Which geological principle, championed by Lyell, influenced Darwin's thinking?

The geological principle championed by Lyell that influenced Darwin’s thinking was uniformitarianism. It held that the same natural processes observable today also operated in the past, gradually reshaping Earth over immense spans of time.

Lyell presented this approach most influentially in Principles of Geology, published in three volumes between 1830 and 1833. Its famous practical implication was that the present could serve as a key to understanding the geological past. Slow erosion, sedimentation, volcanic activity, and uplift could produce dramatic changes when acting over millions of years.

Darwin received the first volume from Robert FitzRoy before the Beagle voyage began in 1831. He applied Lyell’s geological reasoning to South American landscapes, earthquakes, raised beaches, and coral reefs. Lyell’s uniformitarian geology helped Darwin accept deep time, which was essential for natural selection to accumulate small inherited differences. Uniformitarianism is often confused with the claim that Earth’s surface never changes; in fact, it explains large changes through ordinary processes operating continuously or repeatedly.