Survival of the Fittest
Charles Darwin, in The Origin of Species (1859), developed a theory that life on earth evolved through a process of natural selection. Those creatures that were strong and were able to adapt to their environment were the ones that survived. The process as seen by Darwin was ruthless and amoral; there was no beneficent God overseeing it and ensuring justice or tempering it with mercy. London appears to have had Darwin in mind when he wrote The Call of the Wild. (When London himself went to the Klondike in 1897, he took a copy of Darwin's book with him.)
As soon as Buck is kidnapped from Judge Miller's ranch, he learns that a new law applies to life, quite different from the "law of love and fellowship" he has known before. London calls this the "law of club and fang," a succinct phrase that describes in a nutshell the survival of the fittest. The law of the club is that man, having access to greater force, is the master of the dogs. The stronger wins control, and the weaker must submit and serve the will of the conqueror, or be killed. Buck understands this very quickly, after he is beaten by the man in the red sweater, and he adapts to it. Adaptation is the key. He also adapts to the "law of fang," which applies to the dog world. He first sees this at work when his friend Curly naively tries to make friendly advances to the other dogs. She is torn to pieces within minutes. Buck realizes, "So that was the way. No fair play. Once down, that was the end of you" (chapter 2). He resolves never to go down. This sets the pattern for the remainder of the novel. Buck proves himself to be the strongest, the most resourceful, and the most courageous. Since he is the fittest, he is the one who survives in the "ruthless struggle for existence" (chapter 2).
The primitive law of the survival of the fittest also applies to the human world. Perrault and Fran´┐Żois, Buck's first owners, are shrewd men who have adapted to the demands of their environment. They are a match for whatever challenges they encounter. In contrast, Charles, Hal and Mercedes, who have recently arrived in the north from the more civilized south, fail to adapt to its demands, and as a result, they perish.
Heredity and Environment
London was a determinist. That is, he believed that life was conditioned entirely by heredity and the environment. These beliefs are apparent in the novel. As long as Buck lives a comfortable life at Judge Miller's ranch, he remains a domestic dog. His deeper, inherited instincts remain hidden because the environment he lives in does not bring them out. But he is rapidly changed by his exposure to a new environment. Within a short time he is unrecognizable from the tame pet he was at the ranch. The environment shapes him, but heredity also plays a large part in the transformation. Buck adapts so well because he inherits from his ancestors the fiercer instincts he needs to survive: "[N]ot only did he learn by experience, but instincts long dead became alive again. The domesticated generations fell from him. In vague ways he remembered back to the youth of the breed, to the time the wild dogs ranged in packs through the primeval forest and killed their meat as they ran it down" (chapter 2).
This is a continuing theme in Buck's regression (or development, it could be argued) into a wild animal. Something deep inside him, inherited from the distant past, teaches him how to fight and how to use all the tricks of survival that the breed to which he belongs possessed of old. In the final chapter, for example, when he makes friends with the wolf, "Old memories were coming upon him fast, and he was stirring to them as of old he stirred to the realities of which they were the shadows."
The novel is concerned not only with dogs but with their changing relations to humans. This relationship has a long history, going back to the earliest times, as is shown by Buck's vision of the cave-man who is accompanied everywhere by a dog, for protection against beasts of prey.
During his life, Buck experiences many different kinds of relationships with men. The relationship between him and Judge Miller is one of companionship and mutual respect, "a stately and dignified friendship" (chapter 6). With the judge's sons, whom Buck accompanied on hunting trips, he had "a working partnership," and with the grandsons, "a sort of pompous guardianship." But when Buck is owned by Perrault and Fran´┐Żois, and later by the Scotch half-breed, there is a harsher relationship between man and dog, governed by the realities of power-they are stronger and Buck must obey them. But still there is an underlying concept of fairness.
With Charles and Hal, the man-dog relationship deteriorates to its lowest point, into cruelty and abuse of power. These men have no understanding of the needs of their dogs, and their incompetence imposes on the animals an unnecessary hardship.
It is under John Thornton that the relationship between man and dog achieves its highest expression, based on a degree of love that Buck has never known before. Each saves the other's life, and they appear to be willing to do anything for each other. Buck sees Thornton as the "ideal master."