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Figure 1.
A single display feather of a peacock, showing a fully developed “eye,” or “ocellus.” Figure 54 from The Descent of Man, and Selection in Relation to Sex.

A single display feather of a peacock, showing a fully developed “eye,” or “ocellus.” Figure 54 from The Descent of Man, and Selection in Relation to Sex.4

Figure 2.
Three ocelli in an intermediate stage of formation with the double halo still incomplete. Figure 60 from The Descent of Man, and Selection in Relation to Sex.

Three ocelli in an intermediate stage of formation with the double halo still incomplete. Figure 60 from The Descent of Man, and Selection in Relation to Sex.4

Figure 3.
Gradations in development of ocelli as seen on a single peacock feather. At the right are alternating dark and light stripes. At the top are small light spots with a dark halo. Below them is an ocellus whose double border surrounds more than half of the perimeter of the central spot, and below that is an ocellus in which the halo is more complete, almost a complete “ball and socket” or “perfect” ocellus. Figure 61 from The Descent of Man, and Selection in Relation to Sex.

Gradations in development of ocelli as seen on a single peacock feather. At the right are alternating dark and light stripes. At the top are small light spots with a dark halo. Below them is an ocellus whose double border surrounds more than half of the perimeter of the central spot, and below that is an ocellus in which the halo is more complete, almost a complete “ball and socket” or “perfect” ocellus. Figure 61 from The Descent of Man, and Selection in Relation to Sex.4

1.
Browne  J Charles Darwin: The Power of Place. Vol 2. Princeton, NJ Princeton University Press2002;7- 13
2.
Burkhardt  FPorter  DMDean  SA  et al.  The Correspondence of Charles Darwin. Vol 15. Cambridge, England Cambridge University Press2005;338
3.
Burkhardt  FSecord  JADean  SA  et al.  The Correspondence of Charles Darwin. Vol 17. Cambridge, England Cambridge University Press2009;
4.
Darwin  C The Descent of Man, and Selection in Relation to Sex. 2nd ed. London, England John Murray1874;441
5.
Helmholtz  H Popular Lectures on Scientific Subjects. Atkinson  E New York, NY D Appleton1873;218- 220- 227- 269- 390
6.
Fishman  RS Evolution and the eye: the Darwin bicentennial and the sesquicentennial of Origin of Species. Arch Ophthalmol 2008;126 (11) 1586- 1592
PubMedArticle
7.
Voss  J Darwin's intelligent design. http://www.mpg.de/english/illustrationsDocumentation/multimedia/mpResearch/2008/heft01/010/index.html. Max Planck Institute Magazine. 2008. Accessed January 18, 2010
Special Article
Sepetmber 2010

Darwin and Helmholtz on Imperfections of the Eye

Author Affiliations

Author Affiliation: Dr Fishman is retired.

Arch Ophthalmol. 2010;128(9):1209-1211. doi:10.1001/archophthalmol.2010.189

Charles Darwin was curious about the workings of the eye and corresponded with William Bowman and Cornelis Donders about its structure and function. In his On the Origin of Species in 1859, he took care to make the case that even such a complex organ as the eye could arise by natural selection. In other contexts, Darwin also gave particular weight to observations that revealed imperfections in development of organic features. It was exactly this line of inquiry that was so telling against the creationist credo of supernatural design. Darwin made a point of this in the second edition of his The Descent of Man, and Selection in Relation to Sex (1874) in which he made special reference to the views of Hermann Helmholtz about the imperfections of the eye.

DARWIN'S CORRESPONDENCE ABOUT THE EYE

Even more than most scientists of his generation, Darwin relied a good deal on personal correspondence. (More than 14 000 letters to or from him still survive, and these are probably only half of the total.1) In this way, Darwin cultivated a wide range of experts and was not shy in using their expertise to further his own ideas. He corresponded actively with other naturalists and experts in many fields, from farmers, gardeners, and pigeon breeders to diplomats, army officers, and physicians, including ophthalmologists. He exchanged letters with William Bowman and in 1867, while working on The Expression of the Emotions in Man and Animals, Darwin quizzed Bowman about the relation between contraction of the brow and tearing. Bowman doubted any relation but did write: “Be sure no one more appreciates your labours than myself—I think your views the only rational ones on the grand subject of the course of life on our planet.”2

In September of 1869 when Franciscus Cornelis Donders was visiting London, Bowman brought him to visit Darwin at his home at Down House. Darwin wrote that he found Donders “a very pleasant, jolly man & good ‘Darwinian’ (!).” Darwin then referred many times to the opinions of Donders in The Expression of the Emotions in Man and Animals (1872).3 Darwin found the physiology of the pupil “very complex,” a sentiment shared by generations of ophthalmologists ever since.

Darwin also read widely and was knowledgeable about who were authorities in the sciences. As we will see, he was well aware of Helmholtz's preeminence in visual science.

SEXUAL SELECTION

On the Origin of Species in 1859 had avoided discussing what Darwin knew would be the contentious issue of human evolution, making just a glancing reference that his theory would cast light on the question without, for the time being, dealing with it directly. He did grasp the nettle explicitly with Descent of Man, and Selection in Relation to Sex in 1871, with a second edition coming in 1874. Here he elaborated on his theory of sexual selection as a subset of natural selection, using it to account for such conspicuous features—usually but not always in males—as bright coloration, courtship behaviors, and especially the display plumage of birds, particularly the peacock's feathers. These ostentatious features appeared to be useless, wasteful, even deleterious—until one thought of them as somehow related to how animals evaluated the desirability of a potential mate.

THE PEACOCK'S FEATHERS

Other naturalists had waxed lyrical about the perfection and beauty of pheasant and peacock feathers in which the circular “eye,” or “ocellus,” was particularly striking (Figure 1). Instead of following this track, Darwin took the occasion to write an extended, almost obsessive discussion of how these ocelli were often formed in a rudimentary or less-than-full expression of the ideal shape. Here Darwin was writing in the language of ornithologists of the time, who assumed that their own standards of beauty and that of birds were alike. The “perfect” ocellus would have to be round and completely surrounded by a double circular halo—what he termed a ball and socket configuration—if it were to achieve its greatest beauty. In reality, however, primitive and intermediate ocelli—oval, conjoined, and incomplete—appeared in some species, as well as on feathers of an individual who also sported more perfect ocelli at other sites of its display. These could be considered way stations or gradations in the development of the perfect ocellus (Figure 2 and Figure 3). Even so, male pheasants sporting these imperfect features were evidently still attractive to the females.

Why was Darwin so taken with the need to show that ocelli existed in different states than what the adherents of natural theology, in their anthropocentric way, thought to be the ultimate, perfect form? Here he was addressing the basic tenets of natural theology, the creationist or intelligent design creed of the time. Perfection was implicit in the very idea of a supernatural creator or designer. It would be sacrilegious to suppose that such a force would deliberately introduce imperfections in design or to have to experiment to find the perfect design.

This concern reflected a major battleground in the very idea that species could change in the course of time to become different species, instead of being complete and perfect from the first week of creation. Immutable perfection was not the way evolution worked. Natural selection operated on chance variations, tinkering with them in such a way as to occasionally enhance the organism's reproductive success. Sexual selection was just another way of recognizing this: “We have, however, no right to expect absolute perfection in a part rendered ornamental through sexual selection, any more than we have in a part modified through natural selection for real use [ie, adaptations with direct or immediate survival value]; for instance in that wondrous organ the human eye.”4 Here, almost as an afterthought that had just occurred to him and hardly a culmination of the whole labored discussion, Darwin suddenly introduced . . . the imperfections of the human eye: “And we know what Helmholtz, the highest authority in Europe on the subject, has said about the human eye; that if an optician had sold him an instrument so carelessly made, he would have thought himself fully justified in returning it.”

HELMHOLTZ ON THE EYE

What was it that Helmholtz actually wrote and why did Darwin remember it at this particular point? In 1873, an English translation of Hermann Helmholtz's Popular Lectures on Scientific Subjects was published in New York. In it Helmholtz devoted several chapters to the eye and the physiology of vision, describing the many ways in which the human eye is less than a perfect optical instrument. He cited afterimages, spherical aberration, astigmatism, vitreous opacities, the physiological blind spots due to the optic nerve head and its associated retinal vessels, and the less than perfect clarity of the cornea and lens even when no actual opacities were present:

Now it is not too much to say that if an optician wanted to sell me an instrument which had all these defects, I should think myself quite justified in blaming his carelessness in the strongest terms, and giving him back his instrument. Of course I shall not do this with my eyes, and shall be only too glad to keep them as long as I can—defects and all. Still, the fact that, however bad they may be, I can get no others, does not at all diminish their defects, so long as I maintain the narrow but indisputable position of a critic on purely optical grounds. . . . 

We have now seen that the eye in itself is not by any means so complete an optical instrument as it first appears: its extraordinary value depends on the way in which we use it: its perfection is practical, not absolute, consisting not in the avoidance of every error, but the fact that all its defects do not prevent its rendering the most important and varied services. . . . 

From this point of view, the study of the eye gives us a deep insight into the true character of organic adaptation generally. And this consideration becomes still more interesting when brought into relation with the great and daring conceptions which Darwin has introduced into science, as to the means by which the progressive perfection of the races of animals and plants has been carried on.5

Helmholtz showed that he appreciated the core idea of evolutionary theory, that natural selection crafts features that are useful in adaptation, that although the eye may not be perfect it surely helps the animal to survive and prosper.

DARWIN'S USE OF HELMHOLTZ'S VIEWS

The English translation of Helmholtz's book became available in 1873 while Darwin was preparing his second edition of Descent of Man. Darwin obviously knew Helmholtz's reputation as an authority on the eye, but was not at ease in German, writing to his German correspondents in standard Victorian English.1 Because he did not as a rule read the primary German scientific literature, this English translation of Helmholtz was of particular value to him.

Of all bodily organs, the eye had a special fascination for Darwin. He had already in the Origin gone to some length to make the case that even the intricate construction of the eye was still consistent with natural selection.6 Now he took the different tack of using the eye's very imperfections for the same purpose. The human eye as well as peacock plumage may have its faults, but imperfections in organic design actually made an even stronger case for natural selection.

In the words of Julia Voss:

Darwin discovered something exceptional in every organism, his view steadfastly fixed on the imperfections and inadequacies in the natural world; he peeked into the crevices and cracks of what was, until then, considered to be flawless creation until a door opened, where evolution stood waiting to be let in. If the naturalists before him supposed that organisms were machines, then Darwin discovered the grinding and rattling in the works, the scrap and waste. The question that haunted him was not: Why are animals and plants so perfect? His question was: Why are they so terribly imperfect?7

Darwin knew why, and now we do, too.

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Article Information

Correspondence: Ronald S. Fishman, MD, 47880 Cross Manor Rd, Saint Inigoes, MD 20684 (rsfishman@earthlink.net).

Submitted for Publication: August 12, 2009; final revision received December 7, 2009; accepted December 8, 2009.

Financial Disclosure: None reported.

References
1.
Browne  J Charles Darwin: The Power of Place. Vol 2. Princeton, NJ Princeton University Press2002;7- 13
2.
Burkhardt  FPorter  DMDean  SA  et al.  The Correspondence of Charles Darwin. Vol 15. Cambridge, England Cambridge University Press2005;338
3.
Burkhardt  FSecord  JADean  SA  et al.  The Correspondence of Charles Darwin. Vol 17. Cambridge, England Cambridge University Press2009;
4.
Darwin  C The Descent of Man, and Selection in Relation to Sex. 2nd ed. London, England John Murray1874;441
5.
Helmholtz  H Popular Lectures on Scientific Subjects. Atkinson  E New York, NY D Appleton1873;218- 220- 227- 269- 390
6.
Fishman  RS Evolution and the eye: the Darwin bicentennial and the sesquicentennial of Origin of Species. Arch Ophthalmol 2008;126 (11) 1586- 1592
PubMedArticle
7.
Voss  J Darwin's intelligent design. http://www.mpg.de/english/illustrationsDocumentation/multimedia/mpResearch/2008/heft01/010/index.html. Max Planck Institute Magazine. 2008. Accessed January 18, 2010
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