One book convinced Darwin the Earth was old enough for evolution

Charles Lyell's Principles of Geology argued that slow, ordinary processes -- erosion, sedimentation, uplift -- acting over immense stretches of time could explain the planet's features, without invoking sudden catastrophes. Darwin read it aboard the Beagle, and Lyell's deep timescale gave evolution by natural selection the time it needed to work.

— Charles Lyell, Principles of Geology — 3 volumes, 1830-1833
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