which are devoured by
insects.

The main facts and many of the details, as to the relation of insects to
flowers, were discovered by Sprengel in 1793. He noticed the curious
adaptation of the structure of many flowers to the particular insects
which visit them; he proved that insects do cross-fertilise flowers, and
he believed that this was the object of the adaptations, while the
presence of nectar and pollen ensured the continuance of their visits;
yet he missed discovering the _use_ of this cross-fertilisation. Several
writers at a later period obtained evidence that cross-fertilisation of
plants was a benefit to them; but the wide generality of this fact and
its intimate connection with the numerous and curious adaptations
discovered by Sprengel, was first shown by Mr. Darwin, and has since
been demonstrated by a vast mass of observations, foremost among which
are his own researches on orchids, primulas, and other plants.[143]

By an elaborate series of experiments carried on for many years Mr.
Darwin demonstrated the great value of cross-fertilisation in increasing
the rapidity of growth, the strength and vigour of the plant, and in
adding to its fertility. This effect is produced immediately, not as he
expected would be the case, after several generations of crosses. He
planted seeds from cross-fertilised and self-fertilised plants on two
sides of the same pot exposed to exactly similar conditions, and in most
cases the difference in size and vigour was amazing, while the plants
from cross-fertilised parents also produced more and finer seeds. These
experiments entirely confirmed the experience of breeders of animals
already referred to (p. 160), and led him to enunciate his famous
aphorism, "Nature abhors perpetual self-fertilisation".[144] In this
principle we appear to have a sufficient reason for the various
contrivances by which so many flowers secure cross-fertilisation, either
constantly or occasionally. These contrivances are so numerous, so
varied, and often so highly complex and extraordinary, that they have
formed the subject of many elaborate treatises, and have also been amply
popularised in lectures and handbooks. It will be unnecessary,
therefore, to give details here, but the main facts will be summarised
in order to call attention to some difficulties of the theory which seem
to require further elucidation.

_Modes of securing Cross-Fertilisation._

When we examine the various modes in whic