A metaphor is a comparison of two unalike things, where one thing is said to be another. One of the most powerful metaphors in the book is spoken by the giant King of Brobdingnag while he speaks with Gulliver about Gulliver's home country of England. He says, in part,
"[...] I cannot but conclude the bulk of your natives to be the most pernicious race of little odious vermin that nature ever suffered to crawl upon the surface of the earth."
Thus, the king uses a metaphor to compare humans, but especially the residents of England, to vermin. Vermin is a word typically applied to small and common harmful animals, or animals that tend to disgust humans; rats, mice, fleas, lice, cockroaches, and so on, are the types of animals that are frequently referred to as vermin. Despite Gulliver's pride in his home and species, the king finds humanity's violence and apparent lack of morality to be major faults—flaws that make them seem inhuman and more animalistic.