Final answer:
In Jack London's 'The Call of the Wild,' Hal, Charles, and Mercedes are the antithesis of Buck's previous mushers, showcasing the theme of humanity's nature versus nurture through their mistreatment and incompetence.
Step-by-step explanation:
The characters of Hal, Charles, and Mercedes in Jack London's The Call of the Wild serve as the antithesis to the types of mushers that Buck has encountered in the first half of the book. Unlike Buck's earlier owners, who are competent and treat their dogs relatively well, these new characters display cruelty, inexperience, and a lack of understanding of the way of life in the Yukon. Their negative example highlights the theme of struggle between humanity's nature versus the nurturing forces of society and the raw survivalist instinct that Buck embraces as he adapts to his new, harsh environment.