While Darwin didn't explicitly focus on uniformitarianism in his writings, one key observation he made during his voyage on the Beagle was the presence of marine fossils high up in the Andes mountains. This observation, consistent with the work of James Hutton and Charles Lyell, supported the idea that geological processes operating today (like erosion, uplift, and deposition) have been working at the same rate and intensity over vast periods of time.

Here's how it connects to uniformitarianism:

* Observation: Darwin found marine fossils, indicating that the area was once underwater, high in the Andes Mountains.

* Uniformitarianist Interpretation: This suggests that over a very long time, the land was slowly uplifted, lifting these marine sediments and fossils with it. This process, similar to how mountains are formed today, is a gradual and continuous process, not a sudden catastrophic event.

This observation, along with others Darwin made about the gradual changes in geology, contributed to his understanding of the long, slow process of evolution and helped him develop his theory of natural selection.