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Darwin investigated the transmutation of species and conceived his theory of natural selection in 1838. Although he discussed his ideas with several naturalists, he needed time for extensive research and his geological work had priority. He was writing up his theory in 1858 when Alfred Russel Wallace sent him an essay which described a similar theory, prompting immediate joint publication of both of their theories.

Wallace had once briefly met Darwin, and was one of the correspondents whose observations Darwin used to support his own theories. Although Wallace's first letters to Darwin have been lost, he carefully kept the letters he received. In the first letter, dated 1 May 1857, Darwin commented that Wallace's letter of October 10th which he'd recently received as well as Wallace's paper "On the Law that has regulated the Introduction of New Species" of 1855 showed that they were both thinking alike and to some extent reaching similar conclusions, and said that he was preparing his own work for publication in about two years time. The second letter, dated 22 December 1857, said how glad he was that Wallace was theorising about distribution, adding that "without speculation there is no good and original observation" while commenting that "I believe I go much further than you". Wallace trusted Darwin's opinion on the matter and sent him his February 1858 essay, "On the Tendency of Varieties to Depart Indefinitely From the Original Type", with the request that Darwin would review it and pass it on to Charles Lyell if he thought it worthwhile.

On 18 June 1858, Darwin received the manuscript from Wallace. While Wallace's essay did not employ Darwin's term "natural selection", it did outline the mechanics of an evolutionary divergence of species from similar ones due to environmental pressures. In this sense, it was very similar to the theory that Darwin had worked on for twenty years, but had yet to publish. Darwin sent the manuscript to Charles Lyell with a letter saying "he could not have made a better short abstract! Even his terms now stand as heads of my chapters… he does not say he wishes me to publish, but I shall, of course, at once write and offer to send to any journal." Distraught about the illness of his baby son, Darwin put the problem to Charles Lyell and Joseph Hooker, who decided to publish the essay in a joint presentation together with unpublished writings which highlighted Darwin's priority. Wallace's essay was presented to the Linnean Society of London on 1 July 1858, along with excerpts from an essay which Darwin had disclosed privately to Hooker in 1847 and a letter Darwin had written to Asa Gray in 1857.

Wallace accepted the arrangement after the fact, happy that he had been included at all. Darwin's social and scientific status was far greater than Wallace's, and it was unlikely that, without Darwin, Wallace's views on evolution would have been taken seriously. Lyell and Hooker's arrangement relegated Wallace to the position of co-discoverer, and he was not the social equal of Darwin or the other prominent British natural scientists. However, the joint reading of their papers on natural selection associated Wallace with the more famous Darwin. This, combined with Darwin's (as well as Hooker's and Lyell's) advocacy on his behalf, would give Wallace greater access to the highest levels of the scientific community. The reaction to the reading was muted, with the president of the Linnean remarking in May 1859 that the year had not been marked by any striking discoveries; but, with Darwin's publication of On the Origin of Species later in 1859, its significance became apparent. When Wallace returned to the UK, he met Darwin and the two remained friendly afterwards. Over the years, a few people have questioned this version of events. In the early 1980s, two books, one written by Arnold Brackman and another by John Langdon Brooks, even suggested that not only had there been a conspiracy to rob Wallace of his proper credit, but that Darwin had actually stolen a key idea from Wallace to finish his own theory. These claims have been examined in detail by a number of scholars who have concluded that they are not credible.

Doesn’t sound like plagiarism to me. It sounds more like scientists working independently within the same field of interest and then sharing their findings and papers for peer review and critique and publishing both jointly and separately. It happens all the time in the scientific community and it’s certainly not plagiarism. Thomas Edison has been accused of much worse I might add.