In the final essay the _Reflex Theory_ is discussed; and here once more the biological point of view rectifies the error of an analysis which has led to the denial of Sensibility in reflex actions, because that analysis has overlooked the necessary presence of the conditions which determine Sensibility. In these chapters are reproduced several passages from the _Physiology of Common Life_. According to my original intention, this volume was to have included an exposition of the part I conceive the brain to play in physiological and psychological processes, but that must be postponed until it can be accompanied by a survey of psychological processes which would render the exposition more intelligible. THE PRIORY, March, 1877. CONTENTS. PROBLEM I. THE NATURE OF LIFE. CHAPTER I. PAGE THE PROBLEM STATED 3 (_The Position of Biology_) 4 (_Organisms_) 8 (_Vital Force_) 14 (_Vital Force controlling Physical and Chemical Forces_) 16 CHAPTER II. DEFINITIONS OF LIFE 24 CHAPTER III. ORGANISM, ORGANIZATION, AND ORGANIC SUBSTANCE 37 (_Organism and Medium_) 45 (_The Hypothesis of Germinal Matter_) 57 (_Organisms and Machines_) 67 CHAPTER IV. THE PROPERTIES AND FUNCTIONS 70 (_Does the Function determine the Organ?_) 78 CHAPTER V. EVOLUTION 89 (_Natural Selection and Organic Affinity_) 115 (_Recapitulation_) 152 PROBLEM II. THE NERVOUS MECHANISM. CHAPTER I. SURVEY OF THE SYSTEM, 157 (_The Early Forms of Nerve-Centres_), 168 (_The Peripheral System_), 171 (_Ganglia and Centres_), 172 CHAPTER II. THE FUNCTIONAL RELATIONS OF THE NERVOUS SYSTEM, 176 CHAPTER III. NEURILITY, 189 (_Origins of Nerve-Force_), 201 (_The Hypothesis of Specific Energies_), 207 CHAPTER IV. SENSIBILITY, 211 CHAPTER V. ACTION WITHOUT NERVE-CENTRES, 227 CHAPTER VI. WHAT IS TAUGHT BY EMBRYOLOGY?, 237 CHAPTER VII.