A fine quote from Marcus Aurelius, gleaned from the excellent pages of today's New York Times Book Review: "A limit of time is fixed for thee, which if thou dost not use for clearing the clouds from thy mind, it will go and thou wilt go, and it will never return." It comes in a Reading to Live: a review of "The Whole Five Feet"--the length of the full shelf of the Harvard Classics, 51 volumes of indispensable reading.
Otherwise... I invite you to consider the truly frightening column by Nicholas D Kristof, It's Time to Learn From Frogs. It bodes ill for our species.