I want to know the answer to the following question:
Whose letter does the introduction to On the Origin of Species refer?
All the background I'm provided with is that "The Introduction establishes Darwin's credentials as a naturalist and author, then refers to John Herschel's letter suggesting that the origin of species "would be found to be a natural in contradistinction to a miraculous process":". Is that enough to answer the question?
Answer: yes