From 1888 to his death in 1921, Russian Prince Peter Kropotkin forced biologists to ask themselves whether natural selection inevitably led to a dog-eat-dog world, or whether pro-social behavior could also be a product of the evolutionary process. In this historical vignette, I focus on Kropotkin's theory of “mutual aid,” with emphasis on the role that empathy played in that theory, and the unexpected source—economist Adam Smith's 1759 book, The Theory of Moral Sentiments—of Kropotkin's ideas on empathy in animals. J. Exp. Zool. (Mol. Dev. Evol.) 320B: 407–411, 2013. © 2013 Wiley Periodicals, Inc.