George and Lennie's Farm
The farm George and Lennie hope to own is a symbol of the American Dream. Like a mirage, the farm leads George, Lennie, and other ranchers like Candy and Crooks, to indulge in the dream of living "off the fatta the lan.'" George's elaborate description of the farm's abundant plants and animals also makes it seem like a symbol of paradise.
Lennie's dream is to tend the rabbits on the farm that he and George hope to one day own. This dream establishes Lennie's complete innocence. But Lennie loves the rabbits because of their soft fur, and his love of touching soft things leads to his doom. The rabbits, then, symbolize not only innocence, but also the downfall of innocence in a harsh world.
Candy's once powerful sheepdog is now old and useless. Carlson's killing of the dog makes it clear that during the Depression only the strong survive. The way in which Carlson kills the dog—with a gunshot to the back of the head—foreshadows Lennie's death and likens Lennie to Candy's dog: they're both powerless, innocent, and doomed.
Just as Lennie is dependent on George, Lennie's puppy is entirely dependent on Lennie. Like Lennie, the puppy symbolizes the fate of the weak in the face of the strong.