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Show 12G SEXUAL SELECTION : BIRDR. PAn:r Jr. "having the whole of the feathers blue, while others "have the eight central ones tipped with beautiful "green." It does not appear that intermediate gradations have been observed in this or the following cnses. In the males alone of one of the Australian parral•eet. " the thighs in some are scarlet, in others "grass-green." In another parrakeet of tho same country "some individuals have the baud across the "wing-coverts bright-yellow, while in others the same "part is tinged with red." 34 In the United States some few of the males of the Scarlet Tanager ( Tanag1·a rubra) have "a beautiful transverse band of glowing "reel on the smaller wing-coverts ; " 35 but this variation seems to be somewhat 1·are, so that its preservation through sexual selection would follow only under unusually favourable circumstances. In Bengal the Honey Luzzard (Pernis cristata) bas either a small rudimental crest on its head, or none at all; so slight a difference however would not have been worth notice,. had not this same species possessed in Southern India "a well-marked occipital crest formed of several gra" duated feathers." 36 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 confined to the Feroe Islands. It is not very rare there, for Graba saw during his visit from eight to ten living specimens. Although the characters of this. variety are not quite constant, yet it has been named by several distinguished ornithologists as a distinct. species. The fact of the pied birds being pursued and 34 Gould, • II:1nubook of Birus of AuslroJia,' vol. ii. p. 32 and G8. 3·, Audubon, • Omitholog. Biography,' I838, vol. iv. p. 389. ar. Jerclon, 'Birds of Indio,' vol. i. p. I08; and 1\fr. Blyth, in' Lan<l and Water,' I8G8, p. 381. CHAP. Xl\". VARIABILITY. 127 persecuted with much clamour by the other ravens of tho island. was tho chief cause which led Briinnich to conclude that it was spocjfically distinct; but this is now known to be an error.37 In various parts of the northern seas a remarkable variety of the common G ui1lomot ( Uria trot'le) is found; and in Feroe, one out of every :five birds, according to Graba's estimation, consists of this variety. It is characterised 38 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 shewn,39 transmitted either unaltered 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 great advantage over the other, it would soon have been multiplied to the exclusion of the latter~ If, for instance, the male pied ravens, instead of being persecuted and driven away by their comrades, had been highly attractive, like the pied peacock before mentioned, to the common black females, their numbers would have rapidly increased. And this would have been a case of sexual selection. 37 Gmba, 'Tagebueh, Heise nach Fiiro,' I830, s. 51-54. 1\facgillivrny, ''!Jist. British Birds,' YOl. iii. p. 7-.1:5. 'Ibis,' vol. v. I G3, p. 4G9. a~ Graba, ibiJ. s. 51. Macgillivray, ibid. vol. v. p. 327. JJ 'Variation of Animals antl Plant~:; untler Domc::;tication,' rol. ii. p. 92. |