"Ralph had stopped smiling and was pointing into the lagoon. Something creamy lay among the ferny weeds.
A stone.
No. A shell.”
This quote is significant because of how important the conch becomes. It represents power, respect, and most importantly, civilization. Although the boys eventually lose respect for the conch; at one point they respected it and its power and it kept them all alive in the beginning of the story.
“Maybe there is a beast… maybe it's only us.”
The boys had been so concerned about the "beastie" that could hurt them on the island that they were forgetting what was really important. They were turning into evil, descisive brats, and yet they never seemed to realize that the beast was themselves.
"The greatest ideas are the simplest."
While surviving on the island, the boys cared too much about the small things, They never thought about the simple fact that they must learn to cooperate and live peacefully among eachother on the island in order to survive.
“What are we? Humans? Or animals? Or savages?”
The boys first arived on the island as normal boys, that thought they would just have to temporarily survive until help came. After they stayed on the island, they lost hope that they would ever be saved, so they started acting wild. They would become more animal-like, and forget their morals and many human-like things. Eventually, it got to the point that they were so savage that they enjoyed seeing death, and not even wild animals would enjoy this. The boys were not even acting like humans anymore, and they would definitely not be fit to go back into the real world after this experience.