SIGNIFICANCE OF SEXUAL REPRODUCTION, 



ETC. 



-♦♦- 



CONTENTS. 



PAGE 



1. Can we dispense with the principle of natural selection? . . 261 



2. Nageli's theory of transformation from internal causes . . 262 



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



changing idioplasm ........ 264 



4. Conclusive importance of ' adaptations ' ..... 265 

 6. The structure of whales as an example of adaptation . . 267 



6. Transformation takes place by the smallest steps . . .271 



7. The foundation of such minute changes depends upon individual 



variability .......... 272 



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



continuity of the germ-plasm ...... 273 



9. Previous theories by which variability has been accounted for . 274 



10. Non-transmission of acquired characters ..... 275 



11. Nageli's and Alexis Jordan's experiments . .... 276 



12. Germ-plasm is only altered with great difficulty . . . 278 



13. The source of individual variation lies in sexual reproduction . 279 



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



reproduction takes place ....... 280 



15. Origin of variability in unicellular organisms .... 284 



16. Sexual reproduction effects combination ..... 286 



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



as a process of rejuvenescence ...... 289 



18. Theoretical objections to such a view ..... 290 



19. Original significance of conjugation ...... 293 



20. Preservation of sexual reproduction by means of heredity . 294 



21. It is lost in parthenogenesis for reasons of utility . . . 296 



22. Parthenogenesis prevents further transformations . . . 298 



23. It excludes Panmixia and thus prevents disused organs from 



becoming rudimentary ........ 298 



24. Final considerations ......... 302 



APPENDICES. 



I. Further considerations which oppose Nageli's expla- 

 nation OF Transformation as due to internal causes 306 

 II. Nageli's Explanation of Adaptation ..... 308 



III. Adaptations in Plants .... .... 317 



IV. On the Supposed Transmission of Acquired Characters 319 



1. Brown-Sequard's experiments on Guinea-pigs . . . 319 



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



of acquired characters ....... 328 



V. On the Origin of Parthenogenesis ..... 332 



VI. W. K. Brooks' Theory of Heredity. . . . . 335 



