The well-named "primordial beast" grows in Buck for two intertwined reasons. One is what happens to him (the plot). He encounters violence, has to adapt to an environment, and, eventually, does violence and joins the wolves. For a domestic dog to kill (beating the human killers) makes him more of a beast, as does running with the wolves. On the thematic level, this essential nature is part of London's point. He's making an argument through this change about what nature, including human nature, is. Therefore, Buck has to become this beast because that's his essence.