Final answer:
James Hutton proposed that Earth's geologic features could be explained by gradual mechanisms that operate the same way now as they did in the past. Charles Lyell later popularized this idea through the principle of uniformitarianism, which also impacted Darwin's theory of evolution.
Step-by-step explanation:
The person who proposed that Earth's geologic features could be explained by gradual mechanisms still operating today was James Hutton, a Scottish geologist and naturalist from the eighteenth century. Hutton's idea was that geologic change occurred gradually, accumulating small changes from processes operating as they are today over long periods. This concept was in contrast to the belief at the time that Earth's geology was the result of catastrophic events from a relatively brief past.
Hutton's views were later advanced and popularized by Charles Lyell in the nineteenth century, who argued for uniformitarianism--the principle that the same geological processes we see today have been at work in the past. Lyell's works, specifically his three-volume treatise titled Principles of Geology, had a profound impact on the scientific community, altering the perceived age of the Earth from thousands to millions of years and influencing Charles Darwin's evolutionary thought with the principle of gradualism.