For reference purposes, I’ll be citing a PDF version of the text (linked below).
While Darwin’s influences are seen in the physical side of Henry Jekyll’s terrifying transformation, you’ll find Freud’s influence in his psychological changes.
Darwin’s theories are well known today in the concept of evolution—the physiological change over time of living organisms, from simple to complex, which has resulted in the large variety of species on our planet. The idea that man is descended from beast, and yet intellectually beyond beasts, is an idea that fascinated the Victorian era. This particular story encapsulates the idea that a bit of that beast is still living within every man, and Henry Jekyll just happened to come up with the formula to release his own to its full potential. Hyde is often described in beastly terms, and he appears to be hairier and less well-groomed than the gentlemanly Jekyll. In Jekyll’s own description of himself in the final chapter, he remarks that he finds himself suddenly looking at
“the hand that lay on my knee [which] was corded and hairy” (89).
When Utterson encounters Hyde for the first time, Hyde’s laughter is described as an animal “snarl” (19).
Similarly, Freud’s theories of psychology presented Victorians with another explanation of their beastly tendencies: the superego and the id (sometimes symbolized in media as the angel and devil on one's shoulders, respectively) battling it out for control of the ego. The ego is the closest thing to one’s “self,” as Freud understood it, though that self is developed by a myriad of different experiences. The superego comes from society and is often loaded with ideas of supernatural or religious codes of behavior. The superego is concerned with consequences and punishment. This is the source of guilt: the part of you that says, "to do this would be a bad idea." On the other hand, the id is the seat of all impulse and desire. It does not think about consequences.
The id is clearly depicted in Hyde, though I would argue that there is no superego present. Religion is absent from the picture. In fact, Jekyll acknowledges that a spiritual side of him exists, but he doesn’t tend to it or nurse it in the way that he has nurtured the impulsive id from which Hyde has developed. Before his shocking transformation in Regent’s park,
“I sat in the sun on a bench; the animal within me licking the chops of memory; the spiritual side a little drowsed, promising subsequent penitence, but not yet moved to begin” (89).
It is after this moment that Jekyll transforms, unwillingly, into Hyde—full evidence that the id did indeed win out over the nearly ignored superego.