Charles Darwin, a British naturalist, proposed the theory of biological evolution by natural selection and defined evolution as "descent with modification." He argued that all existing creatures descended from a small number of original or progenitor species and compared the history of life to a branching tree. His theory of evolution by natural selection is one of the most solid theories in science, supported by evidence of geographical distribution and natural selection. Darwin's book "On the Origin of Species" presented his idea of natural selection and chronicled his studies, including observational data that strengthened his theory. He used multiple lines of evidence, such as fossils and similarities between related living organisms, to support his theory.