CHAPTER I. INTRODUCTORY 1 CHAPTER II. CLASSIFICATION 23 CHAPTER III. MORPHOLOGY 50 CHAPTER IV. EMBRYOLOGY 98 CHAPTER V. PALÆONTOLOGY 156 CHAPTER VI. GEOGRAPHICAL DISTRIBUTION 204 CHAPTER VII. THE THEORY OF NATURAL SELECTION 251 CHAPTER VIII. EVIDENCES OF THE THEORY OF NATURAL SELECTION 285 CHAPTER IX. CRITICISMS OF THE THEORY OF NATURAL SELECTION 333 CHAPTER X. THE THEORY OF SEXUAL SELECTION, AND CONCLUDING REMARKS 379 * * * * * APPENDIX TO CHAPTER V. 421 NOTE A TO PAGE 257 443 NOTE B TO PAGE 295 445 NOTE C TO PAGE 394 448 INDEX 451 LIST OF ILLUSTRATIONS FIG. PAGE 1. Successive forms of Paludina, from the Tertiary deposits of Slavonia 19 2. Skeleton of Seal 52 3. Skeleton of Greenland Whale 53 4. Paddle of Whale compared with Hand of Man 54 5. Wing of Reptile, Mammal, and Bird 56 6. Skeleton of _Dinornis gravis_ 61 7. Hermit crabs compared with the cocoa-nut crab 64 8. Rudimentary or vestigial hind-limbs of Python 67 9. _Apteryx Australis_ 69 10. Illustrations of the nictitating membrane in various animals named 75 11. Rudimentary, or vestigial and useless, muscles of the human ear 76 12. Portrait of a young male gorilla 78 13. Portrait of a young male child 79 14. An infant, three weeks old, supporting its own weight 81 15. Sacrum of Gorilla compared with that of Man, showing the rudimentary tail-bones of each 82 16. Diagrammatic outline of the human embryo when about seven weeks old 83 17. Front and back view of adult human sacrum 84 18. _Appendix vermiformis_ in Orang and in Man 85 19. The same, showing variation in the Orang 85