Darwin's manuscript might have remained locked away till his death but for an alarming blow that arrived from the Far East in the early summer of 1858 in the form of a packet containing a friendly letter from a young naturalist named Alfred Russel Wallace and the draft of a paper, On the Tendency of Varieties to Depart Indefinitely from the Original Type, outlining a theory of natural selection that was uncannily similar to Darwin's secret jottings. Even some of the phrasing echoed Darwin's own. "I never saw a more striking coincidence," Darwin reflected in dismay. "If Wallace had my manuscript sketch written out in 1842, he could not have made a better short abstract."
Wallace didn't drop into Darwin's life quite as unexpectedly as is sometimes suggested. The two were already corresponding, and Wallace had more than once generously sent Darwin specimens that he thought might be of interest. In the process of these exchanges Darwin had discreetly warned Wallace that he regarded the subject of species creation as his own territory. "This summer will make the 20th year (!) since I opened my first note-book, on the question of how & in what way do species & varieties differ from each other," he had written to Wallace some time earlier. "I am now preparing my work for publication," he added, even though he wasn't really.
In any case, Wallace failed to grasp what Darwin was trying to tell him, and of course he could have no idea that his own theory was so nearly identical to one that Darwin had been evolving, as it were, for two decades.