Birds played an important role in Darwin’s thinking about evolution, both from his observations of bird variation and biogeography and his observation of breeding birds in England. But they also played a very important role in his development of one of his most important theories: Sexual Selection. I’d like to address this development over the course of a few different posts, and to start out with here are a few selections of text from The Descent for you to push into your gullet and mush up with stones for a while:
The nictitating membrane, or third eyelid, with its accessory muscles and other structures, is especially well developed in birds, and is of much functional importance to them, as it can be rapidly drawn across the whole eye-ball. It is found in some reptiles and amphibians, and in certain fishes, as in sharks. It is fairly well developed in the two lower divisions of the mammalian series, namely, in the monotremata and marsupials, and in some few of the higher mammals, as in the walrus. But in man, the quadrumana, and most other mammals, it exists, as is admitted by all anatomists, as a mere rudiment, called the semilunar fold.
As dogs, cats, horses, and probably all the higher animals, even birds, as is stated on good authority,14 have vivid dreams, and this is shewn by their movements and voice, we must admit that they possess some power of imagination.
Sense of Beauty.—This sense has been declared to be peculiar to man. But when we behold male birds elaborately displaying their plumes and splendid colours before the females, whilst other birds not thus decorated make no such display, it is impossible to doubt that the females admire the beauty of their male partners. As women everywhere deck themselves with these plumes, the beauty of such ornaments cannot be disputed. The Bower-birds by tastefully ornamenting their playing-passages with gaily-coloured objects, as do certain humming-birds their nests, offer additional evidence that they possess a sense of beauty. So with the song of birds, the sweet strains poured forth by the males during the season of love are certainly admired by the females, of which fact evidence will hereafter be given. If female birds had been incapable of appreciating the beautiful colours, the ornaments, and voices of their male partners, all the labour and anxiety exhibited by them in displaying their charms before the females would have been thrown away; and this it is impossible to admit. Why certain bright colours and certain sounds should excite pleasure, when in harmony, cannot, I presume, be explained any more than why certain flavours and scents are agreeable; but assuredly the same colours and the same sounds are admired by us and by many of the lower animals.
Darwin, C. R. 1871. The descent of man, and selection in relation to sex. London: John Murray. Volume 1. 1st edition.
This is a drawing of the Grey Peacock-Pheasant, Polyplectron bicalcaratum a.k.a. Burmese Peacock. This drawing is from the 2nd edition of The descent of man (1882).
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