Charles Darwin's studies at the University of Cambridge included two other texts by Paley, and in his final exams in January 1831 he did well in questions on these texts. He had to stay on until June, and read Paley's "Natural Theology" as well as John Herschel's "Preliminary Discourse on the Study of Natural Philosophy" and Alexander von Humboldt's "Personal Narrative": these books inspired "a burning zeal" to research natural history. After the "Beagle" voyage he began development of his theory of natural selection, and in 1838 opened a notebook listing "books to be read", including "Paley's Nat. Theology". In 1859, on completing "On the Origin of Species", he told a friend "I do not think I hardly ever admired a book more than Paley's "Natural Theology": I could almost formerly have said it by heart."