How does Marlow's journey into the "heart of darkness" symbolize a voyage of self-discovery?
Charlie Marlow's search for Kurtz in Heart of Darkness symbolizes a voyage of self-discovery because Marlow is forced to come to terms with who he is and who he is not as he travels farther and farther up the Congo River. Joseph Conrad, the author, contrasts Marlow's choices with those of Kurtz to show how the two men emerge on the other side of their experiences with different conclusions.
Kurtz and Marlow represent two different possibilities of what a man can be. Their relationship is first shown when Marlow and Kurtz are said to belong to the same gang; this is because they were both vouched for by the same people to get their jobs. Marlow remains fixated on Kurtz even before they meet.
As Marlow learns more about Kurtz, he discovers more about himself. The trip up the Congo takes him away from the world he's inhabited in the past and gives him the opportunity to become something else. It would be relatively easy for him to go mad like Kurtz—but he doesn't. The deeper he gets into the wilderness, the more Marlow is forced to confront his own character. Kurtz had decided to civilize everyone in Africa; instead, he's consumed by the jungle and driven to madness. After talking with Kurtz before the man dies, Marlow decides to return home.
Marlow gets close enough to the edge to see the darkness that consumes Kurtz. However, he doesn't let it consume him, even though Kurtz's final words haunt him. Marlow says:
Since I had peeped over the edge myself, I understand better the meaning of his stare, that could not see the flame of the candle, but was wide enough to embrace the whole universe, piercing enough to penetrate all the hearts that beat in the darkness. He had summed up—he had judged. "The horror!"
In the end, Kurtz's darkness is something that Marlow has to turn away from. Kurtz is the other side of the coin—an illustration of what could have been in Marlow's heart. This leads Marlow to self-discovery, as Marlow recognizes that he doesn't have to be like Kurtz. His nature is shown in the end when he talks about Kurtz with his fiancee in England.
Kurtz's fiancee doesn't know about his descent into madness or his evil acts. She knows him as a good and kind person, saying that "Men looked up to him—his goodness shone in every act." Marlow doesn't contradict her. Instead, he agrees with her and lets her maintain her false image of the man.
Though Kurtz's last words were actually, "The horror! The horror!" Marlow doesn't tell Kurtz's fiancee this. Instead, he says that Kurtz's last words were her name. This shows the kindness in him; though he would have preferred to be honest and dissuade her of her notions of Kurtz's goodness, he chooses instead to comfort her and give her something to hold onto.
Marlow's journey into the enigmatic, dangerous Congolese jungle in search of Kurtz symbolically represents his discovery of mankind's inherent wickedness. While Marlow literally travels from Europe to Kurt'z station located in the depths of the Congolese jungle, he metaphorically embarks on a journey to discover mankind's inherent nature. At the beginning of his journey, Marlow has naive conceptions of European culture and believes that Europeans are the beacons of civilization, educating and helping "savages" become modernized. As Marlow's journey progresses, he witnesses the inefficiency, greed, and incompetence of the Company and its employees. Despite the negative perception of the Company, Marlow believes that Kurtz is the exception and anticipates meeting a civilized, refined individual. Unfortunately, Marlow discovers that Kurtz is a debased, fanatical person, who rules as diety in the Congolese jungle. His lack of civility and unrestrained desire indicate his corrupted spirit. By the end of his journey, Marlow gains insight into the wicked nature of mankind and realizes that civility is simply a thin veil covering a corrupted, sinful heart.
The Heart of Darkness is to be understood as a literal journey into the Congo (a physical darkness) as well as a metaphorical journey into the heart of the individual. In the novella, the individual is Marlow. We have evidence of this being a journey of self discovery by the comments that Marlow inserts while narrating the literal journey. This metaphorical journey is double layered as Marlow tells the reader the story of Kurtz. Immediately Marlow identifies himself with Kurtz. The difference being that Marlow does not succumb to the darkness which may be found in every man's soul as Kurtz has.
Marlow's discovery of Kurtz is shrouded in mystery. Slowly Marlow learns who Kurtz was and who Kurtz became as well as learning about himself. He gets close enough to the edge to peer into the darkness but does not fall into it. They mystery of Kurtz and the process of learning about him can also symbolize the process of learning about one's self.
In addition to the answers above, an important aspect to consider is the effect of the Congo on isolating Marlow, in the same way that it isolated Kurtz. One of the themes of Heart of Darkness is what happens when someone leaves civilization and ventures into the unknown (in this case, the wild forests of Africa, which are juxtaposed with European civilization). Most of the time, our inhibitions are guided by civilization and its restraints. What happens when those restraints are removed? The self must then observe who it truly is.
Like the previous answer suggested, Marlow initially had thought of himself as representing the European "beacon" of reason and enlightenment (along with Kurtz, a fellow European). This "reason" or a feeling of civilized superiority was initially the justification for the European colonization of Africa. However, when Marlow actually went to Africa, he realized that in fact, this justification was a lie. Kurtz, the European man who was supposed to have symbolized such reason, was found to be even more savage, greedy, bloodthirsty and exploitative than the "savages" the Europeans were supposedly colonizing.
In this way, Marlow was forced to confront not only himself, when placed in a foreign environment, but also the dubious basis of his European imperialist mindset.