How does Steinbeck use symbolism to convey his theme in of Mice and Men?

Steinbeck uses two animal symbols to convey his theme of innocence lost in the novel Of Mice and Men: the rabbit and the mouse.
Rabbits play an important role in this novel. They symbolize freedom on the farm as they are able to survive and flourish in their cages, but they also represent cuddly warmth and happiness to Lennie. Rabbits, however, do not exist on the farm to be cuddled; rather, they are a food source that happens to be fun to pet, but only while they are alive, which isn't for long. Rabbits represent a sort of innocent fantasy life for Lennie, a life that is pleasant, but only temporary.
Mice are also significant, especially as they are mentioned in the title of the novel. Lennie carries a dead mouse in his pocket because he likes to stroke it, and in this slightly grotesque image, the reader can observe Lennie's attraction to what is soft and vulnerable. The dead mouse portends Lennie's own death, which he accidentally brings upon himself because he, in his childlike innocence, can't help but overpower soft and vulnerable things.