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"Near the same tree two more bundles of acute angles sat with their legs drawn up. One, with his chin propped on his knees, stared at nothing, in an intolerable and appalling manner, his brother phantom rested its forehead, as if overcome with a great weariness; and all about others were scattered in every pose of contorted collapse, as in some picture of a massacre of a pestilence. While I stood horror-struck, one of these creatures rose to his hands and knees, and went off on all-fours towards the river to drink. He lapped out of his hand, then sat up in the sunlight, crossing his shins in front of him, and after a time let his woolly head fall on his breastbone." (1.39-41)
Here we see the true consequences of imperialism—mistreated and overworked slaves who are left to die on their own. They're given no food, care, or medicine, and are left to die outdoors. But notice how Marlow calls them "bundles," "creatures," and phantoms"? They're treated so inhumanely that Marlow can't even see them as fully human.
"It was unearthly, and the men were—No, they were not inhuman. Well, you know, that was the worst of it—this suspicion of their not being inhuman. 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. Ugly. Yes, it was ugly enough; but if you were man enough you would admit to your self that there was in you just the faintest trace of a response to the terrible frankness of that noise, a dim suspicion of there being a meaning in it which you—you so remote from the night of first ages—could comprehend." (2.8)
Marlow begins to feel a teeny, tiny sense of kinship with the native Africans, and he even says that he's starting to understand their screams. At this point, Marlow is turning away from the traditional views of imperialists, who do not see the conquered native Africans as human.
"And between whiles I had to look after the savage who was fireman. He was an improved specimen; he could fire up a vertical boiler. He was there below me, and, upon my word, to look at him was as edifying as seeing a dog in a parody of breeches and a feather hat, walking on his hind-legs. A few months of training had done for that really fine chap. He squinted at the steam-gauge and at the water-gauge with an evident effort of intrepidity—and he had filed teeth too, the poor devil, and the wool of his pate shaved into queer patterns, and three ornamental scars on each of his cheeks. He ought to have been clapping his hands and stamping his feet on the bank, instead of which he was hard at work, a thrall to strange witchcraft, full of improving knowledge. He was useful because he had been instructed; and what he knew was this—that should the water in that transparent thing disappear, the evil spirit inside the boiler would get angry through the greatness of his thirst, and take a terrible vengeance. So he sweated and fired up and watched the glass fearfully (with an impromptu charm, made of rags, tied to his arm, and apiece of polished bone, as big as a watch stuck flatways through his lower lip), while the wooded banks slipped past us slowly…." (2.8)
Marlow may not be a total racist jerk, but he still doesn't consider the native Africans his equal. He sees them instead as animals, calling this fireman "a dog in a parody of breeches and a feather hat, walking on his hind-legs." They don't bother teaching him anything about hydraulics or engineering; they just tell him that an evil spirit will take revenge if the boiler ever becomes empty. This is essentially the same as getting your kid to stay in bed by telling her that there's a monster under it.