If he had a passport Charles Robert Darwin might have been described as a naturalist but he was one of the greatest of scientists, a titan among biologists. His theory of evolution by natural selection unified science, providing a coherent explanation for how the diversity of life came to be.
John Burdon Sanderson Haldane had no formal education in any scientific subject and yet he became a commanding figure in biochemistry, evolutionary theory, genetics, and physiology, publishing over 400 papers and 24 books.