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Darwin Correspondence Project

From C. G. Semper   26 April 1877

Würzburg

26th April 1877.

Dear Sir

You will have noticed perhaps with some astonishment that my photograph is wanting in the Album, which has been presented to You by German Naturalists.1 I would be very sorry if You should have taken this fact as proving, that I did not belong to Your admirers and followers and I trust You will allow me a few words in explanation thereof.

The reasons why I did not send in my photograph were the following two. Firstly, I am of opinion that a man ought not to make a present with such bad portraits as photographs are, especially if he has not been asked for them; secondly, the proposed plan of adorning or embellishing such bad pictures did not seem to my sense of modesty very tasteful.

It appears to me that it had been a much better token of gratitude and devotion, if we German Naturalists would have presented to You on Your birthday a work containing new and original researches. The best photograph of a scientific man is to my understanding his scientific work.

In this sense I venture today to ask You the great favour of accepting the dedication of a little work “On eyes of the Vertebrate type on the back of Molluscs”, which will soon be published as an appendix to the third volume of my “Reisen etc”.2 I have just received a copy of a short preliminary communication on this subject in the “Archiv f. mikrosk. Anat”, which I have the pleasure to send You together with some microscopic—unluckily not very good—sections through these eyes and also some adult animals, on whose back You will easily be able to distinguish the eyes with a pocket-lens.3 I subjoin a MS. copy of the introduction of the work, which as I earnestly hope may be judged by You as deserving the honour, which You will confer on it by accepting my dedication.

I remain dear Sir | Yours devotedly | C. Semper.

[Enclosure]4

Ueber Sehorgane vom Typus der Wirbelthieraugen auf

dem Rücken von Schnecken.

Von C. Semper

Einleitung. Es besteht bekanntlich zwischen den Augen der Wirbelthiere und denen der Wirbellosen ein fundamentaler Gegensatz in der Schichtfolge der Retina; bei jenen bilden die Opticusfasern immer die innerste, bei diesen immer die äußerste Retinaschicht. Darwin hat die große Bedeutung dieses Gegensatzes anerkannt, indem er in der 6ten. Auflage seines “Origin of Species” (englische Ausgabe 1875 p. 152) auf ihn hinwies.5

Bei der anatomischen Durcharbeitung von 19 Arten von Onchidium, einer meist in tropischen Ländern lebenden Gattung schalenloser Pulmonaten, habe ich nun aber Augen aufgefunden, welche, obschon im Einzelnen wesentlich einfacher gebaut, doch fast alle Elemente des Wirbelthierauges in absolut mit diesem übereinstimmender Schichtfolge aufweisen. Auf die stark convexe Cornea folgt eine aus mehreren Schichten von Zellen bestehende Linse; diese wird an ihrem vorderen Abschnitt durch einen Ciliarring festgehalten, sodaß eine wahrscheinlich auch erweiterbare Pupille entsteht; die eng sich an die Linse anschließende Retina weist hart an dieser die Faserschicht auf, darauf folgt die einfache Retinazellenlage, und dann eine Stäbchenschicht, die mit ihren Stäbchenenden an eine Pigmentumhüllung herantritt. Der Sehnerv durchbohrt die äußeren Lagen der Retina, um sich dicht hinter der Linse becherförmig zur Faserschicht auszubreiten; in Folge davon entsteht ein echter blinder Fleck, wie bei den Wirbelthieren.

Dies allein schon würde, wie mir scheint, eine eingehende Untersuchung solcher Augen rechtfertigen. Sie gewinnen aber noch an Interesse durch die Art ihres Vorkommens. Sie stehen nemlich nicht auf den Tentakeln der Onchidium-arten, sondern auf ihrem Rücken, während die gleichfalls vorhandenen Tentakelaugen nach dem gewöhnlichen Typus des Molluskenauges gebaut sind. Es kommen hier also zweierlei Augentypen auf demselben Thiere vor. Die Rückenaugen stehen ferner durch ihre Nerven nicht mit dem Cerebralganglion in Verbindung, sondern mit dem Visceralganglion, während die Optici der Tentakelaugen sich an jenes ansetzen. Es zeigen weiter diese Rückenaugen eine auffallende Variabilität der Zahl selbst innerhalb der Species; so schwankt bei O. verruculatum die Menge derselben bei den einzelnen Individuen zwischen 12 und 82.6 Und es stehen endlich verschiedene Entwickelungsstufen der Augen auf dem Rücken desselben erwachsenen Individuums nebeneinander.

Keine einzige Schneckengattung ist bekannt, welche solche Augen trüge. Bergh,7 der unstreitig beste Kenner der Nacktmollusken des Meeres, schreibt mir, es kämen bei diesen Mollusken keine solchen Augen vor; ich selbst, der ich meiner Jugendliebe zu den Landmollusken nie ganz untreu geworden bin, habe bei diesen so wenig, wie bei Kiemenschnecken derlei Organe gefunden; die beiden Gattungen Fissurella und Haliotis, bei welchen ich aus anderen Gründen noch am Ehesten glaubte Mantelaugen erwarten zu können, besitzen sie nicht, wenigstens nicht bei den mir momentan vorliegenden Arten.8 Es scheinen also diese Rückenaugen in der Gattung Onchidium selbst enstanden zu sein; dafür spricht ferner der Umstand, daß es auch gänzlich derselben entbehrende Arten derselben Gattung giebt.

Diese Thatsachen mußten den Wunsch rege machen, auch die Entwicklungsweise der Rückenaugen zu untersuchen. An jungen Thieren oder Larven dies zu thun, war mir leider nicht möglich. Dagegen fand ich bald, daß auf demselben Thier verschieden große Augen nebeneinander vorkämen, und daß dann immer die kleinsten auch am einfachsten gebaut seien. Parallel damit ging eine Entwicklungsreihe innerhalb der ausgewachsenen Augen selbst, indem manche von ihnen auf verschiedenen Entwicklungsstufen beharrt zu haben schienen. Durch Ausdehnung der Untersuchung auf die Entwickelung der die Augen tragenden Rückenpapillen selbst gelang es mir schließlich den allgemeinsten Entwicklungstypus Augen mit großer Wahrscheinlichkeit aufzufinden und zu constatiren, daß das hier innerhalb der Gattung jetzt in Bildung oder Umbildung begriffene Sinnesorgan direct auf die einfachsten überhaupt und überall in den Papillen vorkommenden Bildungen zurückzuführen sei. Durch hypothetische Verknüpfung der Lebensverhältnisse dieser Thiere mit der Entwicklungsreihe ihrer Rückenaugen ergab sich dann schließlich auch die physiologische Möglichkeit ihrer Entstehung, Fixirung und weiteren Ausbildung.

Footnotes

CD had been presented with an album of photographs of German and Austrian naturalists (see letter from Emil Rade, [before 16] February 1877 and n. 3).
Semper’s work Über Sehorgane von Typus der Wirbelthieraugen auf dem Rücken von Schnecken (On visual organs of the vertebrate eye type on the backs of slugs; Semper 1877b) appeared in July 1877 (see letter from C. G. Semper, 13 July 1877). The dedication read ‘Herrn Charles Darwin | in aufrichtiger Verehrung | gewidmet | vom Verfasser’ (Dedicated to Mr Charles Darwin with sincere devotion from the author). Semper had published on land molluscs of the Philippine archipelago in the third volume of Reisen im Archipel Philippinen. Zweiter Theil: Wissenschaftliche Resultate (Travels in the Philippine archipelago. Second part: scientific results; Semper et al. 1868–1916), which was published in 1870. Semper 1877b was a supplement to this volume.
Semper’s paper ‘Ueber Schneckenaugen vom Wirbelthiertypus nebst Bemerkungen über einige andere histologische Eigenthümlichkeiten verschiedener Cephalophoren’ (On vertebrate-type slug eyes with remarks on some other histological characteristics in various gastropods; Semper 1877a) was published in the Archiv für mikroskopische Anatomie. CD’s copy is in the Darwin Pamphlet Collection–CUL.
For a translation of this enclosure, see Appendix I. The printed version of the introduction is unchanged from this manuscript (see Semper 1877b, pp. 1–2).
The sixth edition of Origin was published in 1872 and various printings were made up to 1876 when a sixth edition with additions and corrections was published. The edition Semper had, published in 1875, was the fifteenth thousand (Freeman 1977). In Origin 6th ed., p. 152, CD had discussed the differences in the structure and formation of vertebrate and invertebrate eyes. He concluded: As two men have sometimes independently hit on the same invention, so in the several foregoing cases it appears that natural selection, working for the good of each being, and taking advantage of all favourable variations, has produced similar organs, as far as function is concerned, in distinct organic beings, which owe none of their structure in common to inheritance from a common progenitor.
Onchidium verruculatum is a synonym of Peronia verruculata, a species of air-breathing marine slug (pulmonate gastropod) in the family Onchidiidae.
Rudolph Bergh was the author of the malacological volumes of Semper et al. 1868–1916 (for more on the subdivisions of Bergh’s work within Semper et al. 1868–1916 and dates of their publication, see Winckworth 1946).
Fissurella is a genus of limpets in the family Fissurellidae (keyhole limpets); Haliotis is a genus of abalone in the family Haliotididae.

Bibliography

Freeman, Richard Broke. 1977. The works of Charles Darwin: an annotated bibliographical handlist. 2d edition. Folkestone, Kent: William Dawson & Sons. Hamden, Conn.: Archon Books, Shoe String Press.

Origin 6th ed.: The origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. 6th edition, with additions and corrections. By Charles Darwin. London: John Murray. 1872.

Origin: On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. By Charles Darwin. London: John Murray. 1859.

Winckworth, R. 1946. On Bergh’s Malacologische Untersuchungen. [Read 12 April 1946.] Proceedings of the Malacological Society of London 27 (1946–9): 20–2.

Translation

From C. G. Semper   26 April 1877

[Enclosure]1

On visual organs of the type of vertebrate eyes on the back of slugs

By C. Semper

Introduction. It is well known that there is a fundamental contrast in the sequence of retinal layers in the eyes of vertebrates and those of invertebrates; in one, the filaments of the optic nerve always form the innermost, in the other always the outermost layer of the retina. Darwin recognised the great significance of this difference by stressing it in the 6th edition of his “Origin of Species” (English edition 1875 p. 152).2

In working out the anatomy of 19 species of Onchidium, a genus of mostly tropical shell-less pulmonates, I have now, however, found eyes which, while in finer points of significantly simpler construction, still display almost all the elements of the vertebrate eye with exactly the same succession of layers. On the strongly convex cornea there is a lens consisting of several layers of cells; this is attached to the anterior section by a ciliary ring, so that a pupil that is probably expandable is formed; the resulting retina, in close proximity to the lens, has a hard fibrous layer, on which lies a simple layer of retina cells, and then a layer of rods, whose ends touch a pigment epithelium. The optic nerve pierces the external layers of the retina and spreads out into a cup-like shape that is connected with the fibrous layer near the lens. As a result, there is a genuine blind spot as in vertebrates.

This alone, it seems to me, would already justify a detailed examination of such eyes. However, they attract even more interest because of the way in which they occur. For they do not sit on the tentacles of the Onchidium species, but on their backs, while the tentacle eyes that occur as well are built according to the ordinary type of mollusc eyes. Thus, there are two kinds of eyes on the same animal. The dorsal eyes, moreover, are connected by their nerves not to the cerebral ganglion, but to the visceral ganglion, while the optic nerves of the tentacle eyes are linked to the former. Furthermore, the number of these dorsal eyes varies to a striking degree even within the species itself; thus, in O. verruculatum the number of the eyes varies in individual animals between 12 and 82.3 Finally, on the back of the same adult individual eyes at different stages of development can be found next to each other.

Not a single genus of snails is known to have such eyes. Bergh,4 indisputably the greatest authority on marine naked molluscs writes to me that these molluscs possess no such eyes. I myself, who I have never become entirely unfaithful to my youthful love of land molluscs, have found such organs neither in them nor in gill snails; the two genera Fissurella and Haliotis, in which for other reasons I felt I could most likely expect mantle eyes, do not possess them, at least not the species that are available to me at the moment.5 Thus, these dorsal eyes appear to have developed in the genus Onchidium itself; a further point in favour of this is the fact that there are species of the same genus that lack these eyes entirely.

These facts inevitably awakened the desire to examine also the manner of development of dorsal eyes. Unfortunately I was unable to do so in young animals and larvae. However, I soon discovered that eyes of different size occurred next to one another on the same animal, and that the smallest of them were always the simplest. Parallel with this was a developmental series within the full-grown eyes, in that some of them appeared to have been arrested at different developmental stages. By extending the investigation to the development of the dorsal papillae that carry the eyes, I eventually succeeded in identifying the most general type of development of the eye with a greater degree of probability, and in establishing that this sense organ, which is in the process of developing or transforming within this genus, goes back directly to the simplest developments that generally take place all over the papillae. The hypothetical comparison of the living conditions of these animals with the developmental sequence of their dorsal eyes, eventually resulted in information on the physiological potentiality of their origin, establishment and further development.

Footnotes

For a transcription of this enclosure in its original German, see pp. QQQQ.
The sixth edition of Origin was published in 1872 and various printings were made up to 1876 when a sixth edition with additions and corrections was published. The edition Semper had, published in 1875, was the fifteenth thousand (Freeman 1977). In Origin 6th ed., p. 152, CD had discussed the differences in the structure and formation of vertebrate and invertebrate eyes. He concluded: As two men have sometimes independently hit on the same invention, so in the several foregoing cases it appears that natural selection, working for the good of each being, and taking advantage of all favourable variations, has produced similar organs, as far as function is concerned, in distinct organic beings, which owe none of their structure in common to inheritance from a common progenitor.
Onchidium verruculatum is a synonym of Peronia verruculata, a species of air-breathing marine slug (pulmonate gastropod) in the family Onchidiidae.
Rudolph Bergh was the author of the malacological volumes of Semper et al. 1868–1916 (for more on the subdivisions of Bergh’s work within Semper et al. 1868–1916 and dates of their publication, see Winckworth 1946).
Fissurella is a genus of limpets in the family Fissurellidae (keyhole limpets); Haliotis is a genus of abalone in the family Haliotididae.

Bibliography

Freeman, Richard Broke. 1977. The works of Charles Darwin: an annotated bibliographical handlist. 2d edition. Folkestone, Kent: William Dawson & Sons. Hamden, Conn.: Archon Books, Shoe String Press.

Origin 6th ed.: The origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. 6th edition, with additions and corrections. By Charles Darwin. London: John Murray. 1872.

Origin: On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. By Charles Darwin. London: John Murray. 1859.

Winckworth, R. 1946. On Bergh’s Malacologische Untersuchungen. [Read 12 April 1946.] Proceedings of the Malacological Society of London 27 (1946–9): 20–2.

Summary

Explains why he did not add his photograph to the album presented by German naturalists to CD. Instead he wishes to dedicate to CD his work on the vertebrate-type eyes on the back of some Mollusca. [Enclosed is a MS introduction to this work, Über Sehorgane von Typus der Wirbelthieraugen auf dem Rücken von Schnecken.].

Letter details

Letter no.
DCP-LETT-10942
From
Carl Gottfried Semper
To
Charles Robert Darwin
Sent from
Würzburg
Source of text
DAR 177: 137
Physical description
3pp, encl 2pp (German)

Please cite as

Darwin Correspondence Project, “Letter no. 10942,” accessed on 19 May 2021, https://www.darwinproject.ac.uk/letter/DCP-LETT-10942.xml

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