Alfred Russel Wallace conceived his theory of evolution by natural selection whilst suffering from fever in the village of Dodinga on the Indonesian island of Halmahera (known to Wallace as Gilolo) in February 1858. When he had recovered sufficiently he wrote an essay about it, which he posted together with a letter, to Charles Darwin probably in early March from the neighbouring island of Ternate. To cut a long story short: when Darwin received the essay he was horrified as he realised that Wallace had independently conceived 'his' theory. He implored his close friend Charles Lyell to help, and Lyell, Joseph Hooker and Darwin agreed to publish Wallace's essay together with some previously unpublished writings on the subject by Darwin, in order to demonstrate that Darwin had had the idea 20 years before Wallace. This 'joint' paper was read to a meeting of the Linnean Society of London on the 1st July 1858, and then published in August of that year. For a detailed account of the curious events which led to its publication see my article Alfred Russel Wallace and natural selection: the real story.
What few people seem to realise is that Wallace's original essay and his covering letter mysteriously disappeared long ago. Historians do not know what Wallace wrote in the letter, and the disappearence of it and the envelope it and the essay were posted in have given rise to a conspiracy theory, which has been the subject of a number of books and research papers (see e.g. https://onlinelibrary.wiley.com/doi/full/10.1111/j.1095-8312.2011.01858.x). Leaving the conspiracy aside, what do we know about the fate of the letter?