Read the excerpt from "An Image of Africa: Racism in Conrad’s Heart of Darkness.”

Conrad, after all, did sail down the Congo in 1890 when my own father was still a babe in arms. How could I stand up more than fifty years after his death and purport to contradict him? My answer is that as a sensible man I will not accept just any traveller's tales solely on the grounds that I have not made the journey myself. I will not trust the evidence even of a man's very eyes when I suspect them to be as jaundiced as Conrad's.

What type of evidence does Achebe use to support his claim in this passage?

geographical evidence
direct quotations
anecdotal evidence
summaries of text