Consider the contrast between "The Law of Love and Fellowship" that London claims civilized society is governed by and "The Law of Club and Fang" that he claims creatures in the wild live by. Those who follow the "The Law of Love and Fellowship" are "civilized" and those who follow the "The Law of Club and Fang" are "wild." Buck changes in the course of the book from fully civilized to fully wild. Some characters exhibit both "civilized" and "wild" characteristics, and some are purely one or the other. The key is to use the two laws as your criteria, and to show how the characters you discuss live by one or the other or both, depending on their circumstances.