XI 
THE SPECIAL COLOURS OF PLANTS 
309 
as they are more complex and more varied in form and 
structure; yet there is some parallelism between them in both 
respects. Flowers are frequently adapted to attract insects 
as fruits are to attract birds, the object being in the former to 
secure cross-fertilisation, in the latter dispersal; while just 
as colour is an index of the edibility of fruits which supply 
pulp or juice to birds, so are the colours of flowers an indica¬ 
tion of the presence of nectar or of pollen 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-fertilisa¬ 
tion of plants Avas 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 
shoAvn by Mr. Danvin, and has since been demonstrated by a 
vast mass of observations, foremost among which are his own 
researches on orchids, primulas, and other plants. 1 
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 Avas 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, 
1 For a complete historical account of this subject with full references to 
all the works upon it, see the Introduction to Hermann Muller’s Fertilisation 
of Flowers, translated by D’Arcy W. Thompson. 
