Heuvelmans' books made reference to literary sources. He was influenced by Jules Verne and Arthur Conan Doyle's book "The Lost World" (1912). Though earlier interested in zoological oddities, he credits a 1948 "Saturday Evening Post" article, "There Could be Dinosaurs" by Ivan T. Sanderson, with inspiring a determined interest in unknown animals. Sanderson discussed the possibility of dinosaurs surviving in remote corners of the world. Heuvelmans was also influenced by the work of Anthonie Cornelis Oudemans, who had defended the existence of the sea serpent.