In Heart Of Darkness, Marlow changes in the sense that he moves towards self,whilst moving towards the centre of earth, taking on a whole new perspective as he does so. His character develops as he is able , on his voyage on the sea, to perceive himself and the whole of society with a sharper clarity than he has envisaged it before. The infinite blackness or dark at the earth's heart throws all this into relief for him, the 'scales fall from his eyes' (as it says in the bible) and all of a sudden he 'sees the light.' His long travails down into the earth represent many similar voyages in literature that require a downward spiral to the pits or bowels of existence/earth before the rise like a phoenix from the ashes - and a triumphant seeing/acquisition of some sort of light, usually spiritual. This triumph needs to have a deep and fundamental 'change of heart' by the traveller.