Darwin did not say that only the strongest survive but that the fittest survive. An animal or species can survive by adaptation. The dinosaurs were definitely stronger than mammals, but the mammals survived while the dinosaurs went extinct. The mammals were able to survive in new conditions apparently caused by the earth being hit by a gigantic meteor. There are many ways for the weak to survive. Rabbits are weak and timid creatures, but they are a plague in Australia because they reproduce in large numbers and are adapted to survival in the harsh conditions. Predators would not survive if they were able to eat all the prey in their environment; the predators would then starve to death.
Darwin’s theory of Natural Selection basically said that evolution works because only the fittest survive because “some differences may improve the chances of survival of a particular individual” (enotes, natural selection)
Zaroff tells Rainsford that he hunts “more dangerous game” than buffalo. Humans actually make “capital hunting” for him, “the most exciting hunting in the world.”
When Rainsford tells Zaroff that what he is doing is murder, Zaroff disagrees.
The general laughed with entire good nature. He regarded Rainsford quizzically. "I refuse to believe that so modern and civilized a young man as you seem to be harbors romantic ideas about the value of human life. Surely your experiences in the war--"
Zaroff believes that he is the strongest, and therefore he should survive. The men he hunts are weak.
"Life is for the strong, to be lived by the strong, and, if needs be, taken by the strong. The weak of the world were put here to give the strong pleasure. I am strong. Why should I not use my gift?
Of course, this is a distortion of natural selection. Zaroff has all of the advantages that he has built into the game, not those from nature. Yes, he is intelligent and strong. But he also has guns, dogs, and knowledge of the island. The men he hunts are at a disadvantage.