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As a boy, Charles Lyell collected butterflies. This hobby might seem a far cry from his later professional work in geology, but in fact the two were closely linked. At that young age, he was already a keen observer of nature with excellent instincts for comparison. Throughout his life, Lyell traveled and collected observations on natural phenomena as he had once collected butterflies. These observations convinced him even as a young man that nature was to be understood through genuinely natural processes rather than as the result of supernatural forces or catastrophic events. Thus was born the theory of uniformitarianism, which maintained that the same geological processes had been at work in the same way throughout Earth's history, and that major features such as mountains showed that the Earth was very old, since geological processes worked very slowly. To convince people of his notions, Lyell needed to back them up with facts, and the three volumes of his pathbreaking Principles of Geology (1830–1833) are notable for being chock-full of geological facts. This principled method of thinking and marshaling evidence, no less than his radical ideas about the Earth's history, was itself revolutionary for the time (something perhaps not appreciated today). By claiming that the Earth was many millions rather than a few thousands of years old, Lyell opened up vast new possibilities for other thinkers, most notably Charles Darwin, whose theory of evolution by natural selection also required time for slow, incremental changes in the history of life.