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I think there are many themes in the story depending on how you interpret it, but I believe there are two main general themes. The first one would be that we all change and get experience as time passes by. We aren't the same person we were years ago because of the things we've learned and have gone through. Throughout the story Buck had to deal with terrible experiences and this is what shaped him to become what he's now. The second theme: What doesn´t kill you makes you stronger. This is literally what happened to Buck in many parts of the story. He had been close to death caused by several things like bad treatment, starvation, tiredness, fighting with other dogs or humans, etc. Buck started as a proud pet in a wonderful home and ended up as a creature way stronger and fierce than both, dogs and wolves living in the wild. Buck completely changed his life to be the strongest of them all.