By 1858, Charles Darwin had been developing and gathering evidence for his theory of evolution by national selection for over twenty years. What eventually persuaded him to publish his theory, however, was the fact that Alfred Russel Wallace, a naturalist who had been working extensively in the Amazon Basin and the Malay Archipelago, had independently come to the same conclusions about the mechanics of species evolution. Eventually, descriptions of his and Darwin's theories were published in the same year, and Darwin's On the Origin of Speciescame out the following year.
Editors. "Charles R. Darwin and Alfred Wallace independently propose natural selection theories of evolution". The Literary Encyclopedia. First published 30 August 2013
[http://www.litencyc.com/php/stopics.php?rec=true&UID=6316, accessed 21 September 2014.]