When Lennie is explaining to Curley's wife what happened to the puppy, she responds, "Don't you worry none. He was jus' a mutt. You can get another one easy. The whole country is fulla mutts." What is the symbolism in her statement?

The statement speaks to the theme of discardable life. Candy's dog was deemed "useless" and Candy feels the same way about himself. The puppy too was of no importance and Curley's wife points this out. This foreshadows the end of Lennie's life. To most people, Lennie (like the puppy) is of little importance.