Like the human eye, the ocellus (the circular patterns on a peacock feather), seem to provide a counter-example that argues against any theory of evolution, Lamarkian or Darwinian. In the following passage from chapter 14 of his book on sexual selection, Darwin, who was always eager to tackle such apparently strong arguments against his theories, shows that exant, easily found examples of such patterns provide a step-by-step development that ends in the ocellus. Passages like this, we should observe, also function to provide authorial ethos (or Aristotleian credibility) that Darwin uses, like other Victorian sages, such as Carlyle, Ruskin, and Arnold, to demonstrate virtuouso acts of interpretation. If Darwin can defend his theories from (and by means of) such apparently powerful counter-examples, then ethos implies, we should give the rest of his argument a patient hearing. —George P. Landow.
As no ornaments are more beautiful than the ocelli on the feathers of various birds, on the hairy coats of some mammals, on the scales of reptiles and fishes, on the skin of amphibians, on the wings of many Lepidoptera and other insects, they deserve to be especially noticed. An ocellus consists of a spot within a ring of another colour, like the pupil within the iris, but the central spot is often surrounded by additional concentric zones. The ocelli on the tail-coverts of the peacock offer a familiar example, as well as those on the wings of the peacock-butterfly (Vanessa). Mr. Trimen has given me a description of a S. African moth (Gynanisa isis), allied to our Emperor moth, in which a magnificent ocellus occupies nearly the whole surface of each hinder wing; it consists of a black centre, including a semi-transparent crescent-shaped mark, surrounded by successive, ochre-yellow, black, ochre-yellow, pink, white, pink, brown, and whitish zones.
Left to right: Figures 53,57, 58, 59 (a) Peacock feather about half-size.. (b) Basal part of the Secondary wing-feather, nearest the body.. (c) part of the Secondary wing-feather, nearest the body (i.e, essentially, a detail of the previous image) (d) Portion of feather showing perfect ball-and-socket pattern. [Click on these images for larger pictures.]
Although we do not know the steps by which these wonderfully beautiful and complex ornaments have been developed, the process has probably been a simple one, at least with insects; for, as Mr. Trimen writes to me, "no characters of mere marking or coloration are so unstable in the Lepidoptera as the ocelli, both in number and size." Mr. Wallace, who first called my attention to this subject, shewed me a series of specimens of our common meadow-brown butterfly (Hipparchia janira) exhibiting numerous gradations from a simple minute black spot to an elegantly-shaded ocellus. In a S. African butterfly (Cyllo leda, Linn.), belonging to the same family, the ocelli are even still more variable. In some specimens (A, Fig. 53) large spaces on the upper surface of the wings are coloured black, and include irregular white marks; and from this state a complete gradation can be traced into a tolerably perfect ocellus (A1), and this results from the contraction of the irregular blotches of colour. In another series of specimens a gradation can be followed from excessively minute white dots, surrounded by a scarcely visible black line (B), into perfectly symmetrical and large ocelli (B1). (48. This woodcut has been engraved from a beautiful drawing, most kindly made for me by Mr. Trimen; see also his description of the wonderful amount of variation in the coloration and shape of the wings of this butterfly, in his 'Rhopalocera Africae Australis,' p. 186.) In cases like these, the development of a perfect ocellus does not require a long course of variation and selection.
With birds and many other animals, it seems to follow from the comparison of allied species that circular spots are often generated by the breaking up and contraction of stripes.
Bibliography
Darwin, Charles. A Naturalist's Voyage Round the World: The Voyage Of The Beagle. Project Gutenberg EBook #3704 produced by Sue Asscher. August 6, 2008. The e-version is based on the 1890 11th edition. (The book first appeared in 1839.)
Last modified 25 March 2012