652 THE DESCENT OF MAN 



halves of the 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 part black which is whit© 

 in the parent-species." 



Formation and Variability of the Ocelli or Eyelike 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 re"ptiles and fishes, 

 on the skin of amphibians, on the wings of many Lepidop- 

 tera and other insects, they deserve to be especially noticed. 

 An ocellus consists of a spot within a ring of another color, 

 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, aa 

 well as those on the wings of the peacock-butterfly (Va- 

 nessa). Mr. Trimen has given me a descriptioit of a South 

 African moth {Gyananis 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, sur- 

 rounded 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 Lepi- 

 doptera as the ocelli, both in number and size." Mr. Wal- 

 la,ce, who first called my attention to this subject, showed me 

 a series of specimens of our common meadow-brown butter- 

 fly (^Hipparchia janira) exhibiting numerous gradations from 

 a simple minute black spot to an elegantly shaded ocellus. 

 In a South African butterfly (Cyllo leda, Linn.), belonging 

 to the same family, the ocelli are even still more variable. 



■*' Bechstein, "Uaturgesohiehte Deutsehlands," B. iv., 1195, b. 31, on a 

 Bub-variety of the Monck pigeoD. 



