Charles Darwin initially shared the belief that nature was perfect and harmonious: after graduating as a student at the University of Cambridge in 1831, he was convinced by William Paley's "Natural Theology or Evidences of the Existence and Attributes of the Deity" which saw adaptation as purposeful design and presented population pressure optimistically; "it is a happy world after all". By mid January 1832, early in the "Beagle" voyage, Darwin saw geology from Charles Lyell's viewpoint. When the second volume of Lyell's "Principles of Geology" was delivered to the "Beagle" that November, Darwin accepted its argument that the "struggle for existence" disproved transmutation of species. He was reminded of Malthusianism when his sisters sent him out pamphlets by Harriet Martineau. Lyell had been unable to show the mechanism for introducing new species, and towards the end of the voyage Darwin noted that the distribution of mockingbirds found on the Galápagos Islands raised doubts that species were fixed.