Fitzroy a supporter, Darwin an abolitionist. Also disagreed on religion. Suppose Darwin tried to refute argument from design. Not a very appealing God. Adam Smith's argument transferred to nature.
Evolution means, literally, progress.
Darwin's theory was nonprogressive. Instead, Natural Selection is only about adaptation to local environments. Slides illustrating our misequation of Evolution with inherent progress. He is obviously afraid. Not of exposing his belief in evolution.