Personification
Although he is quite clearly a dog, we start thinking of Buck as person. After all, he has thoughts, emotions, and a complex, human-like personality. Buck is characterized by this complexity. By taking on the importance and dimensionality of a human, Buck becomes a real protagonist despite his animal form.
London makes extensive use of physical descriptions to make clear what a transformation Buck undergoes. We're frequently told of Buck’s size, his strength, his muscles, and his body’s hardness. This, of course, reflects the other, non-physical changes Buck has undergone in adapting to the wilderness, which makes it a good tool of characterization.
