TABLE OF CONTENTS XV 



PAGE 



The Distribution of Marsupials. A. R. Wallace 174 



The Distribution of Birds. A. R. Wallace 175 



Summary of Mammalian Dispersal. Hans Gadow ...... 177 



Summary of the Argument for Evolution as Based on Geographic 



Distribution 178 



PART III. THE MECHANISM OF EVOLUTION (GENETICS) 



Chapter XIII. Introductory Statement 183 



The Nature and Scope of Genetics 183 



Prerequisites for the Study of Genetics 184 



The Mechanism of Evolution 185 



The Main Causal Factors of Evolution 186 



Chapter XIV. The Biological Background of Genetics . . 190 



Races and Individuals 190 



The Cellular Make-up of Individuals 190 



A Typical Cell and Its Component Parts 191 



Cell Division — Mitosis 194 



Differentiation 195 



The Origin of New Individuals 196 



Modes of Reproduction 196 



The Origin of Gametes 200 



The Maturing of Gametes 203 



The Union of Gametes — Fertilization 207 



Chapter XV. Introduction to the Study of Persistence Factors 209 



Persistence and Diversity Mechanisms Contrasted 210 



A Short Lesson in Biometry 210 



Chapter XVI. Heredity in Puke Lines 212 



The Nature of Pure Lines 212 



The Pure-Line Experiments of Johannsen 212 



Other Examples of Pure Lines 215 



Exactly What Is Inherited? 217 



Chapter XVII. Sex Determination and Sex Differentiation . 219 



Sex Determination 219 



The Chromosomal Mechanism of Sex Determination . . . 221 



Sex Differentiation 227 



Chapter XVIII. Mendel's Laws of Heredity 232 



Mendel's Life and Character. /. Arthur Thomson 232 



Mendel's Discoveries 232 



Mendel's Explanations. John M. and Merle C. Coulter .... 23S 

 Illustrations of Simple Mendelian Inheritance in Both Animals and 



Plants. /. Arthur Thomson 245 



