Darwin’s reliance on Lyell’s Principles of Geology shows how scientific theories build upon one another, in some cases producing a revolution in scientific thought. Darwin predicts that Lyell’s theory of gradual land change will create a “revolution” in science. Lyell’s theory provides examples of land formation changes, such as rising and lowering sea levels, which Darwin later uses as evidence in his evolution theory. The notion of gradual geological change over time is also central to Darwin’s overall theory, not only accounting for gaps in the geological record but also supporting his notion of gradual change in species over time. Whether it was Lyell or Darwin who produced the ultimate revolution in science is a matter of debate. Darwin’s discussion of Lyell’s work illustrates how one important shift in scientific thinking can breed another. In this case, the notion of gradual land change over time helps to form and bolster the theory of natural selection.