Final answer:
Charles Lyell's concept of uniformitarianism, which suggests Earth's geological processes happen slowly over vast time scales, greatly influenced Charles Darwin's evolutionary theory by indicating the need for long periods for the evolution of species.
Step-by-step explanation:
In Charles Lyell's Principles of Geology, the idea that had a significant impact on Charles Darwin's thinking was the concept of uniformitarianism. Lyell's work proposed that geological processes that happened in the past were not different from those that occur in the present and these processes happened very slowly over a long period of time. This theory suggested that the Earth was much older than the commonly held belief of 6,000 years, providing the vast time scales necessary for the evolution of species. Darwin applied this conception of vast geological time to biological evolution, understanding that the gradual change required for the evolution of species from common ancestors needed similarly vast periods of time.