The Call of the Wild is an adventure novel. Adventure fiction, which became popular in the 19th and early 20th centuries, generally follows a character and their development through fantastical and dangerous events that are beyond the scope of what someone would usually experience in day-to-day life. Buck's "ordinary" life is interrupted by his kidnapping and subsequent adventures in the Yukon. Although London himself spent time in the Yukon, the gold rush and the harsh conditions of life Buck experiences would have seemed outlandish and extraordinary to most of his readers.
Despite the fact that the novel depicts its main character in extraordinary circumstances, it still qualifies as a work of naturalism. This term does not refer to the novel's vivid nature scenes (although there are many of these). Rather, it refers to the way circumstances determine the course of Buck's life. Naturalism was a literary movement that developed in the wake of the Industrial Revolution, largely in response to the sense that people's lives and the choices available to them depended on the environments into which they were born or ended up. Buck's life takes an abrupt turn when he is kidnapped away from the comfortable life of his youth. Contact with wild animals awakens the wild animal within him, and the different treatment he receives at the hands of different masters affects his relationship to humans. London explores Buck's innate instincts because of his species, but he still imagines that nurture is a big part of developing identity: there is a real sense that Buck may have been a different dog if he had found himself in a different environment.
What's more, London uses naturalism to comment on the working conditions that many poor, working-class laborers experienced in the late 1800s and early 1900s. London himself came from a poor background and eventually realized that his life was being determined by rich and abusive employers. He grew angry with this system and became a socialist, agitating for change so that people like him could have more options in life.
In the course of making this point, London also makes the troubling choice to imitate tropes from the genre of enslaved people's narratives. In Chapter 1, for example, Buck's kidnapping is reminiscent of the way some enslaved and formerly enslaved people have described their own experiences:
For two days and nights this express car was dragged along at the tail of shrieking locomotives; and for two days and nights Buck neither ate nor drank. In his anger he had met the first advances of the express messengers with growls, and they had retaliated by teasing him. When he flung himself against the bars, quivering and frothing, they laughed at him and taunted him.
Buck's discomfort—his deprivation of food and water, his cramped confinement—seems to draw on the experiences of people kidnapped in Africa and transported across the Atlantic Ocean to be sold as enslaved laborers in the Americas. Free Black Americans were even kidnapped on American soil and forced to endure similar conditions as they were re-enslaved or enslaved for the first time. Solomon Northrup describes this experience in Twelve Years A Slave, the memoir on which the 2013 Steve McQueen film of the same name is based. The cruel taunting of the "express messengers" resembles the dehumanizing cruelty of the men who kidnap Northrup from Washington, D.C. and traffic him in the South. London did not believe that Black Americans deserved the same rights as the white laborers for whom he advocated, and yet he borrows conventions from Northrup and other Black writers to tell the story of Buck and the white laborers he represents.