The juxtaposition of Ralph and Jack is the clearest example of the theme.  Ralph represents civility and Jack represent savagery.  Putting these two characters against one another on the island in the constant struggle for authority and leadership is one way Golding brought out the theme of man's inherent evil nature.  The reader sees how the lack of civilization and its rules brought out the savage in Jack, and subsequently, his followers. Other characters are juxatposed too: on Ralph's side there are Piggy and Simon, both symbolic of the intellect, those who think things through before coming to a conclusion or acting; on Jack's side there are Maurice and Roger, both symbolic of the follower who does not think, but of those who just act.  Another example of the theme through setting items in contrast to one another is fire.  In chapter two, fire gets out of control and becomes savage, killing one of the children.  It shows what can happen when there are no restraints, which is what Golding says happens to people without society's constraints.  Later, the fire is tended and it becomes the symbol of civilization as the boys use it as a signal fire in their attempts to get rescued.