From the very outset of the play, civilization among the boys is under attack. It is represented in many ways by Ralph, by the conch, by the choir boys in uniform marching down the beach, and by Piggy and his desire to think things through and be rational.
As the boys get hungrier and hungrier and begin to descend into a more animalistic state, the civilization they've established quickly starts to unravel as do its representatives in Ralph and Piggy and the authority of the conch.
Eventually the entire idea of a civilized society is destroyed as the boys become a tribe of animals, driven to hunt and to kill, as they kill Simon, then Piggy, then bring about the destruction (at least symbolically) of the island by starting a fire that consumes much of it and perhaps many of the boys as well.
The theme of civilization is carried through-out the book. It begins with the destruction of a civilization (their country) by some unknown force that results in the boys being sent off to safety. On the way their transport crashes and ends up on the island with the boys. The pilot is killed.
The boys Ralph and Piggy find the cnch and when it is blown it calls the rest of the boys who come together. This is the formation of the island's civilizaton. The conch is then used to help the group to behave in a civilized manner by being used to state that the only one who speaks and is the one to be listened to is the one who holds the conch.
Meetings are called to order with the conch. As civilization begins to deteriorate the conch becomes less important. Piggy and Ralph go to talk o Jack and the boys who have taken Piggy's glasses. He is the last one left wanting to protect what he knows of civilization. The conch falls to the ground shattered and Piggy is sent flying off onto the rocks and killed after Roger launches a boulder at him.
The breaking of the conch and Piggy's death demonstrate an end to civilization on the island.