Which line in the following excerpt from joseph conrad’s heart of darkness indicate a flashback?

he paused.

"mind," he began again, lifting one arm from the elbow, the palm of the hand outwards, so that, with his legs folded before him, he had the pose of a buddha preaching in european clothes and without a lotus-flower—"mind, none of us would feel exactly like this. what saves us is efficiency—the devotion to efficiency. but these chaps were not much account, really. they were no colonists; their administration was merely a squeeze, and nothing more, i suspect. they were conquerors, and for that you want only brute force—nothing to boast of, when you have it, since your strength is just an accident arising from the weakness of others. they grabbed what they could get for the sake of what was to be got. it was just robbery with violence, aggravated murder on a great scale, and men going at it blind—as is very proper for those who tackle a darkness. the conquest of the earth, which mostly means the taking it away from those who have a different complexion or slightly flatter noses than ourselves, is not a pretty thing when you look into it too much. what redeems it is the idea only. an idea at the back of it; not a sentimental pretense but an idea; and an unselfish belief in the idea—something you can set up, and bow down before, and offer a sacrifice to. . "
he broke off. flames glided in the river, small green flames, red flames, white flames, pursuing, overtaking, joining, crossing each other—then separating slowly or hastily. the traffic of the great city went on in the deepening night upon the sleepless river. we looked on, waiting patiently—there was nothing else to do till the end of the flood; but it was only after a long silence, when he said, in a hesitating voice, "i suppose you fellows remember i did once turn fresh-water sailor for a bit," that we knew we were fated, before the ebb began to run, to hear about one of marlow's inconclusive experiences.