The most influential evolutionary theories prior to Darwin were those of Lamarck and Geoffroy St. Hilaire, developed between 1794 and 1830. Lamarck suggested that species evolve through the use or disuse of particular organs. In the classic example a giraffe that stretches its neck slightly to reach higher leaves will gain in neck length, and this small gain would be passed on to its offspring. Geoffroy on the other hand suggested that the change was discontinuous, large in magnitude, and occurred at the production of offspring. However, these theories of evolution were based on prior explanations that offered no demonstrated mechanism.

Darwin's theory of evolution differs in that it

and drawing as well as the knowledge of comparative anatomy. It was only much later when Darwin returned from the voyage, and when the fossils were identified by Owen, that Darwin began to examine them as zoological, rather then geological, phenomena.

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the book (on a scale three to four times as large as when it was later published), when one day a nasty surprise awaited him. On June 18, Darwin received a manuscript from the English naturalist Wallace. In the manuscript Wallace described the theory of natural selection, and asked Darwin to comment on his ideas. Darwin thought that the only honorable thing to do was to recommend the paper for publication. Fortunately, for Darwin, Lyell suggested (and Wallace and Darwin accepted) that both Wallace's paper and extracts from Darwin's sketch of 1844 be published simultaneously, thus establishing the rights of both to priority. Interestingly, later on at the fiftieth anniversary meeting of their joint publication, Wallace made it clear that although the idea of natural selection came to both of them independently, Darwin's contributions outweighed his by twenty to one because Darwin had the credit of twenty years of priority and work.

In 1831 Darwin graduated from Cambridge, and as he was pondering his future he received a proposal to join a scientific expedition that would survey the southern coast of Tierra del Fuego. Darwin accepted the proposal, and sailed from England aboard the famed Beagle on December 27, 1831. His job was to collect and catalogue new species so that they could be sent back for further research in England. It is commonly thought that Darwin used the voyage to test his theory of evolution, but this is highly unlikely. At the time Darwin's interests were purely geological as can be seen by his correspondence with his sister. For instance, writing about the fossils that he discovered he said, "All the interest which I individually feel about these fossils is their connection with the geology of the Pampas". Furthermore, Darwin himself confessed that he could not have appreciated the significance of his findings while on the voyage, because he lacked the necessary training in dissection!

The voyage turned out to be very productive for Darwin, who upon his return in 1836 began to work on the conversion of the diary, which he kept during the voyage, into a journal suitable for publication. The Journal was first published in 1839 under the title "Journal and Remarks", as Volume III of the Narrative of the Surveying Voyages of H.M.S. Adventures and Beagle. However, enough people thought that Darwin's work was sufficiently important to warrant a separate publication, and in 1845 a second edition was published under the name Journal of Research into the Natural History and Geology of the Countries Visited during the Voyage of H.M.S. Beagle Round the World (henceforth referred to as the Journal).

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