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"It was a moment of triumph for the wilderness, an invading and vengeful rush which, it seemed to me, I would have to keep back alone for the salvation of another soul." (3.51)
The tables turn! Instead of the explorers "invading" the wilderness, the wilderness is invading the explorers. Hm. Doesn't feel so good when someone does it to you, does it, Marlow?
"[…] she [the Intended] went on, and the sound of her low voice seemed to have the accompaniment of all the other sounds, full of mystery, desolation, and sorrow, I had ever heard - the ripple of the river, the sighing of the trees swayed by the wind, the murmurs of the crowds, the faint ring of incomprehensible words cried from afar, the whisper of a voice speaking from beyond the threshold of an eternal darkness." (3.61)
Marlow associates the Intended's low voice with sounds of the wilderness. Sure, these are slightly friendlier sounds than we're used to hearing from the wilderness—trees swaying, rivers rippling—but they still make us wonder if the line between civilization and the natural word is all that firm.
[Marlow]: "Mind, none of us would feel exactly like this. What saves us is efficiency—the devotion to efficiency. But these chaps were not much account, really. They were no colonists; their administration was merely a squeeze, and nothing more, I suspect. They were conquerors, and for that you want only brute force—nothing to boast of, when you have it, since your strength is just an accident arising from the weakness of others. They grabbed what they could get for the sake of what was to be got. It was just robbery with violence, aggravated murder on a great scale, and men going at it blind—as is very proper for those who tackle a darkness. The conquest of the earth, which mostly means the taking it away from those who have a different complexion or slightly flatter noses than ourselves, is not a pretty thing when you look into it too much. What redeems it's the idea only. An idea at the back of it; not a sentimental pretence but an idea; and an unselfish belief in the idea." (1.13)
Aaaand, now Marlow undermines everything he just said about the nobility and good intentions of the explorers. He's seen how messed up colonization really is, and he knows that the colonizing countries care only about efficiency and profit. The explorers aren't heroes; they're robbers and murderers who just wanted to bring home profit.