Charles Darwin was a British naturalist whose theory of evolution by natural selection revolutionized the biological sciences. During his five-year voyage on the HMS Beagle, Darwin observed diverse species in the Galapagos Islands, noting how they adapted to different environments. In 1859, he published 'On the Origin of Species,' arguing that all life on Earth descended from common ancestors and that traits helpful for survival are passed to the next generation. This process, which he called 'natural selection,' provided a scientific explanation for the diversity of life without resorting to supernatural causes. While initially controversial, his work is now the unifying principle of modern biology. Darwin's legacy is one of the most profound shifts in human thought, changing our understanding of our place in the natural world.