On the Origin of Species (1859) by Charles Darwin is a work of scientific literature considered to be the foundation of evolutionary biology. The book introduced the theory that populations evolve over the course of generations through a process of natural selection, backed by a body of evidence that the diversity of life arose by common descent through a branching pattern of evolution. The evidence was gathered on the Beagle expedition in the 1830s and the author’s subsequent findings from research, correspondence, and experimentation. Darwin’s theory that species derive from other species by a gradual evolutionary process and that the average level of each species is heightened by the “survival of the fittest” stirred controversy and opposition. Darwin's concept of evolutionary adaptation through natural selection has become the unifying concept of the life sciences.