Although people now take for granted the idea that gradual processes acting over long periods can have dramatic effects — think, for example, of the slow erosion by the Colorado River that led to the formation of the Grand Canyon — this idea was at odds with the prevailing view in the 1800s that the Earth was very young. Then along came the field of stratigraphy, which deals with the horizontal banding patterns that you can observe in the faces of cliffs or when a highway is cut through deep rock.
By Darwin's day, detailed geological mapping of Europe had revealed that a reproducible sequence of bands was spread across a large geographical area and that these bands contained fossils. Even in the absence of detailed information about the absolute ages of the different bands, scientists concluded that the ones on the bottom were typically older than the ones on the top. The very existence of these bands and the fossils that were found within them hinted at a process of gradual change.
If the new geological views about gradualism were correct — that is, that the Earth formed over long periods, as indicated by the banding patterns of different geological eras — scientists could imagine that the changes in the biological community were also the result of small changes occurring over a large period. Turns out that they were right on both counts.
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