Part 26

M. Nussbaum, ‘Sitzungber. der Niederrheinischen Gesellschaft fur Natur- und Heilkunde.’ Dec. 15, 1884.

Footnote 120:

A. Gruber, ‘Biologisches Centralblatt,’ Bd. IV. No. 23, and V. No. 5.

Footnote 121:

According to the observations of Nussbaum and van Beneden, the egg of _Ascaris_ departs from the ordinary type, but I think that the latter observer goes too far when he concludes from the form of the nuclear spindle (of which the two halves are inclined to each other at an angle) that we have before us a process entirely different from that of ordinary nuclear division.

Footnote 122:

Trinchese, ‘I primi momenti dell’ evoluzione nei molluschi,’ Atti Acad. Lyncei (3) vii. 1879, Roma.

Footnote 123:

M. Nussbaum, ‘Archiv für Mikroskopische Anatomie,’ Bd. XVIII und XXIII.

Footnote 124:

Valaoritis, ‘Die Genesis des Thier-Eies.’ Leipzig, 1882.

Footnote 125:

Kölliker, ‘Die Bedeutung der Zellkerne,’ etc.; Zeitschr. f. wiss. Zool. Bd. XLII.

Footnote 126:

‘Compt. rend.’ Tom. LIV. p. 150.

Footnote 127:

‘Entwicklung der Dipteren.’ Leipzig, 1864.

Footnote 128:

‘Zeitschr. f. wiss. Zool.’ Bd. XVI. p. 389 (1866).

Footnote 129:

‘Compt. rend.’ Nov. 13, 1882.

Footnote 130:

Grobben, ‘Arbeiten d. Wien. Zool. Instituts,’ Bd. II. p. 203.

Footnote 131:

Bütschli, ‘Zeitschrift f. wiss. Zool.’ Bd. XXIII. p. 409.

Footnote 132:

‘Science,’ vol. iv. No. 90, 1884.

Footnote 133:

Among unicellular organisms, encysted individuals are often called germs. They sometimes differ from the adult organism in their smaller size and simpler structure (_Gregarinidae_), but they represent the same morphological stage of individuality.

Footnote 134:

Compare Bütschli in Bronn’s ‘Klassen und Ordnungen des Thierreichs,’ Bd. I. p. 777.

Footnote 135:

Gustav Jäger, ‘Lehrbuch der Allgemeinen Zoologie,’ Leipzig, 1878; II. Abtheilung. Probably on account of the extravagant and superficial speculations of the author, the valuable ideas contained in his book have been generally overlooked. It is only lately that I have become aware of Jäger’s above-mentioned hypothesis. M. Nussbaum seems to have also arrived at the same conclusion quite independently of Jäger. The latter has not attempted to work out his hypothesis with any degree of completeness. The above-mentioned observations are followed immediately by quite valueless considerations, as, for instance, that the ontogenetic and phyletic groups are in concentric ratio! The author might as well speak of a quadrangular or triangular ratio!

Footnote 136:

[Facts of the same kind are also known in the Vascular Cryptogams, Muscineae, Characeae, Florideae, etc.—S. S.]

Footnote 137:

Weismann, ‘Die Entstehung der Sexualzellen bei den Hydromedusen.’ Jena, 1883.

Footnote 138:

[I adopt this term, suggested by E. Ray Lankester and G. C. Bourne, as the name of the supporting lamina of Coelenterata. See ‘Quart. Journ. Microsc. Sci.’ Jan. 1887, p. 28.—E. B. P.]

Footnote 139:

Dr. Clemens Hartlaub, ‘Ueber die Entstehung der Sexualzellen bei Obelia.’ Freiburg, Inaugural Dissertation: see also ‘Zeitschrift für wissenschaftliche Zoologie.’ Bd. XLI. 1884.

Footnote 140:

English translation, by H. Marshall Ward. Oxford, 1887, Clarendon Press.

Footnote 141:

[Such gland-cells are known in both animals and plants. See W. Gardiner and Tokutaro Ito, On the structure of the mucilage-secreting cells of _Blechnum occidentale_ L., and _Osmunda regalis_ L., ‘Annals of Botany,’ vol. i. p. 49.—S. S.]

Footnote 142:

Thus in 1877 Bütschli thought that ‘the chief significance of the formation of polar bodies lies in the removal of part of the nucleus of the egg, whether this removal is effected by simple expulsion or by the budding of the egg-cell.’ ‘Entwicklungsgeschichtliche Beiträge;’ Zeitschrift für wissenschaftliche Zoologie, Bd. XXIX. p. 237, footnote.

Footnote 143:

C. S. Minot, ‘Account, etc.;’ Proc. Boston Soc. Nat. Hist. vol. xix. p. 165, 1877.

Footnote 144:

E. van Beneden and Boveri have recently, quite independently of each other, made a more exact study of these ‘Polkörperchen’ (‘Centrosoma,’ Boveri). They show that nuclear division starts from these bodies, although the mode of origin of the latter is not yet quite clear.—A. W., 1888.

Footnote 145:

The existence of polar bodies in sponges has been recently proved by Fiedler: Zool., Anzeiger., Nov. 28, 1887.—A. W., 1888.

Footnote 146:

They have now been observed in many species, so that their general occurrence in insects is tolerably certain. Compare bibliography given in Weismann and Ischikawa, ‘Weitere Untersuchungen zum Zahlengesetz der Richtungskörper,’ ‘Zoolog. Jahrbücher,’ vol. iii. 1888, p. 593.—A. W., 1888.

Footnote 147:

Van Beneden, even in his last work, considers these bodies to have only the value of nuclei; l. c., p. 394.

Footnote 148:

I purposely abstain from using a more precise term, for the complicated terminology employed in spermatogenesis hardly contributes anything to the elucidation of the phenomena themselves. Why do we not simply speak of sperm-cells and spermatoblasts, and distinguish the latter by numbers when they occur in successive generations of different form? Moreover, all the names which have been suggested for successive stages of development, can only be applied to the special group of animals upon which the observations have been made. Hence great confusion results from the use of such terms as spermatoblasts, spermatogonia, spermatomeres, spermatocysts, spermatocytes, spermatogemmae, etc.

Footnote 149:

Fol, ‘Sur l’origine des cellules du follicule et de l’ovule chez les Ascidies.’ Compt. rend., 28 mai, 1883.

Footnote 150:

Roule, ‘La structure de l’ovaire et la formation des œufs chez les Phallusiadées.’ Ibid., 9 avril, 1883.

Footnote 151:

Balbiani, ‘Sur l’origine des cellules du follicule et du noyau vitellin de l’œuf chez les Géophiles.’ Zool. Anzeiger, 1883, Nos. 155, 156.

Footnote 152:

Will, ‘Ueber die Entstehung des Dotters und der Epithelzellen bei den Amphibien und Insecten.’ Ibid., 1884, Nos. 167, 168.

Footnote 153:

[It is almost certain that this vesicle is not derived from the nucleus, but from the cytoplasm of the sperm-mother-cell. See Douglas H. Campbell, ‘Zur Entwicklungsgeschichte der Spermatozoiden’ in Berichte der deutschen botanischen Gesellschaft, vol. v, 1887, p. 122.—S. S.]

Footnote 154:

Bütschli, ‘Gedanken über die morphologische Bedeutung der sogenannten Richtungskörperchen,’ Biolog. Centralblatt, Bd. VI. p. 5, 1884.

Footnote 155:

F. M. Balfour, ‘Comparative Embryology,’ vol. i. p. 63.

Footnote 156:

The formation of a polar body in parthenogenetic eggs has now been proved: see note at the end of this Essay; see also Essay VI.—A. W., 1888.

Footnote 157:

R. Leuckart,—article ‘Zeugung,’ in R. Wagner’s ‘Handwörterbuch der Physiologie,’ 1853, Bd. IV. p. 958. Similar observations were made by Max Schultze. These observations appear however to be erroneous, for Pflüger has since shown that the eggs of frogs never develope if the necessary precautions are taken to prevent the access of any spermatozoa to the water.—A. W., 1888.

Footnote 158:

Oellacher, ‘Die Veränderungen des unbefruchteten Keims des Hühncheneies. ‘Zeitschrift für wissenschaftliche Zoologie,’ Bd. XXII. p. 181. 1872.

Footnote 159:

Hensen, ‘Centralblatt,’ 1869, No. 26.

Footnote 160:

Weismann, ‘Beiträge zur Naturgeschichte der Daphnoiden,’ Leipzig, 1876-79, Abhandlung VII, and ‘Zeitschrift für wissenschaftliche Zoologie,’ Bd. XXXIII.

Footnote 161:

Weismann, ‘Beiträge zur Kenntniss der ersten Entwicklungsvorgänge im Insectenei,’ Bonn, 1882, p. 106.

Footnote 162:

W. Roux, ‘Ueber die Bedeutung der Kerntheilungsfiguren.’ Leipzig, 1883.

Footnote 163:

We now know that the number of loops varies considerably in different species, even when they belong to the same group of animals (e.g. Nematodes).—A.W., 1888.

Footnote 164:

This expression is used by bee-keepers, for instance by the well-known Baron Berlepsch. Of course, it would be more accurate to say that the queen, seeing the cell of a drone, is stimulated to lay an unfertilized egg, and that, on the other hand, she is stimulated to lay a fertilized egg when she sees the cell of a worker, or that of a queen.

Footnote 165:

E. Bessels, ‘Die Landois’sche Theorie widerlegt durch das Experiment.’ Zeitschrift für wissenschaftliche Zoologie, Bd. XVIII. p. 124. 1868.

Footnote 166:

‘Daphniden,’ Abhandlung, vi. p. 324.

Footnote 167:

l. c., p. 150.

Footnote 168:

Carl Düsing, ‘Die Regulirung des Geschlechtsverhältnisses.’ Jena. 1884.

Footnote 169:

I intend to publish these experiments elsewhere in connexion with other observations.

Footnote 170:

Weismann, ‘Daphniden,’ Abhandlung, VII. p. 329; Herbert Spencer, ‘The Principles of Biology,’ 1864, vol. i. pp. 229, 230.

Footnote 171:

The same fact has since been ascertained in species belonging to several groups of animal.

Footnote 172:

Brooks, ‘The Law of Heredity.’ Baltimore, 1883, p. 73.

Footnote 173:

‘Zeitschrift für wissenschaftliche Zoologie,’ Bd. XXXIII. p. 107. 1873.

Footnote 174:

Valaoritis, l. c., p. 6.

Footnote 175:

I quote from Falkenberg, in Schenk’s ‘Handbuch der Botanik,’ Bd. II. p. 219. He further states that these are the only instances hitherto known in which undoubted male cells have proved to be capable of further development when they have been unable to exercise their powers of fertilization. It must be added that the two kinds of germ-cells do not differ in appearance, but only in behaviour; the female germ-cells becoming fixed, and withdrawing one of their two flagella, while the male cells continue to swarm. But even this slight degree of differentiation requires the supposition of internal molecular differentiation.

V.

THE SIGNIFICANCE OF SEXUAL REPRODUCTION IN THE THEORY OF NATURAL SELECTION.

1886.

SIGNIFICANCE OF SEXUAL REPRODUCTION, ETC.

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._

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

VI. W. K. Brooks’ Theory of Heredity 326

V.

THE SIGNIFICANCE OF SEXUAL REPRODUCTION IN THE THEORY OF NATURAL SELECTION.

During the quarter of a century which has elapsed since Biology began to occupy itself again with general problems, at least one main fact has been made clear by the united labours of numerous men of science, viz. the fact that the Theory of Descent, the idea of development in the organic world, is the only conception as to the origin of the latter, which is scientifically tenable. It is not only that, in the light of this theory, numerous facts receive for the first time a meaning and significance; it is not only that, under its influence, all the ascertained facts can be harmoniously grouped together; but in some departments it has already yielded the highest results which can be expected from any theory, it has rendered possible the prediction of facts, not indeed with the absolute certainty of calculation, but still with a high degree of probability. It has been predicted that man, who, in the adult state, only possesses twelve pairs of ribs, would be found to have thirteen or fourteen in the embryonic state: it has been predicted that, at this early period in his existence, he would possess the insignificant remnant of a very small bone in the wrist, the so-called _os centrale_, which must have existed in the adult condition of his extremely remote ancestors. Both predictions have been fulfilled, just as the planet Neptune was discovered after its existence had been predicted from the disturbances induced in the orbit of Uranus.

That existing species have not arisen independently, but have been derived from other and mostly extinct species, and that on the whole this development has taken place in the direction of greater complexity, may be maintained with the same degree of certainty as that with which astronomy asserts that the earth moves round the sun; for a conclusion may be arrived at as safely by other methods as by mathematical calculation.

If I make this assertion so unhesitatingly, I do not make it in the belief that I am bringing forward anything new nor because I think that any opposition will be encountered, but simply because I wish to begin by pointing out the firm ground on which we stand, before considering the numerous problems which still remain unsolved. Such problems appear as soon as we pass from the facts of the case to their explanation; as soon as we pass from the statement ‘The organic world has arisen by development,’ to the question ‘But how has this been effected, by the action of what forces, by what means, and under what circumstances?’

In attempting to answer these questions we are very far from dealing with certainties; and opinions are still conflicting. But the answer lies in the domain of future investigation, that unknown country which we have to explore.

It is true that this country is not entirely unknown, and if I am not mistaken, Charles Darwin, who in our time has been the first to revive the long-dormant theory of descent, has already given a sketch, which may well serve as a basis for the complete map of the domain; although perhaps many details will be added, and many others taken away. In the principle of natural selection, Darwin has indicated the route by which we must enter this unknown land.

But this opinion is not universal, and only recently Carl Nägeli[176], the famous botanist, has expressed decided doubts as to the general applicability of the principle of natural selection. According to Nägeli, the co-operation of the external conditions of life with the known forces of the organism, viz. heredity and variability, are insufficient to explain the regular course of development pursued by the organic world. He considers that natural selection is at best an auxiliary principle, which accepts or rejects existing characters, but which is unable to create anything new: he believes that the causes of transformation reside within the organism alone. Nägeli further assumes that organisms contain forces which cause periodical transformation of the species, and he imagines that the organic world, as a whole, has arisen in a manner similar to that in which a single individual arises.

Just as a seed produces a certain plant because it possesses a certain constitution, and just as, in this process, certain conditions must be favourable (light, warmth, moisture, &c.) in order that development may take place, although they do not determine the kind or the manner of development; so, in precisely the same way, the tree of the whole organic world has grown up from the first and lowest forms of life on our planet, under a necessity arising from within, and on the whole independently of external influences. According to Nägeli, the cause which compels every form of living substance to change, from time to time, in the course of its secular growth, and which moulds it afresh into new species, must lie within the organic substance itself, and must depend upon its molecular structure.

It is with sincere admiration and real pleasure that we read the exposition in which Nägeli gives, as it were, the result of all his researches which bear upon the great question of the development of the organic world. But although we derive true enjoyment from the contemplation of the elaborate and ingeniously wrought-out theoretical conception,—which like a beautiful building or a work of art is complete in itself,—and although we must be convinced that its rise has depended upon the progress of knowledge, and that by its means we shall eventually reach a fuller knowledge; it is nevertheless true that we cannot accept the author’s fundamental hypothesis. I at least believe that I am not alone in this respect, and that but few zoologists will be found who can adopt the hypothesis which forms the foundation of Nägeli’s theory.

It is not my intention at present to justify my own widely different views, but the subject of this lecture compels me to briefly explain my position in relation to Nägeli, and to give some of the reasons why I cannot accept his theory of an active force of transformation arising and working within the organism; and I must also explain the reasons which induce me to adhere to the theory of natural selection.

The supposition of such a phyletic force of transformation (see Appendix I, p.298) possesses, in my opinion, the greatest defect that any theory can have,—it does not explain the phenomena. I do not mean to imply that it is incapable of rendering certain subordinate phenomena intelligible, but that it leaves a larger number of facts entirely unexplained. It does not afford any explanation of the purposefulness seen in organisms: and this is just the main problem which the organic world offers for our solution. That species are, from time to time, transformed into new ones might perhaps be understood by means of an internal transforming force, but that they are so changed as to become better adapted to the new conditions under which they have to live, is left entirely unintelligible by this theory. For we certainly cannot accept as an explanation Nägeli’s statement that organisms possess the power of being transformed in an adaptive manner simply by the action of an external stimulus (see Appendix II, p. 300).

In addition to this fundamental defect, we must also note that there are absolutely no proofs in support of the foundation of this theory, viz. of the existence of an internal transforming force.