Charles Darwin is best known for establishing a theory of evolution that explains biological changes among the species of the Earth. His belief that most species evolved through the process of natural selection also made him famous because this idea contradicts those of other naturalists of his time.
Darwin's ideas grew from his experience on the HMS Beagle, where he participated in a five-year survey trip around the world. On the journey, he was able to observe the fundamental laws governing botany and zoology. He collected specimens and studied them carefully.
From his observations, he noticed similarities among certain species and theorized that the similarities existed because the species had evolved to adapt to their surroundings. This kind of evolution was basically a means of survival and the species that did survive and adapt were doing so through a process of natural selection.
Darwin's way of thinking ran counter to what other naturalists believed during his time. Most people believed that the species of the Earth had always existed in their present forms from the beginning of time. Darwin's ideas were radical, but that did not stop him from publishing "On the Origin of Species by Means of Natural Selection," a foundational work of scientific thought.