When the young naturalist Darwin set sail on a round-the-world expedition at the end of 1831, it was only with a vague notion that the relationships between all life forms, both present and extinct, were more complex than the Christian version of the world's creation purported them to be. During his five-year voyage of discovery on board HMS Beagle and more than two decades subsequently devoted to research, he painstakingly collected a mass of evidence from across the planet