CHAPTER I PAGE The Problem 1 CHAPTER II Historical 8 CHAPTER III Mendel's Work 17 CHAPTER IV The Presence and Absence Theory 29 CHAPTER V Interaction of Factors 42 CHAPTER VI Reversion 59 CHAPTER VII Dominance 68 {x} CHAPTER VIII Wild Forms and Domestic Varieties 79 CHAPTER IX Repulsion and Coupling of Factors 88 CHAPTER X Sex 99 CHAPTER XI Sex (_continued_) 115 CHAPTER XII Intermediates 125 CHAPTER XIII Variation and Evolution 135 CHAPTER XIV Economical 153 CHAPTER XV Man 170 APPENDIX 187 INDEX 191 * * * * * {xi} ILLUSTRATIONS PLATES PLATE PAGE Gregor Mendel _Frontispiece_ I. Rabbits _To face_ 60 II. Sweet Peas " 64 III. Sheep " 78 IV. Sweet Peas " 80 V. Fowls " 107 VI. Butterflies " 146 FIGURES IN TEXT FIG. 1. Scheme of Inheritance in simple Mendelian Case 21 2. Feathers of Silky and Common Fowl 30 3. Single and Double Primulas 31 4. Fowls' Combs 32 5. Diagram of Inheritance of Fowls' Combs 37 6. Fowls' Combs 39 7. Diagram of F_2 Generation resulting from Cross between two White Sweet Peas 46 8. Diagram illustrating 9 : 3 : 4 Ratio in Mice 52 9. Sections of Primulas 55