In Joseph Conrad’s novel, Heart of Darkness, how is the novel setting a key component in unlocking the greater meaning of the novel?

As the crew make their way up the river, they are traveling into the “heart of darkness.” The contradiction, however, is that Marlow also feels as if he were traveling back in time. When Conrad wrote this story, scientists were learning that Africa is the seat of human civilization, and this knowledge is reflected in the fact that the trees are (almost prehistorically) enormous on the route down the river. The paradox of the novel, however, is that by traveling backwards in time, the crew do not move closer to the innocence and purity of the "noble savage" but farther away from it. Words like “pestilent” and “sordid” are used again and again to describe the natives and their land. Conrad seems to claim that the Christian belief that prehistory was untouched by obscurity or evil is a fallacy. Instead, there is “the horror.” In contrast, it seems, is the more advanced civilization of the colonizers and visitors.