plain, the other lovely. This suggests a passage from one to the other, and if the plain form most resembles the young bird in colouring (which is my own experience), whilst the young bird resembles, more than any old one, an allied plainer species, this seems to make it more than likely that the passage is from the plain to the lovely, and not from the lovely to the plain. Supporting and emphasizing this, we have the absence of those lance-like feathers in the tail of the young bird which give so marked a character to, and add so infinitely to the grace of, the old one. Of what use can this thin projection an inch or so beyond the serviceable fan of the tail be to the bird? Seeing how well every other bird does without it, can we suppose it to be of any service? Its beauty, however—which one misses dreadfully in the young flying bird—is apparent to anyone, and it goes hand in hand with an increased and ascending scale of beauty in colour. All this seems to me to point towards sexual selection, since I am personally a believer in the reality of that power, having never heard or read anything against it so convincing to my mind as what Darwin has said for it, nor seen anything that has appeared to me to be inconsistent either with his facts or his arguments.
Page:The Zoologist, 4th series, vol 6 (1902).djvu/439
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COLOURING OF STERCORARIUS CREPIDATUS.