Frank Norris' novel McTeague is classified as a Naturalistic novel. Naturalists believed that nature was the most powerful element in existence. Therefore, the Naturalistic text illustrates that mankind bows to the power of nature.
This said, the themes of Naturalistic texts mirror this idea. According to the idea presented in the novel, the nature of things exists as the most powerful image and reality. For example, McTeague begins the story as a very simplistic man. As the story moves forward, McTeague begins to change. He believes himself to be far more than the primitive man in the opening of the novel. In the end, McTeague returns to his simplicity (masked by his animalistic nature to simply survive).
Outside of the theme of survival, McTeague illustrates the typical "American Dream" turned wrong. While the story does show the movement from poor to wealthy, it also depicts the tragic fall many face when in search of their dream. Also highlighted in this idea is the concept that wealth equals power. That said, the novel completes the circle of failure when it illustrates the fall from wealth also means the fall from power.
In the end, the novel possesses two major themes: wealth and power.