© 2014 W.H.Freeman and Company [Photos: (top left) Charles Lyell, Principles of Geology, 11th ed. London: John Murray, 1872. Leith Storage P DG L. Reproduced by kind permission of the Syndics of Cambridge University Library; (top middle) Classic Image/Alamy; (top right) Reproduced with permission from John van Wyhe ed., The Complete work of Charles Darwin Online (http://darwin-online.org.uk/); (top right, inset) World History Archive/Alamy; (bottom right) Reproduced with permission from John van Wyhe ed., The Complete Work of Charles Darwin Online (http://darwin-online.org.uk/); (bottom left) Paul D. Stewart/Science Source