This is a revised and enlarged edition of a volume that first appeared in 1930. The author gives its purpose as follows: “I suspect that this book will continue to find its chief use as a second book in the general and experimental courses.” It covers the work of Binet, Terman, Galton, Pavlov, Franz, Lashley, Kohler, Thorndike, Woodworth, Ebbinghaus, Watson, Cannon, Von Helmholtz, Cattell, and Fechner. There are photos of the above individuals, chapter bibliographies, forty-seven figures and an eight-page index.