XI THE SPECIAL COLOURS OF PLAXTS 309 



as they ?.re 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 was a benefit to them ; but the vdde generality 

 of this fact and its intimate connection with the numerous 

 and ciu-ious adaptations discovered by Sprengel, Avas 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.^ 



By an elaborate series of experiments carried on for many 

 years Mr. Dar^^^n 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 diff'erence 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, 



^ For a complete historical account of this subject Avith full references to 

 all the works upon it, see the Introduction to Hermann Miiller's Fertilisation 

 of Floicers, translated by D'Arcy W. Thompson, 



