....

        Charlie Marlow, thirty-two years old, has always "followed the sea", as the novel puts it. His voyage up the Congo river, however, is his first experience in freshwater travel. Conrad uses Marlow as a narrator in order to enter the story himself and tell it out of his own philosophical mind.
        When Marlow arrives at the station he is shocked and disgusted by the sight of wasted human life and ruined supplies . The manager's senseless cruelty and foolishness overwhelm him with anger and disgust. He longs to see Kurtz- a fabulously successful ivory agent and hated by the company manager. More and more, Marlow turns away from the white people (because of their ruthless brutality) and to the dark jungle ( a symbol of reality and truth.) He begins to identify more and more with Kurtz- long before he even sees him or talks to him. In the end, the affinity between the two men becomes a symbolic unity. Marlow and Kurtz are the light and dark selves of a single person. Marlow is what Kurtz might have been, and Kurtz is what Marlow might have become.

http://www.acsu.buffalo.edu/~csicseri/chara01.htm