Bottom line: Darwin did not have a single scrap of empirical fieldwork to document his conjectures.
After reading Malthus out of personal interest, it dawned on Darwin how he might usefully appropriate the Malthusian analogy.
Erasmus even touched on the theory of natural selection when he concluded that the strongest and most active animals would propagate the species.
Catastrophism viewed the planet as having been molded by forces far more powerful than any observable at the present day.
Remarkably, Darwin’s evolutionary ideas did not derive from his empirical observations in the South Seas or anywhere else.