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(Marlow): "Light came out of this river (the Thames) since—you say Knights? Yes; but it's like a running blaze on a plain, like a flash of lightning in the clouds. We live in the flicker—may it last as long as the old earth keeps rolling! But darkness was here yesterday." (1.11)
This is essentially a more poetic way of saying that humankind has really only been around for a fraction of a fraction of Earth's history. Conrad associates "light" with humankind—but then immediately undermines all our warm fuzzy feelings about light by using "knights" and "lighting" to make light seem, well, violent and destructive.
(Marlow): "They were men enough to face the darkness." (1.11)
Darkness here represents the unknown and potentially hostile land. Zzzz. This is so conventional that we have to wonder if Conrad really expects us to believe this—or if Marlow's perspective is as flawed as everyone else's.
[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)
Marlow says that "strength is just an accident arising from the weakness of others." In other words, he totally undermines any sense that these Western explorers are noble or motivated by emotions other than greed: they're just glorified robbers. Shh, don't tell Ayn Rand.