A geneticist pal:
[Quoting me] “… but it was Darwin who wrote the Big Book, and that’s what gets you remembered.”
Also, remember that Darwin had written much of the Big Book before receiving the correspondence from Wallace. In fact, Origin is an abridgement of a much bigger book which never really was completed in the expansive manner that Darwin intended, and that abridgement was prompted by Wallace’s inquiry.
Additionally, obviously Darwin didn’t invent the idea of evolution. It was in the air at the time; rather, he proposed a mechanism, natural selection, which explained how evolution might plausibly occur. There was a minor reference to natural selection in a gardening magazine before Darwin, but I don’t think Darwin saw that reference (I believe the author contacted Darwin after the publication of Origin).
[Me] One reason to air this stuff now, as good a reason as any, is that we are coming up to the 150th anniversary of the day, one of the most momentous days in the history of science, when Darwin received Wallace’s letter and manuscript. We don’t know exactly when that day was (and our ignorance on the point is one of the strands of wool from which the conspiracists spin their theories) but it was very likely either the 17th or 18th of June, 1858.
Here’s Mark Ridley, from the excellent Oxford Reader on evolution:
The mail delivery of 17 June 1858 at Down House, Kent, England, is one reasonable starting-date for the literature of evolution. Charles Darwin (who lived at Down House) then received a letter and manuscript from Alfred Russel [hah!] Wallace, a British naturalist travelling in the Malay archipelago. (The original letter and manuscript are both lost and the 17 June date of receipt is conjectural.) Darwin had invented the theory of evolution by natural selection about twenty years earlier, and since then had studiously avoided publishing a single word of it. He was saving himself to write a Big Work on the subject. Now Wallace, as the mailed manuscript revealed, had invented much the same theory and seemed poised to scoop him. However, an arrangement was made and papers by Darwin and Wallace were presented simultaneously later in 1858 (and printed in 1859). Darwin need not have worried about being scooped. The world ignored the 1858 papers and it was the subsequent 1859 publication of On the Origin of Species that caused the intellectual earthquake.