Alfred Russel Wallace was the co-discoverer of natural selection, the mechanism that drives evolution. He and Charles Darwin corresponded and agreed to share credit for the theory. But it's Darwin we remember, mainly because he published The Origin of Species. Wallace was not upset that Darwin's reputation eclipsed his; he was a fan of Darwin. But it was Wallace who first described natural selection, an idea that came to him as he suffered a malarial fever.
If you want a "Eureka!" moment, that's pretty Eureka-ry.
A.R. Wallace died 100 years ago on November 7th. To mark the anniversary, Flora Lichtmanis and Sharon Shattuck have produced an animated account of Wallace's life and studies for the New York Times.