I don't think that is a very useful question because Darwin's strength comes not so much from what he proved, but from the near-inescapable conclusions that he led us to. He used every means of informing himself about questions that interested him. He is known for his massive and continuous correspondence, always asking pertinent questions of those studying what we should now call model systems or model organisms. Of course, he was also a great natural historian himself, so his own observations pervade his writings. He was then able to integrate observations from one species into a prototype for what we now call the comparative method – he looked across species and showed how similar environments resulted in the development of similar adaptations. When making cross-species comparisons it is important to distinguish between similarity through inheritance from a common ancestor and similarity through independent evolutionary origins.