The People That Time Forgot is the second of three books that Edgar Rice Burroughs wrote about Caspak. The three books are best read together (so I hear), but I had no trouble reading this one out of order.
Caspak is a land populated by beasts and men from Earth's prehistory: dinosaurs, mammoths, saber-toothed cats, and cave men. Caspak is a huge island in the Antarctic Sea; the coasts are vertical cliff walls, completely cutting the island off from the rest of the world. Volcanic activity heats Caspak and maintains a year-round tropical temperature.
Tom Billings leads an expedition to Caspak to rescue his friend Bowen Tyler, who was stranded in that savage land. Billings scouts the terrain in an airplane, and meets with disaster: he is attacked by a pteradactyl and crashes into the jungle. Then he rescues a native girl from a band of savage apes, and together they make their way to the land of the Galus, where Billings hopes to find Bowen Tyler.
On their way north to Galu, the girl (named Ajor) teaches Billings all about Caspak. It is home to beasts from all of Earth's lost ages. The complete evolutionary history of Earth is represented, with the oldest and most savage creatures living in the south of Caspak. The farther north one goes, the more advanced and developed are the denizens of the forest--and more advanced are the people.
For Caspak is peopled by multiple groups of ancient men. First, the Ho-Lu, who are mere apes. Next, the Apu, the speechless men. Next the Bo-Lu, or clubmen, followed by the Band-Lu (spear men), Kro-Lu (hatchet men), and finally, at the pinnacle of evolution, the Galu (rope men). The girl Billings rescued, Ajor, is a Galu.
The most curious thing about Caspak is that the men themselves evolve. The Galu do not have children; instead, the most advanced Kro-Lu evolve and become Galu. Similarly, the Band-Lu become Kro-Lu. And so forth, with each tribe being made up of risen members of the tribe below it.
"[The Band-Lu] are no longer my people," To-Mar replied proudly. "Last night, in the very middle of the night, the call came to me. Like that it came into my head"--and he struck his hands together once--"that I had risen. I have been waiting for it and expecting it for a long time; today I am a Kro-Lu. Today I go into the coslupak" (unpeopled country, or literally, no man's land) "between the Band-Lu and the Kro-Lu, and there I fashion my bows and my arrows and my shield; there I hunt the red deer for the leathern jerkin which is the badge of my new estate. When these things are done, I can go to the chief of the Kro-Lu, and he dare not refuse me."
The basic question is: where do the Ho-Lu come from? Do they have babies? This question Burroughs does not answer--but he leaves clues. The precise nature of the curious accelerated evolution of Caspak is revealed in the next book, Out of Time's Abyss.