Who said Of all the words 'Of Mice and Men' the saddest are it might have been?

Of all the Words of mice and men, the saddest are 'It might have been.' Kurt Vonnegut, Jr. in the book, Cat's Cradle. According to Wikiquote, it is a variation of the poem "Maud Muller" by John Greenleaf Whittier: "For of all sad Words of tongue or pen, The saddest are these: "It might have been!"