William “Strata” Smith’s “principle of faunal succession,” introduced in Chapters 1 and 2, codified one of the key observations in geology: strata of different ages contain unique, age-diagnostic fossils. Today anyone can repeat these observations, either in northern England, where Smith worked, or with any other fossiliferous sequence of rocks. The rock record is clear, life on Earth has changed over time. As one species becomes extinct it is often succeeded by a new species. This is evolution, a simple and straightforward observation. The ability of life to diversify was made possible by the immensity of geological time and was largely driven by the changes in local or global environments. The only theoretical framework needed to start this work is Steno’s principle of superposition, which we explained in Chapter 1. Applying this idea with observations of fossils in different strata clearly show that life has evolved. Observations from other fields of science, including anatomy and physiology, molecular biology, and genetics, are consistent with the data from paleontology and stratigraphy and show that all life is related. The idea that best explains the observed evolution of life over time, as well as how species are related, is the theory of natural selection and is the focus of this chapter.