressed stigma and a corolla of somewhat
different form; they produce a greater number of seeds.

These different forms of flowers were regarded as merely a case of
variation, until Darwin showed "that these heterostyled plants are
adapted for reciprocal fertilisation; so that the two or three forms,
though all are hermaphrodites, are related to one another almost
like the males and females of ordinary unisexual animals." ("Forms
of Flowers" (1st edition), page 2.) We have here an example of
hermaphrodite flowers which are sexually different. There are essential
differences in the manner in which fertilisation occurs. This may
be effected in four different ways; there are two legitimate and two
illegitimate types of fertilisation. The fertilisation is legitimate
if pollen from the long-styled flowers reaches the stigma of the
short-styled form or if pollen of the short-styled flowers is brought
to the stigma of the long-styled flower, that is the organs of the
same length of the two different kinds of flower react on one
another. Illegitimate fertilisation is represented by the two kinds of
self-fertilisation, also by cross-fertilisation, in