In <em>The Strange Case of Dr. Jekyll and Mr. Hyde </em>the duality of human nature is one of the major themes. Stevenson is able to show that humans aren't born inherently bad or good, but somewhere in the middle.
It is a study about the mind of the author and the theories of dualism, and also about human psychology.
The word choice in the sentence adds to the development of the story by providing descriptive imagery. By doing this, the author allows readers the ability to visualize detailed images of the text in their heads.