What are the two natures of man according to Dr. Jekyll?

Robert Lewis Stevenson from what I can gather from his Dr. Jekyll and Mr Hyde novella was a dualist. He adopted a yin/yang approach to moral character. In the moral philosophy of dualism good and evil are two separate and independent entities. This view is seen in Zoroastrianism, Star Wars, and most eastern religions.
According to Dr. Jekyll, he discovered that the dual-nature of Man exists chemically in the brain and can be divided via medical means. This was Stevenson's attempt to link with moral philosophy and psychology. This is of course a very Victorian approach to ideology. The Victorian mind desired to reduce everything to a science, even things as unscientific as morality and metaphysics.
These two natures according to Jekyll are the positive (moral) aspect exemplified by the mild mannered Dr. Jekyll and the negative (immoral) aspect exemplified by the loathsome Mr. Hyde.