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"Perhaps I had a little fever, too. One can't live with one's finger everlastingly on one's pulse. I had often 'a little fever,' or a little touch of other things—the playful paw-strokes of the wilderness, the preliminary trifling before the more serious onslaught which came in due course." (2.14)
Aw, look, nature is a little kitty cat! Oh, wait. Not a little kitty cat; more like a hungry cheetah. Nature is depicted as wickedly playing with Marlow's health for its own amusement before hitting him with a real assault.
"We had just floundered and flopped round a bend, when I saw an islet, a mere grassy hummock of bright green, in the middle of the stream. It was the only thing of the kind; but as we opened the reach more, I perceived it was the head of a long sand-bank, or rather of a chain of shallow patches stretching down the middle of the river. They were discoloured, just awash, and the whole lot was seen just under the water, exactly as a man's backbone is seen running down the middle of his back under the skin." (2.18)
The riverbank, a manifestation of nature, is compared to a man's backbone. This is another instance of Marlow considering the wilderness a live thing.
"I had to lean right out to swing the heavy shutter, and I saw a face amongst the leaves on the level with my own, looking at me very fierce and steady; and then suddenly, as though a veil had been removed from my eyes, I made out, deep in the tangled gloom, naked breasts, arms, legs, glaring eyes—the bush was swarming with human limbs in movement, glistening of bronze colour." (2.21)
Here, the forest swarms with human activity—furthering the association of Nature with the living. Nature's ill will towards the pilgrims is now manifested in the native Africans' surprise attack on Marlow's steamboat. The Africans are depicted as an extension of Nature and minions of her will.