When British naturalist Charles Darwin arrived in the Galápagos Islands in 1835, he was already four years into a round-the-world voyage aboard the survey ship HMS Beagle. His observations on the remote volcanic islands in the eastern Pacific Ocean would help him to develop his famous theory of evolution through natural selection. While studying the creatures here, he concluded that many South American species, mainly reptiles and birds that could cross the vast expanse of water, had migrated to the Galápagos and gradually adapted to their surroundings.