Apart from the natural cycle of day and night, the other element of darkness is the interior of the island itself. It is a wild, jungle-like biotope and very aptly represents the more savage, instinctive side of man. Note that Jack and his hunting gang automatically gravitate towards it whereas the more "civilized" group of boys huddle together for security on the beach. The boys have a hard time finding a balance between these two extremes, and finally the wild, 'survival of the fittest' side wins out.
An interesting side note is that at one point Ralph takes refuge in the underbrush when the boys are hunting him out. In this case, the obscurity the dense foilage offers represents protection and temporary security in a hostile environment.
The final "darkness" on the island comes from the wildfire on the mountain. Ironically, it is the smoke from the fire which attracts the attention of a passing ship and is the reason why the boys were ultimately saved. Paradoxically, it represents both destruction and a means of escape. The boys' distress is finally recognized and they are literally saved from themselves.
Darkness in Lord of the Flies is found on the island in the forest. The darkness is a metaphor for the fear and uncertainty that the boys experience on the island as they imagine a beast. Further, darkness comes to represent the darkness of men's hearts, or evil that lies within human nature, a darkness that gradually manifests itself in the actions of several of the boys.
In Chapter Two the littl'uns imagine that they have seen something in the dark—"the beastie"—that turned into the vines lying in the trees. Then, in Chapter Five, the place of assembly is described as having the beach on two sides, the lagoon behind it, and "in front, the darkness of the island." This darkness that is before them literally foreshadows the evil which will emerge in the boys. The evil that is later manifested is the intimidation that Jack employs in order to control others, the theft of the fire, the beating to death of Simon, the slaying of Piggy, and the destructive hunt for Ralph.
In Chapter Eight, entitled "Gift for the Darkness," Simon encounters the Lord of the Flies, who tells him:
"You knew, didn't you? I'm part of you? Close, close, close! I'm the reason why it's no go? Why things are what they are?"
Further, the pig's head swarming with flies represents evil and corruption, and when the yawning mouth widens, "there was blackness within, a blackness that spread." This "blackness within . . . that spread" represents the evil that exists in man. Simon has looked inside the pig's mouth and recognized the "beast" that is on the island. Then, he tries to explain where the evil lies to the others, but he is killed.
Darkness is often a symbol of evil. The darkness that is said to be creeping over the island is a reference to the evil that is evolving inside each of the boys.