As is typical of London, The Call of the Wild presents a contrast between the "civilized," comfortable life and the harsh, wild environment of the Northland. Buck begins as a domesticated dog and eventually becomes a wild dog, or wolf-dog, as his ancestors were. The message is not so much that the primitive life is better as that all of us, humans and animals, have both characteristics within ourselves, and that we are capable of being both civilized and wild.
Beyond this theme, which is more of a simple observation on London's part than a moral, the message of the story is a basic one regarding the opposite human tendencies to cruelty and kindness. In the Northland, John Thornton is the first person who shows any compassion for Buck, and so, Buck grants him unwavering loyalty and devotion in return. In his time with Thornton, Buck is living a kind of prelapsarian existence, as if all the cruelty of the real world has been wiped away. But it cannot last, just as the Garden of Eden didn't last. The ultimate moral of the story may be that all of us—people and animals—must deal with the reality of both good and evil as they exist in the world. The tragedy of Buck's loss of Thornton, and Buck's leadership of the wolf pack, are together a metaphor for the reality and imperfection of life on earth, from which, London's moral indicates, there's no escape.