What instinct becomes alive in Buck again at the end of chapter 2 that's a reflection of his ancestor, the wolf?

In the end of Chapter Two, the journey grows more difficult as the dogs have to break their own trail. Every night in camp, Buck is exhausted. He is bigger than the other dogs, and though he receives a larger ration, he never feels satisfied. Once a dainty eater, he has lost all fastidiousness after he being robbed for eating slowly. He duplicates the actions of Pike, a new dog, by stealing a whole chunk of bacon. He is not caught, and another, weaker dog is punished in his place. Buck's theft marks him as "fit to survive" in the Northland environment. Property and personal feelings can no longer be respected. He does things because they are necessary. He acts not on reason but on instinct. Buck speedily develops more heightened senses, hardened muscles, and an iron stomach. He becomes the son of his ancestors, and when he howls at the moon he repeats the same ancient song they sung before him.