The Lost World is unified by the themes of discovery and reward. It is an exceptionally fine novel because these themes are worked out in depth. Discovery is the process of the novel. Ostensibly, the expedition to the jungles of Brazil is undertaken for the purpose of furthering knowledge, of discovering new places and long-hidden animal life. The real purposes of the explorers are not so altruistic; they hope for rewards. Challenger's rewards would be fame and the shaming of his detractors, Malone hopes to win the hand in marriage of Gladys Hungerton. Lord John Roston seems restlessly in quest of "masculine virility." And Professor Summerlee hopes to prove that "Professor Challenger is an absolute fraud."

Only Professor Challenger finds what he wants. He is so determined to have his way that he gives the impression that he would have built his own dinosaurs had he not found them. Malone's discoveries are more unexpected. He finds courage in himself, and his loss of Gladys emphasizes that his true reward is greater knowledge of himself.

Another theme that underlies the experiences in The Lost World, named Maple White Land after an earlier explorer, is that of evolution by natural selection. The dinosaurs and other beasts survive on the plateau because they have been cut off for eons from competition from the evolving life in the rain forests below. While they are in Maple White Land, the explorers see the process of natural selection played out. On one end of the oval plateau live Indians; on the other live "ape-men." In a cataclysmic struggle, modern men — the Indians — slaughter the animalistic ape-men, replaying a struggle completed eons earlier in the rest of the world. Darwinism triumphs.