forces of nature. Paley saw a rational proof of God's existence in the complexity and perfect adaptation to needs of living beings exquisitely fitted to their places in a happy world, while attacking the evolutionary ideas of Erasmus Darwin as coinciding with atheistic schemes and lacking evidence. Paley's benevolent God acted in nature through uniform and universal laws, not arbitrary miracles or changes of laws, and this use of secondary laws provided a theodicy explaining the problem of evil by separating nature from direct divine action, drawing directly on the ideas of Thomas Malthus. For Paley, a Malthusian "system of natural hostilities" of animals living on prey was strictly connected to the surplus of births keeping the world appropriately stocked as circumstances changed, and poverty showed that the world was in a "state of probation… calculated for the production, exercise, and improvement of moral qualities, with a view to a future state", even where such divine purpose was not obvious. This convinced Charles and encouraged his interest in science. He later wrote "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." He read John Herschel's new Preliminary Discourse on the Study of Natural Philosophy, learning that nature was governed by laws, and the highest aim of natural philosophy was to understand them through an orderly process of induction, balancing observation and theorising. This exemplified the natural theology that Darwin had learnt in previous years. He also read Alexander von Humboldt's Personal Narrative, and the two books were immensely influential, stirring up in him "a burning zeal to add even the most humble contribution to the noble structure of Natural Science." Voyage of the Beagle Darwin planned a visit to the tropics before settling down as a clergyman, and on Henslow's advice studied geology with Adam Sedgwick, finding about the ancient age of the Earth, then went with him for two weeks surveying strata in Wales. He returned to find that his arrangements had fallen through, but was given the opportunity to join the Beagle survey expedition as a gentleman naturalist and companion to captain Robert FitzRoy. Before they left England FitzRoy gave Darwin a copy of the first volume of Charles Lyell's Principles of Geology, the subject which would be his primary work. Darwin was questioning from the outset, and in his first zoology notes