During a native attack on the steamboat, the helmsman -- also a native -- is mortally wounded by a thrown spear. Marlow takes over the steering, and is shocked by the quiet acceptance of the helmsman in death:
I had to make an effort to free my eyes from his gaze and attend to the steering.
'Can you steer?' I asked the agent eagerly. He looked very dubious; but I made a grab at his arm, and he understood at once I meant him to steer whether or no.
(Conrad, Heart of Darkness, eNotes eText)
Here, the effects of the jungle are seen in Marlow's actions; he is steering the steamboat out of necessity, but as soon as he is able, he gives the job over to someone else so he can change his shoes and not have to think about the dead helmsman. This is the blithe acceptance of death that slowly infects Marlow's mind; before the jungle, he would never treat a death with such disdain. By switching his focus from the reality of death to the pragmatism of steering the steamboat, Marlow shows that he jungle has already started to change his outlook, exposing the darkness that hides in his own heart.