56 The Bird 



have numerous httle ridges extending down the sides, 

 and in some way, by reflection, these change the yellow 

 or black to ])lue. If we take a parrot's feather and 

 pound the blue portion, that colour will disappear and 

 the vane will become black. 



It is surprising to see how the colours of many beau- 

 tiful feathers will vanish when we hold them between 

 our eye and the light. When we look at feathers under 

 the microscope, and see their horny rays, we forget, for 

 a time, the delicac}' and fluffiness which the bird's plumage 

 as a whole exhibits, and we are constanth^ reminded of 

 the scales of reptiles. And in colour we have another 

 similarity between the two: lizards have both pigment 

 and prisms, and the scales of large snakes glow like opals 

 when the sunlight falls on them. 



White never exists as a pigment in the feathers of 

 birds, but is always due to innumerable air-spaces in the 

 substance of the feather, by which the rays of light are 

 reflected and deflected until, as in snow or foam, all 

 coloiu' is lost and white results. 



In any one Order of birds there ma}' often be found 

 a series of species with colour patterns grading into each 

 other and connecting two extremes, perhaps very diverse 

 in a})])earance. But it is seldom that we can examine 

 such a series at once, and, except in a large collection of 

 birds' skins in a museum, these wonderful life-chains, or 

 twig-tips of the tree of evolution seldom a])peal to us 

 very forcibly. But in a feather it is different. We may 

 find on one bird a most delicately graduated series, show- 

 ing every step in the process by which simple unicoloured 



