He goes on to say that Conrad worries not about the differentness of the two rivers, but about their common ancestry. "And this also," said Marlow suddenly, "has been one of the dark places of the earth." The Thames has conquered its darkness and now it's peaceful.
Unfortunately, in saying this, Achebe is missing the point. Africa is the darkness, on the outside, but it is an irony in that the Englishmen who go to Africa and are colonizing there are the ones who are dark and barbarous. They are greedy and have become dark, like the appearance of the Africans. Perhaps the "darkness" of the Congo has brought out that animalistic instinct, but the pagan rites and savage dances are not only done by the Africans, but ironically and hypocritically, by the English as well. C.P. Sarvan states that, "As for pagan rites and savage dances, the Europeans with 'imbecile rapacity' were 'praying' to ivory, that is, to materialism, and one red-haired man 'positively danced,' bloodthirsty at the thought that he and the others 'must have made a glorious slaughter' of the Africans in the bush."