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"I tried to break the spell—the heavy, mute spell of the wilderness—that seemed to draw him to its pitiless breast by the awakening of forgotten and brutal instincts, by the memory of gratified and monstrous passions. This alone, I was convinced, had driven him out to the edge of the forest, to the bush, towards the gleam of fires, the throb of drums, the drone of weird incantations; this alone had beguiled his unlawful soul beyond the bounds of permitted aspirations." (3.29)
Marlow totally gets it: he feels the spell of the wilderness, just like Kurtz does, but he's not going to succumb to it. Really. He promises.
"The long reaches that were like one and the same reach, monotonous bends that were exactly alike, slipped past the steamer with their multitude of secular trees looking patiently after this grimy fragment of another world, the forerunner of change, of conquest, of trade, of massacres, of blessings." (3.38)
Marlow seems to see the wilderness as existing in a separate time from civilization, a sort of eternal land of the lost effect that makes civilization equivalent to change. Wow, that's a lot to get out of "monotonous bends."
"We broke down—as I had expected—and had to lie up for repairs at the head of an island. This delay was the first thing that shook Kurtz's confidence." (3.39)
Marlow's steamboat breaks down again. Gee, it's almost like nature doesn't want them to take Kurtz back to civilization.