In The Strange Case of Dr. Jekyll and Mr. Hyde, Hyde is described as "a troglodyte." To what does this term refer? What was its significance in Victorian England? How does it relate to the themes of the novel?
During the late XIX century, the theories of psychoanalysis that we know today, namely, those of Freud, were just becoming a part of the mainstream mindset of people everywhere. Multiple attempts had been made to study human behavior in hopes of predicting it.
Freud used the terms ego, super ego, and the Id. These were constructs that aimed to explain that every human mind consists on a dual nature. The Id, represented by Mr. Hyde, is the wild Homo Sapiens within all of us. He seeks instant gratification, immediate satiety, is reactive, and out of control.
The Ego is the opposite of the Id. Dr. Jekyll, paused, self-contained, and in complete control, is the social favorite whose poise and sophistication denote his capacity to monitor his thoughts, emotions, and behaviors.
The Super Ego, the Victorian society at large, sets the rules, expects, regulates, controls, governs, and also monitors the behaviors of all citizens. Duality and the inner capacity of adapting to the social rules are, therefore, the central themes of the novel.
This established, let us look at another hugely influential figure of the XIX century. Charles Darwin had already published around 1870 The Descent of Man. Here is where Darwin proposes his evolutionary theory which states that humans came from hairy, wild apes.
Now we see that the novel has two powerful social and psychological theories underlying its theme of duality: The Structural Theory (ego, Id) proposed by Freud, and the Theory of Evolution proposed by Darwin. You can imagine how transcendental it would have been to live in a time and place such as Victorian England, when all of these discoveries were taking place.
Back to the novel, the attribute of troglodyte given to Mr. Hyde summarizes the views just expressed.
There is something more..something troglodytic about him.
His wild nature places him closer to the bottom of the human food chain, as he does not adapt at all to the expectations of the super ego. Moreover, as a bottom feeder when compared to Jekyll, he is also closer to the primitive version of himself than to his evolved self; a troglodyte evolved from the bowels of earth. All of this responds to the theme of dualism that the novel is about.