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Rh collections, first at Cambridge for three months and then in London. His pocket-book for 1837 contains the words: “In July opened first note-book on Transmutation of Species. Had been greatly struck from about the month of previous March [while still on the voyage and just over twenty-eight years old] on character of South American fossils, and species on Galapagos Archipelago. These facts (especially latter) origin of all my views.” From 1838 to 1841 he was secretary of the Geological Society, and saw a great deal of Sir Charles Lyell, to whom he dedicated the second edition of his Journal. On the 29th of January 1839 he married his cousin, Emma Wedgwood, the daughter of Josiah Wedgwood of Maer. They lived in London until September 1842, when they moved to Down, which was Darwin’s home for the rest of his life. His health broke down many times in London, and remained precarious during the whole of his life. The immense amount of work which he got through was only made possible by the loving care of his wife. For eight years (1846 to 1854) he was chiefly engaged upon four monographs on the recent and fossil Cirripede Crustacea ( Roy . Soc., 1851 and 1854; Palaeontograph. Soc., 1851 and 1854). Towards the close of this work Darwin became very wearied of it, especially of the synonymy. For a time he hoped to start a movement which should discourage the habit of appending the name of the describer to the name of the species, a custom which he thought led to bad and superficial work. From this time he was engaged upon the numerous lines of inquiry which led to the great work of his life, the Origin of Species, published in November 1859.

Soon after opening his note-book in July 1837 he began to collect facts bearing upon the formation of the breeds of domestic animals and plants, and quickly saw “that selection was the keystone of man’s success. But how selection could be applied to organisms living in a state of nature remained for some time a mystery to me.” Various ideas as to the causes of evolution occurred to him, only to be successively abandoned. He had the idea of “laws of change” which affected species and finally led to their extinction, to some extent analogous to the causes which bring about the development, maturity and finally death of an individual. He also had the conception that species must give rise to other species or else die out, just as an individual dies unrepresented if it bears no offspring. These and other ideas, of which traces exist in his Diary, arose in his mind, together with perhaps some general conception of natural selection, during the fifteen months after the opening of his note-book. In October 1838 he read Malthus on Population, and his observations having long since convinced him of the struggle for existence, it at once struck him “that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species. Here, then, I had a theory by which to work.” In June 1842 he wrote out a sketch, which two years later he expanded to an essay occupying 231 pages folio. The idea of progressive divergence as an advantage in itself, because the competition is most severe between organisms most closely related, did not occur to him until long after he had come to Down. During the growth of the Origin Sir Joseph Hooker was his most intimate friend, and on the 11th of January 1844 he wrote: “At last gleams of light have come, and I am almost convinced (quite contrary to the opinion I started with) that species are not (it is like confessing a murder) immutable” (l.c. ii. 13). In 1855 he began a correspondence with the great American botanist Asa Gray, and in 1857 explained his views in a letter which afterwards became classical. In 1856, urged by Lyell, he began the preparation of a third and far more expanded treatise, and had completed about half of it when, on the 18th of June 1858, he received a manuscript essay from A. R. Wallace, who was then at Ternate in the Moluccas. Wallace wanted Darwin’s opinion on the essay, which he asked should be forwarded to Lyell. Darwin was much startled to find in the essay a complete abstract of his own theory of natural selection. He forwarded it the same day, writing to Lyell, “your words have come true with a vengeance—that I should be forestalled.” He placed himself in the hands of Lyell and Hooker, who decided to send Wallace’s essay to the Linnean Society, together with an abstract of Darwin’s work, which they asked him to prepare, the joint essay being accompanied by a preface in the form of an explanatory letter written by them to the secretary. The title of the joint communication was “On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection.” It was read on the 1st of July 1858, and appears in the ''Linn. Soc. Journal'' (Zoology) for that year. In this statement of the theory of natural selection, Darwin’s part consisted of two sections, the first being extracts from his 1844 essay, including a brief account of sexual selection, and the second an abstract of his letter to Asa Gray dated the 5th of September 1857. This latter, probably his first attempt to expound natural selection, cannot be surpassed as a clear statement of the theory. Darwin explained at the outset, what he insisted on elsewhere, that the facts of adaptation or contrivance in nature are the real difficulty to be explained by a theory of evolution, the stumbling-block of every previous suggestion. Until he could explain “the mistletoe, with its pollen carried by insects, and seed by birds—the woodpecker, with its feet and tail, beak and tongue, to climb the tree and secure insects,” he was “scientifically orthodox.” Nevertheless he was led to believe in evolution, apart from any possible motive-cause, by “general facts in the affinities, embryology, rudimentary organs, geological history, and geographical distribution of organic beings.” He then proceeds to describe the manner in which he met the difficulty of adaptation by “his notions on the means by which Nature makes her species.” The essentials of the statement are as follows:—I. Man has made his domestic breeds of animals and plants by selection, conscious or unconscious, of very slight or greater variations. II. The material for selection exists in nature, namely, slight variations of all parts of the organism. III. The “unerring power” which sifts these variations is “natural selection ... which selects exclusively for the good of each organic being.” The rate of increase is such that only a few in each generation can live: hence the never sufficiently appreciated struggle for life. “What a trifling difference must often determine which shall survive and which perish!” The remaining heads explain the complex nature of the struggle, the reasons for deficient direct evidence, the advantage of divergence, &c. In the joint essay the phrases “natural selection” and “sexual selection” were first made public by Darwin, the “struggle for existence” by Wallace. Darwin and Wallace had met only once before the departure of the latter for the East. Their rivalry in the discovery of the great principle of natural selection was the beginning of a lifelong friendship. Wallace was lying ill with intermittent fever at Ternate in February 1858 when he began to think of Malthus’s Essay on Population, read several years before: suddenly the idea of the survival of the fittest flashed upon him. In two hours he had “thought out almost the whole of the theory,” and in three evenings had finished his essay. Darwin, also inspired after reading Malthus, in October 1838, did not publish until nearly twenty years had elapsed, and then only when Wallace sent him his essay. Canon H. B. Tristram was the first to apply the new theory, explaining by its aid the colours of desert birds, &c. (Ibis, October 1859).

Acting under the advice of Lyell and Hooker, Darwin then began to prepare what was to become the great work of his life. It appeared on the 24th of November 1859, with the full title, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. The whole edition of 1250 copies was exhausted on the day of issue. The first four chapters explain the operation of artificial selection by man and of natural selection in consequence of the struggle for existence. The fifth chapter deals with the laws of variation and causes of modification other than natural selection. The five succeeding chapters consider difficulties in the way of a belief in evolution generally as well as in natural selection. The three remaining chapters (omitting the recapitulation which occupies the last) deal with the evidence for evolution. The theory which suggested a cause of evolution is thus given the foremost place,