Answer:
Because he was tired and did not have the strength to fight back.
Step-by-step explanation:
Jack London's "The Call Of The Wild" tells the story of a dog named Buck and his life from being a pet to being a creature of the wild. It is a sort of the life experience of Buck, his growing into a whole different being.
Chapter V "The Toil of Trace And Trail" tells the scene where Buck and the other dogs were travelling with their new master Hal, Charles and his wife Mercedes. The ignorant Hal had been driving them hard to keep on going, eve though the dogs were underfed, overworked and tired. But even with the constant nagging and beating, Hal still continued to beat the dogs. But Buck did not fight back as he was out of energy and strength. He just let Hal beat him even though he so wanted to fight back for he had been drained of his energy along the whole journey.