Even before Marlow really begins his tale, he reminisces about the importance of maps to his childhood and how they were a site of intense imagination and adventure. He remembers in particular Africa and describes a river, the biggest on the map, that he sees there:
But there was in it one river especially, a mighty big river, that you could see on the map, resembling an immense snake uncoiled, with its head in the sea, its body at rest curving afar over a vast country, and its tail lost in the depths of the land. And as I looked at the map of it in a shop-window, it fascinated me as a snake would a bird--a silly little bird.
Later on, when he heard about the possibility of employment going up this river, he remarks, "The snake had charmed me." Note the way in which Marlow describes himself as a "silly little bird" being "charmed" by a snake before it is killed. This is clearly foreshadowing something of the tragic events and changes that Marlow will undergo and witness on his voyage. However, at the same time the river seems to be used to symbolise temptation and evil. Marlow will witness just how degraded and unscrupulous humanity can be by going up the river, with its prime example in Kurtz. The snake leads to the heart of darkness in exposing the true evil to which all men are capable of surrendering or being tempted by.