Themes in Heart of Darkness
Like a knight of the Round Table, Marlow sets off in search of strange adventures. He only gradually acquires a grail, as he picks up more and more hints about Kurtz. Like a knight he is frequently tested by signs he must confront, question and interpret. Signs are things you see or experience or are told which have meaning beyond the literal: old women knitting black wool might simply be relatives of the company personnel given some position of respect and usefulness, or the somber color of their wool and clothing, and their serious demeanor, might suggest that they mind the gateway to a mysterious underworld. You might take as signs the following:

Even before he sets out, omens present themselves to Marlow; old women knitting black wool in the Belgian office, the phrenologist measuring Marlow's skull and warning of changes to take place inside, the tale of how his predecessor died in an uncharacteristic dispute over black hens.