CONTENTS 



CHAPTER I. 

 INTRODUCTORY . 



CHAPTER II. 

 CLASSIFICATION 23 



CHAPTER III. 

 MORPHOLOGY 50 



CHAPTER IV. 

 EMBRYOLOGY 98 



CHAPTER V. 

 PALAEONTOLOGY 156 



CHAPTER VI. 

 GEOGRAPHICAL DISTRIBUTION 304 



CHAPTER VII. 

 THE THEORY OF NATURAL SELECTION 251 



CHAPTER VIII. 

 EVIDENCES OF THE THEORY OF NATURAL SELECTION . . 285 



