Uniformitarianism has had two separate meanings, both more prevalent in 19th-century discourse:
religious philosophy, Uniformitarianism ("with a capital U") is the belief that the Universehas existed as it is now for an infinite time and will continue to exist for ever. This view is opposed to traditional theological views and modern science.
scientific philosophy, uniformitarianism ("with a small u") refers to the principle that the same processes that shape the universe occurred in the past as they do now, and that the same laws of physics apply in all parts of the knowable universe. This axiomatic principle, not often referred to as an "-ism" in modern discussions, is particularly relevant to geologyand other sciences that operate on a long timescale such as astronomyand paleontology. The leading geologist of Charles Darwin’s era, a Scot named Charles Lyell(1797 – 1875), incorporated James Hutton’s gradualism into a theory known as uniformitarianism. The term refers to Lyell’s idea that geological processes have not changed throughout Earth’s history. Thus, for example, the forces that build mountains and erode mountains and the rates at which these forces operate are the same today as in the past.
*Campbell, Reece; Biology Sixth Edition; Benjamin Cummings, 2002
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