Robert Louis Stevenson's famous novel added a new phrase to the English language; that of the "Jekyll-and-Hyde" character, meaning an individual who exhibits two contrasting personalities at different times, usually one peaceful and one aggressive. Almost everyone knows at least the outlines of the story: how the mild and perhaps somewhat repressed Dr. Jekyll, wishing to recapture the days of his irresponsible and carefree youth, developes a drug that transformed him into the violent, amoral Hyde. There is also an antidote that enables him to return to the character of Jekyll. The climax might be foreseen: after several uses the antidote eventually fails, he becomes trapped in the persona of Hyde and eventually dies.
This is often taken to be a description of schizophrenia, as indeed the "Jekyll-and-Hyde" phrase implies, but having reread the book I would question this. It is not only Jekyll's personality that changes when he becomes Hyde, his physical appearance and his voice are also altered so radically that even Jekyll's servants and his oldest friends fail to recognise him. His face is much uglier, his form is shrunken and repellant, and the backs of his hands are described as being covered with coarse black hair, which apparently disappears immediately he once more becomes Jekyll. It was this last detail which suggested to me that Stevenson was not dealing with modern notions of schizophrenia, but had actually developed a new, pseudo-scientific variation on a very ancient legend: that of the werewolf. I'm sure no reader ever takes seriously the account of Dr Jekyll's drugs that effect the transformation: they are the least convincing aspect of what is otherwise a gripping story.