What does "survival of the fittest" mean?
The character White Fang is used by the author to show the development of a "half-wolf, half-dog" more wild being into a more tame and loyal domesticated dog. This happens when White Fang meets Scott who is the first human to show kindness to him.
The author, Jack London, explores the theme nature versus civilization. He illustrates the example of White Fang who begins the story in the wild, killing many other animals to survive, and ends the story as the father of a liter of pups living in a household with his loyal friend Scott.