The predominant theme of chapter six, "Beast from the Water," is fear. During the night, a parachute carrying a dead man has dropped onto the island; Samneric mistake the silhouette of the dead man for the beast. In the twins' minds, they saw something "furry. There was something moving behind its head--wings" (100).
The beast represents the sum of all the boys' fears on the island; now that Samneric have identified the beast in its physical form, Ralph determines that they must explore the island to find the beast's whereabouts and relight the signal fire.
Another theme of "Beast from the Air" is authority. Ralph and the conch represent authority in the novel, but as the chapter progresses, Jack begins to challenge Ralph's authority more openly than ever before. He rejects the value of the conch in the boys' meeting by arguing:
"'Conch! Conch!' shouted Jack. 'We don't need the conch! We know who ought to say things. [...] It's time some people knew they've got to keep quiet and leave deciding things to the rest of us'" (101-102).
Ralph orders Jack to sit down, but the tension between the two boys continues on their mission to find the beast. By the time they reach Castle Rock, both Jack and his hunters openly challenge Ralph's authority, refusing to listen to his orders and complaining openly. Finally, Ralph has to shout at them to get the other boys to listen, and narrator concludes: "Mutinously, the boys fell silent or muttering" (108). The connotation of 'mutinously' suggests that the boys, extremely unhappy with Ralph's decision to make them go on toward the mountain, are actively considering a change of leadership. The chapter ends with the image of Jack, not Ralph, leading the boys across the rock bridge.