It opened up the new study areas of pollination research and reproductive ecology, directly related to Darwin's ideas on evolution, and supported his view that natural selection led to a variety of forms through the important benefits achieved by cross-fertilisation.

By showing how practical adaptations develop from cumulative minor variations of parts of the flowers to suit new purposes, Darwin countered the prevailing view that beautiful organisms were the handiwork of a Creator.

[3] The little known book, published in 1793 by Christian Konrad Sprengel but never translated into English, introduced the idea that flowers were created by God to fulfill a teleological purpose: insects would act as "living brushes" to cross-fertilise plants in a symbiotic relationship.

He welcomed its support for his supposition that cross-fertilisation in flowering plants tended to allow their offspring to avoid possible disadvantages resulting from self-fertilisation,[7] and by 1845 he had verified many of Sprengel's observations.

[12] He experimented with insect pollination to investigate whether, by cross-fertilising field crops such as Fabaceae, they would yield more vigorous offspring, and published letters about his inconclusive results in The Gardeners' Chronicle in 1857 and 1858.

[11] In this book, he gave credence to Sprengel's ideas on the advantages of "intercrossing",[14] and noted: "Many of our orchidaceous plants absolutely require the visits of moths to remove their pollen masses and thus to fertilise them".

[15] He introduced his new concept, the process of coevolution, describing the co-adaptation of bumblebees and red clover, and speculating "how a flower and a bee might slowly become, either simultaneously or one after the other, modified and adapted in the most perfect manner to each other, by the continued preservation of individuals presenting mutual and slightly favourable deviations of structure".

[16][17] After On the Origin of Species was published, Darwin became involved in producing revised editions as well as working on Variation of Animals and Plants Under Domestication as the first part of his planned "Big Book".

He investigated other botanical questions raised by his ideas of natural selection, including the advantages of sexual dimorphism in primulas, and the adaptive mechanisms that ensure cross-pollination in orchids.

Darwin's aim was to show how the complex structures and life cycles of the plants could be explained by natural selection rather than by the handiwork of God; he saw the huge variety of flowers as a collection of ad hoc evolutionary adaptations.

Darwin wrote: I have, owing to many interruptions, not been going on much with my regular work (though I have done the very heavy jobs of variation of Pigeons, Fowls, Ducks, Rabbits Dogs &c) but have been amusing myself with miscellaneous work.—I have been very lucky & have now examined almost every British Orchid fresh, & when at sea-side shall draw up rather long paper on the means of their fertilisation for Linn.

[37] He sought advice on obtaining the exotic South American Catasetum, to see it eject pollen masses, as "I am got intensely interested on subject & think I understand pretty well all the British species.

For example, Thomas Henry Huxley was strongly influenced by German idealism and in 1856 had asked if it was "to be supposed for a moment that the beauty of colour and outline ... are any good to the animals? ...

On the recommendation of John Lindley, Darwin wrote to Lady Dorothy Nevill, who responded generously by sending numerous exotic orchids, and requested a signed photograph of him to hang in her sitting room next to portraits of her other notable friends, including Hooker.

[48] He was particularly astounded by the long spur of the Angraecum sesquipedale flowers, one of the orchids sent by the distinguished horticulturist James Bateman,[49] and wrote to Hooker "Good Heavens what insect can suck it[?

]"[50] By November, a specimen of the exotic South American Catasetum orchid Hooker had given to Darwin had shown its "truly marvellous" mechanism, by which it shot out a pollinium at any insect touching a part of the flower "with sticky gland always foremost".

One of Darwin's correspondents told of delight at growing a beautiful specimen of Myanthus barbatus imported from Demerara, then dismay when the plant flowered the next year as a simple Catasetum.

[1] Darwin set out a detailed study of common descent with modifications by expanding on the theme of coevolution between local populations of insects and flowering plants that he had briefly discussed in On the Origin of Species.

[64][65] This wastefulness is familiar in modern terms as the idea of an evolutionary arms race, but was disturbing to biologists of the time who believed that adaptations were the outcome of benevolent divine purpose.

"[72] He was almost exasperated by the inventiveness of forms in nature; "In my examination of Orchids, hardly any fact has so much struck me as the endless diversity of structure,—the prodigality of resources,—for gaining the very same end, namely, the fertilisation of one flower by the pollen of another.

[76][77] Daniel Oliver thought it "very extraordinary", and even Darwin's old beetle-hunting rival Charles Babington, by then professor of botany at the University of Cambridge and inclined to oppose natural selection, called it "exceedingly interesting and valuable ... highly satisfactory in all respects.

"[78][79] George Bentham praised its value in opening "a new field for observing the wonderful provisions of Nature ... a new and unexpected track to guide us in the explanation of phenomena which had before that appeared so irreconcilable with the ordinary prevision and method shown in the organised world.

[55] In his address in 1863 he stated that "Mr Darwin has shown how changes may take place", and described it as "an unimpeachable example of a legitimate hypothesis" in compliance with John Stuart Mill's scientific method.

[84][87] Michael Ghiselin has expressed the view that all studies of coevolution follow directly or indirectly from Darwin's orchid book, which was also the origin of all work on the evolution of extreme specialisation.

By comparing related plant species that he thought had diverged in form from a common ancestor, and testing whether they were visited by butterflies or bees, he was the first to use a combination of morphological and ecological approaches to understand patterns in the evolution of interactions and specialisation.

[88] The early development of ideas on specialisation and coevolution became increasingly focused on the problem of mimicry; Henry Walter Bates had initially raised this issue in a paper read to the Linnean Society of London in December 1861 in Darwin's presence, and published in November 1862.

[89][90] Others basing their studies of reproductive ecology on Darwin's evolutionary approach included Friedrich Hildebrand and Severin Axell in Europe, Asa Gray and Charles Robertson in North America.

[91] In 1874, Asa Gray paid tribute to Darwin's work on orchids for explaining "all these and other extraordinary structures, as well as of the arrangement of blossoms in general, and even the very meaning and need of sexual propagation".

Darwin had been given the use of a hot-house at The Rookery on the other side of the village,[33] and at the end of 1862 he was persuaded by this neighbour's helpful gardener to have his own built at Down House as an extension to the existing cold lean-to greenhouse.

[97] A chance observation "thoroughly aroused" Darwin's attention to a surprising decrease in vigour of the offspring of Linaria vulgaris following only one instance of self-fertilisation, and after eleven years of experimental work he published The Effects of Cross and Self Fertilisation in the Vegetable Kingdom in 1876 as "a complement to the 'Fertilisation of Orchids,' because it shows how important are the results of cross-fertilisation which are ensured by the mechanisms described in that book.