The Cave Girl by Edgar Rice Burroughs
Blueblooded mama’s boy Waldo Emerson Smith-Jones is swept overboard during a south seas voyage for his lifelong ill health. He finds himself on a jungle island. His bookish education has not prepared him to cope with these surroundings, and he is a coward. He is terrified when he encounters primitive, violent men, ape-like throwbacks in mankind’s evolutionary history.