CONTENTS Xlll 



CHAPTER VIII 

 MULTIPLE FACTORS 



PAGE 



The Meaning of the term " Multiple Factors " 172 



Examples of Multiple Factors 172 



Selection and Multiple Factors 195 



CHAPTER IX 

 THE FACTORIAL HYPOTHESIS 



On the Relation between Factors and Characters 208 



1. The Organism-as-a- Whole Objection 211 



2. The Invariability of the Factor and the Variability in the 



Character 212 



3. So-called Contamination of Allelomorphs 214 



4. Fractionation 214 



5. The Presence and Absence Hypothesis 216 



Weismann's Prseformation Hypothesis and the Factorial Theory . 223 



APPENDIX 



Methods of Breeding Drosophila 229 



Acknowledgments 235 



Bibliography 237 



INDEX . . 259 



