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 Brunnich to conclude that they were specifically distinct; but this is now known to be an error. (40. Graba, 'Tagebuch Reise nach Faro,' 1830, ss. 51-54. Macgillivray, 'History of British Birds,' vol. iii. p. 745, 'Ibis,' vol. v. 1863, p. 469.) 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 (41. Graba, ibid. s. 54. Macgillivray, ibid. vol. v. p. 327.) 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 (42. 'Variation of Animals and Plants under Domestication,' vol. ii. p. 92.), 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 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 by their comrades, had been highly attractive (like the above pied peacock) to the black female ravens their numbers would have rapidly increased. And this would have been a case of sexual selection.
With respect to the slight individual differences which are common, in a greater or less degree, to all the members of the same species, we have every reason to believe that they are by far the most important for the work of selection. Secondary sexual characters are eminently liable to vary, both with animals in a state of nature and under domestication. (43. On these points see also 'Variation of Animals and Plants under Domestication,' vol. i. p. 253; vol ii. pp. 73, 75.) There is also reason to believe, as we have seen in our eighth chapter, that variations are more apt to occur in the male than in the female sex. All these contingencies are highly favourable for sexual selection. Whether characters thus acquired are transmitted to one sex or to both sexes, depends, as we shall see in the following chapter, on the form of inheritance which prevails.