132 SEXUAL SELECTION : BIRDS. Part IL 



outer webs of the two outer tail-feathers white ; now 

 there is a sub-variety having a white instead of a blue 

 tail, with precisely that small part black which is white 

 in the parent-species.^* 



Formation and variahiliiy of the Ocelli or eye-like 

 Spots on the Plumage of Birds. — 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 con- 

 sists of a spot within a ring of another colour, like the 

 pupil within the iris, but the central spot is often sur- 

 rounded 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 at least with insects has probably 

 been a simple one ; 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 speci- 

 mens of our common meadow-brown butterfly {Hip- 



'^■^ Bechsteiu, ' Naturgeschichte Deutschlands,' B. iv. 1795, s. 31, on 

 a sub-variety of the Monck pigeoa. 



