Where did the title "Of Mice and Men" come from?

The title for John Steinbeck's novel "Of Mice and Men" comes from a poem written in 1785 by the Scottish poet Robert Burns.  The poem is entitled "To a Mouse, on Turning Her Up in Her Nest, with the Plough.

Perhaps the most famous line from that poem is "The best laid schemes o' mice an' men gang aft agley."  Which means something like "The best laid plans of mice and men often go awry."

The Steinbeck novel focuses on the problems faced by Depression-era farmers hoping to own their own land, but having their hopes shattered.