Move over, Mother Nature: there's a new wilderness in town. In Heart of Darkness, the natural world isn't a place of comfort or pleasure or even mild neutrality: it's dark, frightening, and it will basically eat your face off if you so much as look at it cross-eyed. But is civilization really that much better? Sure, you might get to sleep in a bed—but human nature is the same whether it's shouting "brava" after an operatic aria or chanting along with war drums.
In Heart of Darkness, natural forces have a will of their own: they're hostile to the white "pilgrims," but accepting toward the black "savages."
Conrad suggests that there's no real difference between the natural world and human nature.