Alfred Russel Wallace
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 Natural selection
Wallace is the co-discoverer of natural selection because he wrote an essay about it on the spur of the moment while suffering from malaria (that must have been some good malaria). He sent the essay to Charles Darwin in 1858, who realized that Wallace had come up with the same theory of evolution that he had spent 20 years working on. Not wanting to be scooped, Darwin arranged to have both Wallace's paper and some of his own writing presented at a meeting of the Linnean Society without Wallace's permission.
Wallace didn't publish as extensively on natural selection as Darwin did, but he published other things as well. For example, Wallace was in the Malay Archipelago when he came up with natural selection, dashed off an essay that would ensure his place in history, and then went back to exploring the region.
Wallace wouldn't return to the subject until 1864. Later he expressed doubts about whether human intelligence could be the result of natural selection. He also became a spiritualist in the late 1860s.
Wallace was a good sport about it all, though, and even published a book defending Darwin.