Darwin’s great discover was natural selection, the mechanism of evolution. An article in yesterday’s Science Times goes into some detail about the homology between a bat’s wing and a human hand. But this would have been well known long before Darwin. Many people, including his grandfather Erasmus Darwin, had conceived of evolution, but it was Charles Darwin’s incredible insight that variation and natural selection was what made evolution possible. Darwin didn’t understand this until years after the Beagle returned. He hadn’t even tracked the habitats of the various finches, but realized that the speciation had to be the result of different food sources.