What novel did John Steinbeck originally title "Something That Happened"?

"Of Mice and Men" is a novella written by author John Steinbeck. Published in 1937, it tells the story of George Milton and Lennie Small, two migrant ranch workers, who move from place to place in California in search of new job opportunities during the Great Depression in the United States.

George Milton is an intelligent but uneducated man and Lennie Small is a bulky, strong man but mentally disabled. They hope to one day attain the dream of settling down on their own piece of land. Lennie's part of the dream is merely to tend and pet rabbits on the farm, as he loves touching soft animals, although he always accidentally kills them. This dream is one of Lennie's favorite stories, which George constantly retells. They had fled from Weed after Lennie touched a young woman's dress and wouldn't let go, leading to an accusation of rape. It soon becomes clear that the two are close and George is Lennie's protector, despite his antics.

Steinbeck originally titled it "Something That Happened" (referring to the events of the book as "something that happened" because nobody can be really blamed for the tragedy that unfolds in the story). However, he changed the title after reading Robert Burns's poem "To a Mouse".  Burns's poem tells of the regret the narrator feels for having destroyed the home of a mouse while plowing his field.