Farmer examines the psychological make up of John Clayton (Tarzan's real name in the novels) and his peers, based on close readings of the various Burroughs books, accepting some of Burroughs' concepts and rejecting others in an attempt at greater verisimilitude. Among his conceits is that, since the apes described by Burroughs had a spoken language that Tarzan learned, these animals must have been "pithecanthropoids": "a group of rare hominids who are probably now extinct" and "not great apes".
Farmer attempts to create a more accurate representation of Tarzan's world by analyzing the psychological characteristics of John Clayton and his peers based on Burroughs' books. He proposes that the apes in the novels, having a spoken language learned by Tarzan, must have been "pithecanthropoids," a group of extinct hominids who are not great apes.