SIGNIFICANCE OF SEXUAL REPRODUCTION, ETC. 



CONTENTS. 



PAGE 



1. Can we dispense with the principle of natural selection ? . 255 



2. Nageli's theory of transformation from internal causes . . . 256 



3. A definite course of development is possible without a self-changing idio- 



plasm ............. 258 



4. Conclusive importance of ' adaptations ' 260 



5. The structure of whales as an example of adaptation . . . .261 



6. Transformation takes place by the smallest steps 264 



7. The foundation of such minute changes depends upon individual variability 266 



8. Difficulty in accounting for variability on the supposition of a continuity of 



the germ-plasm ........... 266 



9. Previous theories by which variability has been accounted for . . . 267 



10. Non-transmission of acquired characters ....... 267 



11. Nageli's and Alexis Jordan's experiments ....... 269 



12. Germ-plasm is only altered with great difficulty . . . . .271 



13. The source of individual variation lies in sexual reproduction . . .272 



14. The process of natural selection does not operate when asexual repro- 



duction takes place 274 



15. Origin of variability in unicellular organisms ^;S 



16. Sexual reproduction effects combination 279 



17. E. van Beneden's and V. Hensen's theory of sexual reproduction as a process 



of rejuvenescence ........... 282 



18. Theoretical objections to such a view 283 



19. Original significance of conjugation 286 



20. Preservation of sexual reproduction by means of heredity .... 287 



21. It is lost in parthenogenesis for reasons of utility 289 



22. Parthenogenesis prevents further transformations ..... 290 



23. It excludes Panmixia and thus prevents disused organs from becoming 



rudimentary 291 



24. Final considerations -><J4 



APPENDICES. 



I. FURTHER CONSIDERATIONS WHICH OPPOSE NAGELI'S EXPLANATION OP 



TRANSFORMATION AS DUE TO INTERNAL CAUSES 298 



II. NAGELI'S EXPLANATION OF ADAPTATION 300 



III. ADAPTATIONS IN PLANTS 308 



IV. ON THE SUPPOSED TRANSMISSION OF ACQUIRED CHARACTERS . . .310 



1. Brown-St^quard's experiments on Guinea-pigs 310 



2. A case which at first sight appears to prove the transmission of acquired 



characters 320 



V. ON THE ORIGIN OF PARTHENOGENESIS 323 



VI. W. K. BROOKS' THEORY OF HKHEDITY 326 



