The British naturalist Charles Darwin and his contemporary Alfred Russel Wallace proposed the theory of biological evolution through natural selection. Darwin defined evolution as "descent with modification" and argued that all organisms descended from a small number of original species. His theory became widely accepted and is considered one of the most solid theories in science. Darwin's studies and observations of geographical distribution and adaptations in organisms provided evidence for his theory. His most influential work, "On the Origin of Species," chronicled his findings and remains a landmark in scientific literature. Through his personal observational data and various forms of evidence such as fossils and similarities between living organisms, Darwin developed his theory of natural selection, which explains the complexity and diversity of life.