With respect to the use of "doubles," Joseph Conrad's The Secret Sharer and Robert Louis Stevenson's Dr. Jekyll and Mr Hyde share more similarities than either of those works share with Conrad's Heart of Darkness. In both Secret and Jekyll, the doubles clearly reside in one man and the doubles are clearly delineated--one good and the other evil. In Heart, on the other hand, one could make an argument that both Kurtz and Marlow have doubles within them, but in both Kurtz's and Marlow's cases those doubles are less delineated than in the main characters of Secret and Heart. Still, in both cases, the exploration of doubles finds that within each man there is a force for evil that does battle with the force for good. While the doubles advance the stories in all three cases, the more fascinating aspect of the issue is the psychological and what it shows in each story about the evil within every human being.
Stevenson's story is about the transformation of an eccentric but relatively reasonable doctor into a monster, or, rather, into the evil half of his being. Psychologically and symbolically, Jekyll's experience might be said to illustrate the dangers of denying an important part of one's reality until it is too late, although in the end Jekyll has come to accept, although perhaps too much, his evil half.
A friend of Dr. Jekyll meets Mr. Hyde (who is, in fact, the embodiment of the evil within Jekyll, an evil that has manifested as a result of the Doctor's experimentation with drugs), and that friend reflects on his meeting with the strange Hyde:
`There must be something else,' said the perplexed gentleman. `There is something more, if I could find a name for it. God bless me, the man seems hardly human! Something troglodytic, shall we say? . . . Or is it the mere radiance of a foul soul that thus transpires through, and transfigures, its clay continent? The last, I think; for, O my poor old Harry Jekyll, if ever I read Sata...