Nature and civilization rather decisively represent the dichotomy of "primitive" non-Europeans and "cultured" Europeans. "Civilization" comes to refer to the supposed enlightenment and refinement of the Western world. The British imperialists feel their conceptions of black "savagery" are confirmed when the native Africans attack the pilgrims and reveal their cannibalism. The white Europeans seek to tame these Africans with "civilization," but the notion becomes problematic when "noble altruism" acts as a shield for blind imperialism. The white men who call their actions altruistic are quickly recognized by the readers as hypocrites.
In Heart of Darkness, natural forces have a will of their own and prove themselves hostile towards the white pilgrims.