CONTENTS 



CHAPTER I 



I'AGE 



The Problem . . . . . . . i 



CHAPTER II 

 Historical 7 



CHAPTER III 

 MCndel's Work ^5 



CHAPTER IV 

 The Presence and Absence Theory ... 26 



CHAPTER V 

 Interaction of Factors 39 



CHAPTER VI 



Reversion 54 



vii 



