xiv CONTENTS 



CHAPTER VI 



PAGE 

 COMMBRCIAl VARIETIES 105 



CHAPTER VII 



NATURAL CROSSING 113 



CHAPTER VIII 



HEREDITY. I ■ 127 



CHAPTER IX 



HEREDITY. II 150 



SECTION IV 



CHAPTER X 



ECONOMICS . . 176 



BIBLIOGRAPHY ... ... 181 



