increasingly questioned the fixity of species and eventually became convinced that entirely new species could arise by natural descent from pre-existing ones. For over twenty-five years after Beagle returned to England he worked on the manuscript of what would have been a massive volume on the subject. This never saw the light of day, but in 1859, concerned by news that the British naturalist and biologist Alfred Wallace was about to go into print along similar lines, he rushed out a condensed version