The experiments carried on during several successive generations.
Nature of the relationship of the plants in the later generations.
Uniformity of the conditions to which the plants were subjected.
Some apparent and some real causes of error.
Amount of pollen employed.
Arrangement of the work.
Importance of the conclusions.

There is weighty and abundant evidence that the flowers of most kinds of
plants are constructed so as to be occasionally or habitually
cross-fertilised by pollen from another flower, produced either by the
same plant, or generally, as we shall hereafter see reason to believe,
by a distinct plant. Cross-fertilisation is sometimes ensured by the
sexes being separated, and in a large number of cases by the pollen and
stigma of the same flower being matured at different times. Such plants
are called dichogamous, and have been divided into two sub-classes:
proterandrous species, in which the pollen is mature before the stigma,
and proterogynous species, in which the reverse occurs; this latter form
of dichogamy not being nearly so common as the other.
Cross-fertilisation is also ensured, in many cases, by mechanical
contrivances of wonderful beauty, preventing the impregnation of the
flowers by their own pollen. There is a small class of plants, which I
have called dimorphic and trimorphic, but to which Hildebrand has given
the more appropriate name of heterostyled; this class consists of plants
presenting two or three distinct forms, adapted for reciprocal
fertilisation, so that, like plants with separate sexes, they can hardly
fail to be intercrossed in each generation. The male and female organs
of some flowers are irritable, and the insects which touch them get
dusted with pollen, which is thus transported to other flowers. Again,
there is a class, in which the ovules absolutely refuse to be fertilised