What central idea or theme about humans’ treatment of animals does The Call of the Wild convey

In California, Buck believed he was very powerful, for he was the most important dog in Judge Miller's household. He ruled over all of the other dogs, and he even believed that he ruled over the people. In the Klondike, he learns what a hierarchy really is, and he understands that power is truly the power over life and death. All of the dogs either have power, and must exert it in order to survive, or they give up their power to a bigger and stronger dog and can merely hope that that dog will protect them.

Once Spitz fears Buck's power, Buck realizes that he must exert it. The appearance of power must lead to the assertion of power. The only other option is death. Buck quickly learns one of the most important laws of Club and Fang. When Curly is killed for making a friendly advance to another dog, he recognizes that he is in a world where it is kill or be killed. He immediately begins to see the world in terms of who he has power over and who has power over him.

The issue of power exists both in the relations of the dogs among themselves and in the relation of the dogs and the men. Slowly over the course of the novel Buck learns that human beings do not have intrinsic power over dogs. When he asserts his right to leadership of the sled, he imposes his will on Francois, even though Francois has a club. When he kills the Yeehat Indians, he consciously acknowledges that he need never fear human beings again. In this world, he is more powerful than a human being. In light of this view of power, London suggests that a wild, natural existence is not as free as the reader might imagine. Buck is free because he is the most powerful, but he must never for a moment let down his guard. The natural world is dominated by rules and codes just as the civilized world is, and in this world, Buck can read and understand the subtlest of controls.