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. 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.
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