Eight thousand miles away, on an island called Gilolo, Wallace spent much of February 1858 wrapped in blankets against the alternating hot and cold fits of malaria. He passed the time mulling over the species question, and one day, the same book that had inspired Darwin came to mind—Malthus' Essay on the Principle of Population. "It occurred to me to ask the question, Why do some die and some live?" he later recalled. Thinking about how the healthiest individuals survive disease, and the strongest or swiftest escape from predators, "it suddenly flashed upon me...in every generation the inferior would inevitably be killed off and the superior would remain—that is, the fittest would survive." Over the next three days, literally in a fever, he wrote out the idea and posted it to Darwin.
Less than two years later, on November 22, 1859, Darwin published his great work On the Origin of Species by Means of Natural Selection, and the unthinkable—that man was descended from beasts—became more than thinkable. Darwin didn't just supply the how of evolution; his painstaking work on barnacles and other species made the idea plausible. Characteristically, Darwin gave credit to Wallace, and also to Malthus, Lamarck and even the anonymous "Mr. Vestiges." Reading the book, which Darwin sent to him in New Guinea, Wallace was plainly thrilled: "Mr. Darwin has given the world a new science, and his name should, in my opinion, stand above that of every philosopher of ancient or modern times."
Wallace seems to have felt no twinge of envy or possessiveness about the idea that would bring Darwin such renown. Alfred Russel Wallace had made the postman knock, and that was apparently enough.
Richard Conniff is a longtime contributor to Smithsonian and the author of The Ape in the Corner Office.