In 1831, Charles Darwin received an astounding invitation: to join the HMS Beagle as ship's naturalist for a trip around the world. For most of the next five years, the Beagle surveyed the coast of South America, leaving Darwin free to explore the continent and islands, including the Galápagos. He filled dozens of notebooks with careful observations on animals, plants and geology, and collected thousands of specimens, which he crated and sent home for further study.

Darwin later called the Beagle voyage "by far the most important event in my life," saying it "determined my whole career." When he set out, 22-year-old Darwin was a young university graduate, still planning a career as a clergyman. By the time he returned, he was an established naturalist, well-known in London for the astonishing collections he'd sent ahead. He had also grown from a promising observer into a probing theorist. The Beagle voyage would provide Darwin with a lifetime of experiences to ponder—and the seeds of a theory he would work on for the rest of his life.