How did Buck become part of the wolf pack?

The wolf motions for Buck to come with him, and Buck is wildly happy as they run. He now understands what the call meant, running wild and free with his brothers. Buck remembers Thornton and returns to the camp. Thornton has been killed by Indians but Buck fulfills his loyalties to Thornton by killing the Indians. Buck returns once more to the call of the wolves. Buck asserts himself as dominant with the wolf pack. Obeying the law of club and fang, they accept his fellowship, and he runs with them into the woods.