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 recovered sufficiently he wrote a detailed essay explaining his idea, which he posted to Charles Darwin together with a covering letter, probably in early March, from the neighbouring island of Ternate. To cut a long story short: when Darwin received the essay in June 1858 he was horrified as he realised that Wallace had independently conceived 'his' theory. He implored his close friend the geologist Charles Lyell for help, and Lyell, botanist Joseph Hooker and Darwin discussed the issue and decided to publish Wallace's essay prefixed by some of Darwin's writings on the subject which hadn't been written with publication in mind, in order to demonstrate that Darwin had had the same 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 its covering letter mysteriously disappeared long ago. No one knows what Wallace wrote in the letter, and the disappearence of it and the envelope it was 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 this intrigue aside, what do we know about the fate of the letter, the essay and the envelope? Well, we know nothing about the fate of the envelope, and all we know about the original essay is that Darwin sent it to Lyell in his 'anguished' letter of June 18 1858 asking for it to be returned. There is no record of what then happened; it could have been sent to the Linnean Society for typesetting and then lost; Lyell could have kept it and sent a transcript to the Linnean Society, then returned the original to Darwin, who subsequently lost or destroyed it etc etc. What we do know is that the original was never returned to Wallace (see below). So what became of the letter?