The setting of William Golding's Lord of the Flies is critical to the plot and themes of the novel. These proper English schoolboys are living without an adult on a tropical island. The island is comprised of areas which are referred to as jungle, but it also has a mountain on which craggy rocks are piled in a place which the boys think looks like a fort. Both of these surroundings, coupled with the fact that there is no authority figure on the island, influence the behavior and appearance of the boys.
The island is hot and muggy; many of the boys are eager, from the first morning after the crash, to shed their clothing because of it. Over the course of the novel, the boys are all nearly naked, symbolic of their reversion to a more primitive way of thinking and living. The most civilized of them at the end is Ralph, though even he is annoyed by the effects of his surroundings (sun, sand, salt) on his clothing. The others have long ago given up any semblance of being civilized. They are forced to hunt and scavenge if they want to eat, they go to the bathroom without regard to their food, and their desperation for meat drives the hunters, and especially Jack, into rather predatory behavior.
The rugged rock fort on the mountain is the place where Jack and his tribe eventually settle, and it seems to give those who live in a sense of invincibility and power. It is from here that a boulder is deliberately dropped on Piggy to kill him and the new tribe of savages reigns over the island.
These boys, because of their surroundings and the fact that they have no restraints on their behavior, become savages. Despite that, they do not need to fight one another to save their own lives; however, they do. That aspect of the boys' actions is a choice, not something required of them for survival; but it is certainly made possible by their surroundings.
The surroundings of the characters invoke the basic human instincts in the characters. They are forced to their "fight or flight" instincts in many situations. The island leaves no way for them to escape, and thus by turning to their instincts they become savage.
They are influenced by their surroundings because the only thing that they want to do it survive. So they are going to do anything to survive.