2THEORY OF EVOLUTIONreaction withi7i the mass until the chemical substances that we know today were produced. This is the nebular hypothesis of the astronomer. The astronomer explains, or tries to explain, how this 'evolution took place, by an appeal to the physical processes that have been worked out in the laboratory, processes winch he thinks have existed through all the eons during which this evolution was going on and which were its immediate causes.When the biologist thinks of the evolution of animals and plants, a different picture presents itself. He thinks of series of animals that have Jived in the past, whose bones (fig. 1) and shells have been preserved in the rocks. He thinks of these animals as having in the past given birth, through an unbroken succession of individuals, to the living inhabitants of the earth today. He thinks that the old, simpler types of the past have in part changed over into the more complex forms of today.He is thinking as the historian thiTable of Contents TABLE OF CONTENTS; CHAPTER I; A REVALUATION OF THE EVIDENCE ON WHICH THE THEORY OF EVOLUTION; WAS BASED; page; Preface v; 1 Three Kinds of Evolution 1-7; 2 The Evidence for Organic Evolution 7-27; a The Evidence from Comparative Anatomy 7-14; b The Evidence from Embryology14-23; c The Evidence from Paleontology24-27; t The Four Great Historical Speculations 27-39; a The Environment27-31; Geoffroy St Hilaire; b Use and Disuse31-34; From Lamarck to Weismann; c The Unfolding Principle34-3G; Nàgeli and Bateson; d Natural Selection36-39; Darwin; vii; Vili; CONTENTS; CHAPTER II; THE BEARING 01< MENDEL'S DISCOVERY ON THE ORIGIN OF HEREDITY CHARACTERS; 1 Mendel's Jb'irst Discovery-Segregation41-52; 2 Mendel's Second Discovery-Independent As-; sortment 52-59; 3 The Characters of AVild Animals and Plants; Follow the Same Laws of Inheritance as do the Characters of Domesticated Animals a