How does the theme of "primitivism" work in William Golding's Lord of the Flies?

William Golding says he wrote Lord of the
Flies in "an attempt to trace the defects of society back to the defects of
human nature," and the theme of primitivism suits his theme
perfectly.

Golding starts with a rather blank slate under
the best possible conditions for mankind to be able to prove that it will not, when
given the opportunity, revert to savagery; but of course that is exactly what happens
here. These English schoolboys are the best examples of proper behavior and following
rules; even without an adult or other restraints present, they know how to behave
appropriately and lawfully. Unfortunately, they fail to live up to their best selves
almost from the beginning.

As the story continues, the
deterioration also continues. Soon the rules they establish are broken, and soon after
that the rules of human decency and lawfulness are broken, as well. Jack is the
character most symbolic of this primitivism, as he spends his days doing nothing but
hunting. The final descent into savagery for the entire island begins with one simple
act.

When Jack realizes he has not been able to catch a pig
because they see him, he decides to paint his face. He looks at his reflection in some
water.

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He looked in astonishment, no longer at
himself but at an awesome stranger. He spilt the water and leapt to his feet, laughing
excitedly. Beside the pool his sinewy body held up a mask that drew their eyes and
appalled them. He began to dance and his laughter became a bloodthirsty snarling.
He capered toward Bill, and the mask was a thing on its own, behind which Jack hid,
liberated from shame and self-consciousness. The face of red and white and black swung
through the air and jigged toward Bill. Bill started up laughing; then suddenly he fell
silent and blundered away through the
bushes.

This is a
significant incident, for Jack is no longer restrained either by shame or by his own
conscience. These are the last two defenses against complete barbarism, and they are
gone. It is a quick and noticeable change, as demonstrated by Bill's hasty
departure.

In the final two chapters of the novel, Golding
rarely refers to Jack or any of his tribe (which is all but a few boys) as anything but
"savages." After Simon's and Piggy's murders, as well as the smashed conch, chaos reigns
on the island. It has become a primitive place in every way: naked (or nearly naked)
boys running through the forest with spears, making ululations as they hunt the greatest
prey, another human. 

At the end of the novel, after the
naval officer arrives, Golding makes his final statement about what has been
lost:

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[W]ith filthy body, matted hair, and
unwiped nose, Ralph wept for the end of innocence, the darkness of man’s heart, and the
fall through the air of the true, wise friend called
Piggy.