What are some symbols in The Call of the Wild?

Jack London's fiction typically establishes a dichotomy between civilized and non-civilized worlds. The subtext is generally the question: is one realm "better" than or preferable to the other? In The Call of the Wild, Buck is taken from his home in California and thrust into a new, harsh environment which is a kind of hell on earth for him. Ironically, however, it leads him to fulfill his inner nature that could never have occurred if he had continued his peaceful life as a domesticated dog.
The "Northland," the harsh, forbidding Yukon territory into which Buck is taken and where he is to live out the rest of his life, is a symbol of the wild, uncivilized external world as well as the primitive inner world from which we—both humans and domesticated animals—have presumably escaped.
But it is also a symbol of liberation. Buck is transformed into a new being when he is forced to become a working dog who pulls a sled. When he's rescued by John Thornton, he bonds with him and becomes devoted to a human who has an intensity and a completeness: the kind of person who he would not have found had he remained in the Southland, in California.
Finally, when he does in fact return to "the wild," he is a leader, paradoxically reverting to his ancestral roots but also becoming a "greater" being (both physically and, in some sense, spiritually) as well as the head of a wolf pack. The trajectory of Buck's story is a symbol of this process of liberation.
London's point may not be that the wild world is "superior," but that—just as we humans must progress beyond our initial state of childlike innocence and become adults—Buck, through the suffering he endures (including the loss of John Thornton), has progressed beyond the cocoon of innocence and become a more complete being.
The story of The Call of the Wild is therefore itself a symbol of maturation and of the expulsion from the Garden of Eden into the "real" world—a place where both pain and triumph are possible, as they would not have been in the ignorance of paradise.