In 1862 he brought out his first botanical book, the "Fertilisation of Orchids," more fully entitled, "On the various Contrivances by which Orchids are Fertilised by Insects." These most singular flowers had long attracted great attention owing to their peculiar shapes and often their great beauty, while their marked deviation from typical forms of flowers perplexed botanists extremely. The celebrated Robert Brown, in a well-known paper in the Linnean Society's Transactions, 1833, expressed the belief that insects are necessary for the fructification of most orchids; and as far back as 1793, Christian Sprengel (in "The Newly Discovered Secret of Nature") gave an excellent account of the action of the several parts in the genus Orchis, having discovered that insects were necessary to remove the pollen masses. But the _rationale_ of the process was not fully known until Darwin revealed it, and illuminated it by the light of natural selection. He had, in the "Origin of Species," given reasons for the belief that it is an almost universal law of nature that the higher organic beings require an occasional cross with another individual. He here emphasised that doctrine by a series of proofs from a peculiar and otherwise inexplicable order of plants, and showed that the arrangements by which orchids are fertilised have for their main object the fertilisation of the flowers with pollen brought by insects from a distinct plant. In the group to which our common orchids belong, remarkable adaptations for securing that the pollen masses brought from another flower solely through the visits of insects shall reach their precise destination, were brought to light. "A poet," says Darwin, "might imagine that whilst the pollinia were borne through the air from flower to flower, adhering to an insect's body, they voluntarily and eagerly placed themselves in that exact position in which alone they could hope to gain their wish and perpetuate their race." As he had examined all the British genera, Darwin's conclusions were indubitable. He had patiently watched for hours on the grass to notice insects' visits, had counted the fertilised flowers on many spikes, the fertilised spikes on many plants, had dissected and redissected the flowers till he saw how the fertilisation must absolutely be effected; and utilising the enthusiasm of orchid growers, had excited them to do the same, till his storehouse of facts was full.