At one point in the novel, Hyde is described as a “troglodyte.” To what does this term refer? What was its significance in Victorian England?...

The term "troglodyte" means "cave dweller". It is derived from two Greek words, "trogle" meaning "hole", and "dyein", meaning "to dive into." In Victorian English, it typically refers to prehistoric humans who were presumed to live in caves and carries with it the sense of the barbarian or primitive, roughly equivalent to the way we might now use the term "Neanderthal" colloquially to refer to someone whose attitudes we considered very old-fashioned or crude.

The Strange Case of Dr. Jekyll and Mr. Hyde was published in 1886, some 27 years after the publication of Darwin's On the Origin of Species, which was published in 1859. This use of the term to describe Mr. Hyde has a Darwinian flavor, in which the unrestrained, primitive nature of Mr. Hyde is shown in his looking like a troglodyte, or someone at an earlier stage in human evolution than the erudite and civilized Dr. Jekyll. This relates to the main theme of the novel, that we have a primitive human nature that has been restrained by civilization, and that one of the benefits of civilization is precisely the degree to which it restrains our baser instincts.