Charles Darwin set sail aboard the <em>H. M. S. Beagle</em> in 1831 as the ship's naturalist. Inspired by Lyell's <em>Principles of Geology</em>, which he read on the voyage, Darwin was persuaded by his observations on the Galapagos Islands that existing species evolved from pre-existing ones. He struggled to refine his theory until 1858, when he learned that Alfred Wallace had hit on the same idea.