teleological purpose: insects would act as "living brushes" to cross-fertilise plants in a symbiotic relationship. This functional view was rejected and mostly forgotten, as it contradicted the common belief that flowers had been created for beauty, and were generally self-fertilising. For Darwin, the concept of evolution gave new meaning to Sprengel's research into the mechanisms for cross-fertilisation. 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, and by 1845 he had verified many of Sprengel's observations. A favourite walk took the Darwin family to a spot above the quiet Cudham valley, teeming with orchids including Cephalanthera, Neottia, fly orchids and musk orchids. They called this place "Orchis Bank", and the whole family became involved in Darwin's researches. Darwin observed orchids every summer, but in twenty years, only on two occasions (when he noticed butterflies "sucking O. pyramidalis and Gymnadenia") did he see insects visiting flowers. In 1854, Darwin began to work full-time on the origin of species. He examined orchids and counted how often one or both pollinia (pollen masses) had been removed from their flowers, indicating that they had been visited by insects. 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. He next applied Sprengel's methods to empirical research on orchids. Despite delays caused by recurring illness, he made progress on writing his planned "Big Book" on evolution, but when Alfred Russel Wallace's letter prompted joint publication of both of their theories of natural selection in 1858, Darwin quickly wrote On the Origin of Species as an abstract of his theory, published on 22 November 1859. In this book, he gave credence to Sprengel's ideas on the advantages of "intercrossing", and noted: "Many of our orchidaceous plants absolutely require the visits of moths to remove their pollen masses and thus to fertilise them". 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". This was a