Charles Darwin is widely known for his theory of evolution, which he detailed in his seminal work, "On the Origin of Species," published in 1859. His theory primarily focuses on the process of natural selection, where organisms better adapted to their environment tend to survive and reproduce, passing on advantageous traits to future generations. Darwin's ideas challenged prevailing notions of static species and laid the groundwork for modern evolutionary biology. His work has had profound implications across various fields, influencing not just science, but also philosophy, ethics, and the understanding of life on Earth.