Chapter XVI: Preface

The greater part of the present essay was delivered at the first general meeting of the Association of German Naturalists, at Strassburg, on September 18th, 1885, and is printed in the Proceedings of the fifty-eighth meeting of that Society.

The form of a lecture has been retained in the present publication, but its contents have been extended in many ways. Besides many small and a few large additions to the text, I have added six appendices in order to treat of certain subjects more fully than was possible in the lecture itself, in which I was often obliged to be content with mere hints and suggestions. This appears to be all the more necessary because it is impossible to suppose that many views and ideas upon which the lecture was based would be well known to all readers, although they have been described in my former papers. It was above all necessary to deal with the class of acquired characters, which, as it seems to me, is easily confounded, especially by the medical profession, with the much broader class of new characters generally. Only those new characters can be called ‘acquired’ which owe their origin to external influences, and the term ‘acquired’ must be denied to those which depend upon the mysterious relationship between the different hereditary tendencies which meet in the fertilized ovum. These latter are not ‘acquired’ but inherited, although the ancestors did not possess them as such, but only as it were the elements of which they are composed. Such new characters as these do not at present admit of an exact analysis: we have to be satisfied with the undoubted fact of their occurrence. The transmission or non-transmission of acquired characters must be of the highest importance for a theory of heredity, and therefore for the true appreciation of the causes which lead to the transformation of species. Any one who believes, as I do, that acquired 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._

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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

23. It excludes Panmixia and thus prevents disused 291
organs from becoming rudimentary

24. Final considerations 294

APPENDICES.

I. Further considerations which oppose Nägeli’s 298
explanation of Transformation as due to internal
causes

II. Nägeli’s Explanation of Adaptation 300

III. Adaptations in Plants 308

IV. On the Supposed Transmission of Acquired 310
Characters

1. Brown-Séquard’s experiments on Guinea-pigs 310

2. A case which at first sight appears to prove 320
the transmission of acquired characters

V. On the Origin of Parthenogenesis 323