they seem to be symbolic of something to do with human nature, Conrad's racist views and civilized/uncivilized tribes/groups of people, but i'm really not sure.
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The natives or "savages" are a collective presence throughout the story. They are never described as individuals rather than an idea. They are the mystery and darkness of the Congo. White pilgrims do not attempt to understand them. They remain an enigma: the shadow side of whatever the whites hope to gain from the exploitation of the land.