It would come slowly to one. They howled and leaped, and spun, and made horrid faces; but what thrilled you was just the thought of their humanity -- like yours -- the thought of your remote kinship with this wild and passionate uproar." (Conrad 105). This indicates a gradual shift of viewpoint from the Western, or civilized, to the uncivilized. In this, Marlow's viewpoint shift foreshadows his meeting with Kurtz. The latter is iconic of the completion of this viewpoint. The reader is therefore prepared for an increased contact with darkness as Marlow travels deeper into the physical darkness of Africa towards the ultimate heart of the matter personified in Kurtz. The decay of Kurtz's station indicates not only his absence, but also his lingering influence during the time when he was present at the station. It is central to the novel to note that the barbarian nature of the natives influenced the Western-born Kurtz and Marlow to a greater degree than their civilization. The novel closes with the indication that barbarism waits just beneath the surface of even the highest civilization. In this, Conrad makes his point regarding the true nature of humanity.

Another poignant issue in the novel is the juxtaposition of what is regarded as civilized with the barbaric displays of violence towards those regarded as barbarians. "Civilized" colonists such as Kurtz, for example, brutalize the native tribes into submission. These colonists therefore act barbarically to begin with, and it is therefore not difficult to believe that they can be utterly seduced by the barbarian way of life. This seduction ruined Kurtz's body and Marlow's soul. Indeed,...

When finally meeting Kurtz, Marlow is struck by the decay that seems to have invaded his body: "He rose, unsteady, long, pale, indistinct, like a vapour exhaled by the earth, and swayed slightly, misty and silent before me." (Conrad 142).
The decay in Kurtz's body then signifies the effect of the clash between the civilized and uncivilized viewpoints. Marlow is in the process of being seduced by the darkness when he meets Kurtz. In Kurtz, this seduction is complete and disastrous. Kurtz then represents the viewpoint of ruined civilization and concomitantly ruined humanity. All that remains for him is not only the horror of what he encountered, but also the horror of his own actions. This is the horror that threatens to overwhelm Marlow at the end. In this, Marlow completes his journey and his change of viewpoint. He understands the darkness not only of Africa, but also of his native Western world.

Marlow then comes full circle in his physical journey. However, his soul can never return to its former innocence. His view of the heart of darkness has forever ruined him for both civilization and the uncivilized environment of Africa. In accommodating both viewpoints, Marlow cannot be truly part of either. This is the tragic crux of Conrad's novel.

Sources

Conrad, Joseph. Heart of Darkness. University of Virginia, 1999. Online version:

http://etext.virginia.edu/toc/modeng/public/ConDark.html

Mohring, Brent. "Heart of Darkness." 2007. http://caxton.stockton.edu/brent/stories/storyReader$19