on page 67, when roger gathers stones and throws them, the narrator explains that “Roger’s arm was conditioned by civilization”. What is meant by this? What other characters are also similarly conditioned?

Roger is a natural socio-path. He is a bully who derives pleasure from causing harm to others. The only thing which holds him back is the faint memory of punishment. Golding's point here is human nature's natural inclination to cause harm. Without laws a deterrents boys like Roger have a natural inclination to cause harm. Jack, although much more manipulative than Roger, has similar characteristics.