Charles Darwin is best known for formulating the theory of natural selection, a foundational concept in evolutionary biology. In his seminal work, "On the Origin of Species," published in 1859, Darwin outlined how species evolve over time through the process of natural selection, where organisms better adapted to their environment tend to survive and reproduce. This groundbreaking idea transformed how we understand the development of life on Earth, influencing both science and philosophy. Darwin's contributions to biology have established him as one of history's most significant scientists.