In the Lord of the Flies, the boys find themselves stranded on a deserted island with no adults to supervise them.
At first, the boys begin by forming rules and a structured way of life. In doing so, they attempt to be orderly. It isn't long, however, before these rules quickly begin to be disregarded.
As they begin to disregard the rules, they begin to give into their savage sides. They stop wearing clothes, they cannot keep the signal fire going, and they only want to hunt and chant. They begin to be afraid of a "thing" lurking in the woods (which, of course, is not a monster at all) and even begin worshipping a dead animal. As their savagery grows, the boys do not want to hear anything reasonable and logical.
Piggy tries to be the voice of reason and logic, but the boys (with the exception of a few) don't want to hear it. On a literal level, Piggy's physical appearance (his weight, his asthma, his glasses) make him an outcast. He is forever calling the boys "children," even though he is the same age as them. This is one reason that the boys ignore Piggy.
However, Piggy is ignored for symbolic reasons as well. The boys have given into their dark sides and as a result Piggy is ignored. Their lives are given over to hunting, chanting, and savage dancing. When Piggy is finally killed at the end of the novel, it symbolizes the death of reason in the boys. All civilization in them has been destroyed by their time on the island and the fear it has created within them.
