characters are not transmitted, will be compelled to assume that the process of natural selection has had a far larger share in the transformation of species than has been as yet accorded to it; for if such characters are not transmitted, the modifying influence of external circumstances in many cases remains restricted to the individual, and cannot have any part in producing transformation. We shall also be compelled to abandon the ideas as to the origin of individual variability which have been hitherto accepted, and shall be obliged to look for a new source of this phenomenon, upon which the processes of selection entirely depend. In the following pages I have attempted to suggest such a source. A. W. Freiburg I. Br., _November 22, 1885._ ------------------------------------------------------------------------ SIGNIFICANCE OF SEXUAL REPRODUCTION, etc. CONTENTS. PAGE 1. Can we dispense with the principle of natural 255 selection? 2. Nägeli’s theory of transformation from internal 256 causes 3. A definite course of development is possible 258 without a self-changing idioplasm 4. Conclusive importance of ‘adaptations’ 260 5. The structure of whales as an example of 261 adaptation 6. Transformation takes place by the smallest steps 264 7. The foundation of such minute changes depends upon 266 individual variability 8. Difficulty in accounting for variability on the 266 supposition of a continuity of the germ-plasm 9. Previous theories by which variability has been 267 accounted for 10. Non-transmission of acquired characters 267 11. Nägeli’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 272 reproduction 14. The process of natural selection does not operate 274 when asexual reproduction takes place 15. Origin of variability in unicellular organisms 278 16. Sexual reproduction effects combination 279 17. E. van Beneden’s and V. Hensen’s theory of sexual 282 reproduction as a process of rejuvenescence 18. Theoretical objections to such a view 283 19. Original significance of conjugation 286 20. Preservation of sexual reproduction by means of 287 heredity 21. It is lost in parthenogenesis for reasons of 289 utility 22. Parthenogenesis prevents further transformations 290