The most surprising thing about Stevenson's horror story if you had previously known the Jekyll and Hyde characters only indirectly through popular culture is that the story's so brief. Not just the novella—which is short enough—but the story of that the famous two-sided personality takes up only the last quarter of the text.

Reading it today, we know of course that Jekyll and Hyde are the same person (in a manner of speaking). Is there anyone alive today for whom this comes as a spoiler? And yet it takes most of the novella for the figures we follow—not Jekyll/Hyde but his various friends and colleagues—to discover this fact. It is almost impossible for us in this era when superheroes with secret identities, MPD, personality altering drugs and transformers are cultural tokens to realize what a shocker the concept must have been in the Victorian era when this tale was written. And of course this story itself has become part of our daily reference whenever we talk about changeable characters we know being Jekylls and Hydes.

There's an interesting difference I've noticed though between Stevenson's original idea and the passed-down current concept of the two-faced character. In the book, counter to most movie treatments of Jekyll and Hyde, the latter person is physically smaller, representing only the incomplete backward personality. Dr. Jekyll is the larger, more evolved being, who has progressed beyond the primitive state, while Mr. Hyde is a regression to an earlier stage of development. Hyde is more vital and primal in his appetites because he has cast off the civilizing evolution that restrains the doctor. But still he is only a partial person while Jekyll is the complete man. Unfortunately the complete man yearns to be able to indulge in the lascivious and guiltless activities of the brute within him.

When we talk about Jekyll and Hyde today however, we generally mean a good and a bad character, forming a kind of a yin-yang complementarity, each being only half of the whole. We refer to the good side and the evil side of a person. Hollywood has generally seen the Stevenson story this way too, preferring to sell the idea in vaguely supernatural rather than evolutionary terms. Often too the story is recast as being about a scientist tampering with matters that should be left to God and thereby losing his soul—similar to the cautionary Frankenstein tale.

The nineteenth-century horror tale that Dr Jekyll and Mr Hyde is closest to however may be Bram Stoker's Dracula, which came out only a year later. There is, first of all, some similarity of style, with Stevenson employing multiple narrators, telling the story partly through letters and documents, as Stoker does to a greater extent. And secondly there is of course the similar themes of people giving up their humanity, in the one case with the help of a drug and in the other case due to a blood infection.

Another intriguing coincidence is that both of these stories became wildly popular at a time when ideas of evolution—of species not being fixed but continually transforming—were catching on among the general public (although still considered radical ideas). Both of these literary works at this time look backwards, to more feral, animal pasts, and wonder how much of them is still in us.