By the end of his journey into the Congo, Marlow is so mixed up that he might as well be singing, "I Am the Walrus." Although he starts off with a pretty clear sense of who he is (white, successful, explorer), the jungle and the wilderness pretty quickly get him all mixed up. Is black white? Is civilization actually wild? Is Kurtz really that different from Marlow? And who is the manager, anyway? And are we really all just hollow inside? Heart of Darkness isn't about to say.
How is Africa a place of emptiness from the white European perspective? How are even places of civilization—cities and trading stations—empty in terms of European amenities and values? What characters lack essential human characteristics? How do they show their fundamental emptiness? And are these characteristics actually essentially human? How are the white pilgrims deprived of their senses and reason as they descend deeper and deeper into the interior? What characteristics or understanding do both Marlow and Kurtz lack? How does this affect their interpretation of the events toward the end? It may be helpful to look at their comments about language here.
Men may go into the interior whole and unscathed, but the hostile wilderness quickly drains them of their humanity.
The wilderness only exposes the essential emptiness in every man's heart.