There are three main themes within Frank Norris's naturalistic novel McTeague. These themes are greed, the animalistic nature within mankind, and natural selection.

The first theme, and most obvious, is that of greed. The three main characters (McTeague, Trina, and Marcus) all strive for something better in the world. None are happy with their stature in life. All three come to find out that the cliche "money is the root of all evil" is true. All three find that, in the end, money causes their downfall. Each dies as a result of the personal goal of becoming rich (defined individually). As a result of each individual's greed, the money possessed is never enough, and the search for more money is directly responsible for each of their deaths.

The second theme, the animalistic nature of mankind, is illuminated directly by the author. Numerous times throughout the novel, characters are compared to animals. For example, in the opening chapter and opening description of the protagonist, McTeague is compared to a horse: "McTeague's mind was as his body, heavy, slow to act, sluggish. Yet there was nothing vicious about the man. Altogether he suggested the draught horse, immensely strong, stupid, docile, obedient." Throughout the novel, McTeague is also compared to an elephant and a bull. All of these comparisons speak to the animalistic nature of mankind.

These comparisons help to highlight the final theme: natural selection. Within the comparison to a horse, Norris illuminates McTeague's "docile, obedient" nature. Horses are domesticated in order to work for mankind. Once the horse becomes old or is unable to work, it tends to be forgotten or put down. McTeague is no different. As soon as Trina wins the lottery, her husband (McTeague) is no longer of use to her. She has risen above him, and she no longer needs him to support her. She has become the stronger of the two and should, although she does not, survive on her own. Later, McTeague is compared to an elephant. These creatures seem to be docile, much like the previously mentioned horse, but this can change in a moment. This illustrates the idea of the animal that lies beneath the surface of the human. In the end, the massive elephant is able to take down most enemies, proving itself to be a dominant beast. Lastly, McTeague is compared to a bull. He and Marcus fight in Death Valley over the money McTeague took from Trina (which also illustrates the ideology behind natural selection). Both men circle one another in a literal and figurative battle to the death. The hate the men have for one another is appropriately illustrated as fighting bulls thirsting for the blood of the other.

In the end, all three themes circulate around one another. Greed brings out the animal instincts in man. Man then does whatever it takes in order to survive. While all of the themes can be identified individually, Norris ensures that they are able to exist on their own through the way he highlights each one's importance.