In The Call of the Wild, by Jack London, our main character, a dog by the name of Buck, has several visions of what seems like a caveman by a fire.
In Chapter 4 this vision is described:
This other man was shorter of leg and longer of arm, with muscles that were stringy and knotty rather than rounded and swelling. The hair of this man was long and matted, and his head slanted back under it from the eyes. He uttered strange sounds, and seemed very much afraid of the darkness, into which he peered continually, clutching in his hand, which hung midway between knee and foot, a stick with a heavy stone made fast to the end. He was all but naked, a ragged and fire-scorched skin hanging part way down his back.
In this novel, our main character journeys not only physically, but emotionally. Buck starts out as a pet, but he ends his journey as a wild animal, taking his place in nature. In his visions, he is tapping into his "ancestral" memories as a wild dog by the fire with a caveman-type being.
Because this novel is about Buck's transition from pet to wild being, the author uses different devices to highlight this transition. The vision is one of those devices. At first the vision is vague. However, the further he travels down his path from civilized to primitive, the more vivid the description becomes.