First published in 1903, the novella The Beast in the Jungle has characteristics of both literary realism and literary modernism. Realism was a post-Romantic movement in the late 19th and early 20th centuries. Realist novels represented reality in a faithful and objective way; authors like Henry James were interested in objective knowledge and often used third-person narration to provide insight into the minds and hearts of their characters. This becomes clear in The Beast in the Jungle through James's descriptions of May and Marcher's dynamic.
By contrast, modernism was a literary movement that began in the early 20th century. Modernist stories often focus on individualism, disillusionment, psychological fragmentation, and alienation from society. The Beast in the Jungle illustrates many of these key themes, especially as John Marcher finds himself isolated from other people due to his impending fate. As a result of writing this book and many others, Henry James is often referred to as a key transitional figure between realism and modernism.
Finally, it is interesting to note that The Beast in the Jungle also has undertones of existentialism. While Henry James does not officially belong to the cohort of existentialist writers, the novella's themes of agency, individualism, and the illusion of destiny align with existentialist values. John Marcher exemplifies someone grappling with the same issues as many existentialists: he realizes that there is no set path for one's life, and also that each person must give meaning to their own existence. Marcher's dramatic moment of realization at the end of the text serves as a fictional example of what might happen in someone's mind when they realized that they were the ultimate arbiter of their own existence.