pose to be the way with predators and prey in savage nature. In any case, 

 it is obvious that, as things stand to-day, the very smallest items in aid either 

 of the hunters or the hunted must be of vital importance. Eagles and tigers 

 are not more clever at catching than their quarries are at escaping, hence the 

 slightest additional aid may save a quarry's life. Just such an aid is the mo- 

 mentary deception effected by the contrary movement of a spot of iridescence, 

 as described above. Hindered but for an instant, the pursuer may be wholly 

 balked, for that instant may enable the quarry to slip into cover, or take wing, 

 just in the nick of time. 



But the larger deceptions achieved by iridescence, viz., nearly complete 

 'obliteration,' in one form or another, are still more potent and important. 

 A brightly changeable plumage is like a sumptuous wardrobe, packed into 

 marvelously small compass — many different dresses combined in one, without 

 the loss of their individual identity. The Mallard Duck (Anas boschas), for 

 instance, has in some lights a bright green "speculum" on its wing. In other 

 lights this mark is blue, in still others, purple. In addition to the look of life 

 and motion (like that of water and glittering vegetation) which the change- 

 ableness of this marking gives it, it also makes it far likelier to match the 

 bird's background than any fixed tint could. Water, mirroring whatever is 

 above it, varies interminably in color, and so do foliage-vistas and other land- 

 scape details. Were the Mallard's speculum of a uniform blue, it would 

 serve its full obliterative use only when the bird's background happened to 

 show areas of just that hue. But containing as it does the whole scale of 

 colors from grass-green to reddish purple, displayed one after another by 

 slight changes in the bird's position, it is equipped for perfect color-match- 

 ing, if often only in flashes, with many sorts of background. Indeed, even in 

 most single yiews, and without motion, the speculum shows such a range of 

 lustrous color that some part is likely to be an exact match for one of the back- 

 ground tints. (Although this marking is usually almost hidden while the 

 ducks are swimming, it often comes into full view when they walk or stand, 

 as on river-banks or tussocks, or in reed-grown shallows.) Still more marked 



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