Philosophy of science
With biology, the situation wasn't the same as other parts of science. There were thousands of published observations, and Darwin showed that sense could be made of the observations. He used those ideas to prove evolution had taken place. In a natural history science (such as biology, geology or astronomy), one of the main jobs of science is to explain what has happened and what is seen. Obviously, explanations as well as observations and theories are part of the philosophy of science.