Throughout his life, Stevenson was always interested in mental illness and distortions of the mind and a lot of his novels are based on or involve schizophrenic characters. When growing up he had followed the case of Deacon Brodie - a peaceable cabinet maker by day and murderer by night. In my opinion, he saw the duality as a reflection of his own life; because he too had a split personality. Maybe he was using "Jekyll and Hyde" to portray his own personality.
At the time he wrote "Jekyll and Hyde", Darwin's theories of evolution were just starting to be contemplated and not just discard…
