The author makes no claim for originality, unless it be in the _manner_ of presenting the subject. He has utilized the facts collated by other observers, and sometimes quoted the exact language and expressions of well-known writers on Evolution, and has endeavored to put them together in a way that may be helpful to those who are beginning the study of the Evolution Theory. No attempt has been made to prove the truth of the theory: this is assumed. The arguments in support of it are coextensive with our knowledge of Comparative Anatomy, Embryology, Physiology, Psychology, and many other sciences. In the preparation of the book the author is especially indebted to his friend Prof. Theodore Gill, the eminent ichthyologist, for many valuable suggestions, and more particularly for his aid in constructing the Diagram of Development. He also desires to thank his friend Dr. A. F. A. King for his kindly assistance in preparing the manuscript for the press; and also his friend Dr. L. O. Howard, Chief of the U. S. Bureau of Entomology, for much valuable information and assistance. D. K. SHUTE. August 1, 1899, 1318 L Street, N. W., Washington, D. C. CONTENTS. PAGE INTRODUCTION xiii System of Classification xiv Definition of Evolution xv SECTION I. ORGANIC CELLS: THE VISIBLE UNITS OF LIFE 3 Structure and Composition of Cells 4 Activities of Cells 7 Examples of Unicellular Animals and Plants 10 Mitosis 25 Maturation of the Human Ovum 29 Fertilization of the Human Ovum 29, 42 Segmentation of the Oösperm 31 Gastrulation 32 SECTION II. HEREDITY WITH VARIATION 39 Examples of Variations 39 Illustrations of Heredity 40 Nucleus in Heredity 41 Heritages 47 Pseudo-Heredity 53 Environment and Variations 55 Acquired Characters 65 Pangenesis 66 Continuity of Germ-Cells 67 Modified Pangenesis 68 Heredity Stronger than Environment 75 SECTION III.