The central theme of the three novellas that make up The land That Time Forgot is evolution, and the land of Caspak is a miniature example of the vast evolutionary forces that have shaped life on earth, with each individual creature and human in Caspak living out the basic evolution of primitive species changing into more advanced species. This complex world of rampant evolution provides a wonderful background for exotic adventures, but Burroughs enriches his theme of evolution by portraying the Wieroo in "Out of Time's Abyss."

Burroughs's depiction of evolution is in some ways a naive one. In The Land That Time Forgot, the process of natural selection seems to have little to do with the formation of advanced species from primitive ones, although natural selection was the mechanism Charles Darwin argued created the world's diversity of life. On the other hand, the evolution of Caspak is founded on the basic ideas of modern biological science, that fish evolved into reptiles, which evolved into mammals and birds, and so on, but the mechanism for this is not described. Burroughs enriches his account by introducing the Wieroo and thereby suggesting that evolution could have taken another course than developing into human beings; humanity is not necessarily the inevitable result of evolution. The concept of a world that is continuously living out the progress of evolution is a fascinating one, and it serves as a powerful unifying theme for The Land That Time Forgot,

The Wieroo have for ages practiced eugenics, killing off what they deem to be inferior members of their species. This is a practice abhorred by the Galu, the full human beings of Caspak, and over time the Wieroo and the Galu have become bitter enemies. The result of the Wieroo's eugenics is that they have become a winged species with humanlike bodies and skulllike heads. They are hideous to look upon, and they are regarded with immense fear by the seventh-cycle Galu women. The Wieroo are unable to father females of their own, so they seize the seventh-cycle women because these women can bear babies. The Wieroo reproduce themselves by mating with Galu women, whom they keep captive in their cities on the island of Oo-oh. Ages of eugenic breeding has created an unnatural species that cannot properly reproduce itself, whereas the Galu, who let nature take its course, have evolved into a fully developed species that can reproduce itself without outside help. The Wieroo are a cruel race; their belief in eugenics has made them into treacherous beings. They advance in rank in their society by murdering others of their kind. Their high rulers live in constant fear of death at the hands of one of their subjects. Burroughs seems to suggest by his portrait of the Wieroo that natural processes of human development are better than unnatural ones; that social engineering in which those people who are thought to be inferior are not helped or even killed is cruel and will breed cruel people.