Page:Descent of Man 1875.djvu/440

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424
Part II.
The Descent of Man.

yellow, while in others the same part is tinged with red."[1] In the United States some few of the males of the Scarlet Tanager (Tanagra rubra) have "a beautiful transverse band of glowing red on the smaller wing-coverts;"[2] but this variation seems to be somewhat rare, so that its preservation through sexual selection would follow only under unusually favourable circumstances. In Bengal the Honey buzzard (Pernis cristata) has 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 graduated feathers."[3]

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 persecuted with much clamour by the other ravens of the island was the chief cause which led Brünnich to conclude that they were specifically distinct; but this is now known to be an error.[4] 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 estimation, presents this variation. It is characterised[5] 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,[6] 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

  1. Gould, 'Handbook to Birds of Australia,' vol. ii. pp. 32 and 68.
  2. Audubon, 'Ornitholog. Biography,' 1838, vol. iv. p. 389.
  3. Jerdon, 'Birds of India,' vol. i. p. 108; and Mr. Blyth, in 'Land and Water,' 1868, p. 381.
  4. Graba, 'Tagebuch, Reise nach Färo,' 1830, s. 51–54. Macgillivray, 'Hist. British Birds,' vol. iii. p. 745. 'Ibis,' vol. v. 1863, p. 469.
  5. Graba, ibid. s. 54. Macgillivray, ibid. vol. v. p. 327.
  6. 'Variation of Animals and Plants under Domestication,' vol. ii. p. 92.