In "Heart of Darkness", who said, "Ah, my boy...trust to this-I say, trust to this."

This quote was given by the uncle of the station manager while talking to his nephew and pointing towards the forest. Thus,the word "this" refers to the forest. What he is implying is that his nephew should trust in the dangers and climate of the forest to take care of those he fears, including Kurtz and Marlow.