How does Marlow change throughout the novella Heart of Darkness?

In Joseph Conrad's Heart of Darkness, Marlow is rarely directly involved in the action of the story, and serves mainly as a narrator who relates the story of Kurtz's degradation. Be that as it may, Marlow does not escape the narrative unscathed, and he changes from an idealistic and excited young boy into a man hardened by the evils dwelling within the human heart.

Marlow tells us that he was passionate about maps as a young boy and dreamed of traveling to distinct locales, especially Africa. From this description, we can surmise that Marlow, like many young children, was idealistic and a touch naive. Moreover, we can assume that he considered traveling to be adventurous and full of excitement. By the time Marlow finds Kurtz and witnesses the man's death, however, things have changed. At that point in the story, Marlow has come to recognize the evil corruption governing the European presence in Africa, and he regards it with revulsion. That is not to say, however, that Marlow is entirely embittered by the end of the story. It would be more accurate to say that he regards exploration more realistically and no longer entertains naive boyhood fantasies. As such, we can see that, through the course of the narrative, Marlow matures considerably and is better equipped to view colonization from a critical point of view.