CONTENTS 13 



CHAPTER PAGE 



C. ZYGOPHASE SEXUALITY— continued 



3. Other organisms 223 



4. Phenotypic sex determination and hermaphrodites 224 



5. The Hymenopteran type of sex determination 229 



6. Progamic sex determination 230 



D. THE THEORY OF SEXUALITY 



1. The two sexes 230 



2. The alternative reaction systems 232 



3. The differential sex genes or realizers 235 



4. The meaning of "sex" 236 



PART THREE 



GENETICS AND EVOLUTION 



XI. Processes of Evolution 241 



1. The palaeontological evidence 



(a) Trends 242 



(b) Programme evolution 244 



2. The sudden origin of species : age and area 248 



3. The local origin of important groups of plants 250 



XII. The Genetic Nature of Taxonomic 



Differences 253 



A. CHROMOSOME DIFFERENCES 



1. Polyploid species 254 



2. The experimental production of polyploid species 254 



3. The analysis of natural polyploid species 257 



4. Secondary polyploids 261 



5. Non-polyploid species 263 



6. Translocation, inversion, etc., in species formation 264 



7. Gene differences between species and sub-species 270 



(a) Lymantria 271 



(b) Peromyscus 273 



8. Local races in plants 274 



