482 THE DESCENT OF MAN, 



mental crest on its head, or none at all; so slight a differ- 

 ence, however, would not have been worth notice, had not 

 this same species possessed in Soutliern India a well-marked 

 occipital crest formed of several graduated feathers/'* 



The following case is in some respects more interesting. 

 A pied variety of the raven, with the head, breast, abdomen, 

 and parts of the wings and tail-feathers white is con- 

 fined to the Feroe Islands. It is not very rare there, for 

 Graba saw during his visit from eight to ten living speci- 

 mens. Although the characters of this variety are not 

 quite constant, yet it has been named by several distin- 

 guished ornithologists as a distinct species. The fact of 

 the pied birds being pursued and persecuted with much 

 clamor by the other ravens of the island was the chief 

 cause which led Briinnich to conclude that they were 

 specifically distinct; but this is now known to be an error.} 

 This case seems analogous to that lately given of albino 

 birds not pairing from being rejected by their comrades. 



In various parts of the northern seas a remarkable variety 

 of the common Guillemot ( Uria troile) is found; and in 

 Feroe one out of every five birds, according to Graba's esti- 

 mation, presents this variation. It is characterized^ by a 

 pure white ring round the eye, with a curved narrow white 

 line an inch and a half in length extending back from the 

 ring. This conspicuous character has caused the bird to 

 be ranked by several ornithologists as a distinct species 

 under the name of U. lacrymans, but it is now known to 

 be merely a variety. It often pairs with the common kind, 

 yet intermediate gradations have never been seen; nor is 

 this surprising, for variations which appear suddenly are 

 often, as I have elsewhere shown, transmitted either unal- 

 tered or not at all. We thus see that two distinct forms of 

 the same species may co-exist in the same district, and we 

 cannot doubt that if the one had possessed any advantage 



*Jerdon, "Birds of India," vol, i, p. 108; and Mr. Blytb, in 

 " Land and Water," 1868, p. 381. 



f Graba, "Tagebucli Reise nacb Faro," 1830, ss. 51-54. Macgilli- 

 vray, "Hist. British Birds," vol. iii, p. 745. "Ibis," vol. v, 1863, 

 p. 469. 



if Graba, ibid, s. 54. Macgillivray, ibid, vol. v, p. 327. 



" Variation pf Animals and Plants un(Jer Domestication," vol. ii, 

 p. 9^, 



