In the eyes of Conrad and his European readers of the time, the African Congo must have been seen as the complete opposite of European society, a part of a completely different planet altogether. Savage versus civilized, dark versus light - the duality of these two worlds run throughout Conrad's novella, Heart of Darkness. In that sense, a collision between worlds acts as the catalyst for all of the "horrors" in the story. Conrad does not only use this dualism to illustrate the absurdity of "progressive imperialism" - the idea that Europeans could tame this wild and foreign environment, but also as a cautionary tale to demonstrate the danger that must follow when products of two completely conflicting cultures collide.
Right from the start of the novella we see characterizations of the African and European worlds. Marlow tells his story while drifting down the Thames River, which is described as tranquil and "nearly calm". Conversely, as a British citizen Marlow describes the expedition up the Congo River as a "journey up a prehistoric earth". The juxtaposition of the two rivers is significant because it establishes Europe and Africa as two opposing cultures and paves the way for further comparisons. Similarly, Marlow introduces most characters as accountants or doctors or lawyers, denoting them by only their occupation, because he sees them as anonymous products of the society that created them. When Marlow mentions the Pilgrims or the Cannibals, the reader is instinctively reminded of the world they belong to. Their behaviour comes to characterize Europe and Africa as a whole.
For the most part Europeans and Africans tend to stay in their respective societies, but as Marlow travels deeper into the Congo, he sees examples of what happens when humans are removed from their native environment. A clear example of this is the character of Kurtz. As one of the few characters in the novella who is actually addressed by name, Kurtz is said to represent a product of the pinnacle of Western imperialism. "All Europe contributed to the making of Kurtz" (citation here). A talented painter and musician, Kurtz journeys into the Congo to complete great acts of "humanizing, improving, instructing" (as he explains in his initial report to the Company). But upon leaving his home environment Kurtz is said to have lost all restraints, exploiting the natives and mounting their heads on stakes, as well as committing other atrocities (to be fixed).
Although Marlow seems to dismiss the idea that Kurtz was "shamefully abandoned", there may be some truth in the idea. In a sense, Kurtz was a lone European stranded in the heart of Africa. The final result is the postscriptum added by a Kurtz who had been "caressed", "embraced" and "loved" by his new environment: "Exterminate all the brutes!".
Parallels to Kurtz's descent can be seen in the characters of Freselven and the Helmsman. A European seaman once described "the gentlest,...