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 analyzes the psychological characteristics of John Clayton (Tarzan's real name in the novels) and his peers, based on close readings of the various Burroughs books, incorporating some of Burroughs' ideas while rejecting others to enhance realism. He proposes that since the apes described by Burroughs had a spoken language that Tarzan learned, they must have been "pithecanthropoids": a group of rare hominids who are probably now extinct and not great apes.