In this volume, with a Preface and an Appendix by J Mark Baldwin, Professor in Princeton University, Professor Groos makes a contribution to three distinct but cognate departments of inquiry: philosophical biology, animal psychology, and the genetic study of art. Those who have followed the beginnings of inquiry into the nature and functions of play in the animal world and in children will see at once how much light is to be expected from a thorough-going examination of all the facts and observations recorded in the literature of animal life. Professor Groos makes this sort of examination with great care and thoroughness, and the result is a book which, in my opinion, is destined to have wide influence in all these departments of inquiry.