Charles Darwin was a 19th century British naturalist. He is best known for his theory of evolution by natural selection, which transformed the way scientists view and interpret life on Earth.
Darwin's theory, popularized in his book "On the Origin of Species," states that pressures in the environment determine which members of a species survive long enough to reproduce. The individuals of a species that are best adapted to their environment more often reproduce and pass on their genes to the next generation, spreading certain physical and behavioral traits in the population. Eventually, those traits — such as giraffes' long necks or polar bears' white fur — come to dominate and shape the evolution of the species.