In the frame story that opens the novel, a sailor records what Marlow says as they sit on a boat on the Thames, the river that flows past London to the sea. Marlow muses about the Romans who first came up this river to conquer the Britons. In doing this, he is universalizing Kurtz's saga. What the modern northern Europeans, including the British, have done in Africa to the Africans is no different from what the Romans did to the northern Europeans almost two thousands years ago. The patterns of conquest and cruelty to those that are deemed the "other" have been going on forever, and what we do now is just a "flicker" in a larger pattern.

Marlow says, for example, that "this" place, Britain, has also been a heart of darkness, as Africa is in the time of the novel:

"And this also," said Marlow suddenly, "has been one of the dark places of the earth."

Marlow goes on to imagine what it was like for a Roman commander 1,900 years ago who might have felt surrounded by savagery:

I was thinking of very old times, when the Romans first came here, nineteen hundred years ago—the other day. ... We live in the flicker—may it last as long as the old earth keeps rolling! ... Imagine the feelings of a commander ... ordered suddenly to the north ... Imagine him here—the very end of the world, a sea the color of lead ...cold, fog, tempests, disease, exile, and death,—death skulking in the air, in the water, in the bush. ... in some inland post feel the savagery, the utter savagery, had closed round him,—all that mysterious life of the wilderness that stirs in the forest, in the jungles, in the hearts of wild men. ... He has to live in the midst of the incomprehensible, which is also detestable. And it has a fascination, too, that goes to work upon him. The fascination of the abomination—you know.

By likening Kurtz to the ancient Romans, a culture largely admired by the British, Marlow speaks to his fascination with and admiration of Kurtz for his mad audacity, an admiration that mingles with his sense of horror at what he has done. When Marlow says "All Europe contributed to the making of Kurtz," he speaks of a tradition that goes back to Roman times.

Marlow's experience in the jungle was of a man used to Western civilization placed suddenly out of his comfort zone, into a world where the instinct is more important and morality is a shifting, grey area. He compares his journey to that of the ancient Romans who landed on the British islands; those people were used to a certain type of world and found what was, to them, a harsh environment where they could not prosper according to their normal routines:

"They were men enough to face the darkness. And perhaps he was cheered by keeping his eye on a chance of promotion to the fleet at Ravenna by-and-by, if he had good friends in Rome and survived the awful climate."
(Conrad, Heart of Darkness, gutenberg.org)

His basic point, then, is that the men who journeyed across the ocean to a harsh place were hardy and strong, both in spirit and in body. They were almost designed to be explorers, people who could put aside their own comfort to find a purpose greater than themselves. Despite the removal of civilization, the Romans continued to search and colonize, and did not descend into madness and anarchy. In that fashion, Marlow believes that men who journeyed into the jungle were similar, but decides that he himself is not of the same strength; he survived, but barely, and admires those men who came out stronger instead of being destroyed.