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EVOLUTION OF ANIMALS 



Part V 



uses. A peacock's so-called tail is a gorgeous display of overgrown covert 

 feathers and the real tail is inconspicuous. The male turkey displays a fan of 

 tail feathers but, when they are folded, the tail is a rudder. 



At hatching, chickens, ducklings, and many other birds are covered with 

 small fluffy down feathers that shut in the heat of the young animals. Each 

 one consists of filaments that spring from the tip of a very short quill. Down 

 is often abundant under the contour feathers especially in ducks and geese. 

 Filoplumes or thread feathers are the hairlike ones that remain after all the 

 others are plucked and are removed when a chicken is singed. In the course 

 of evolution these feathers have lost the spreading vane or web and only a 

 weakened shaft is left. Whippoorwills, flycatchers and others have stiff bristles 

 near the base of the beak. A bristle has a short quill, and a slender shaft with 

 a few barbs at its base. 



Colors. White and the colors of feathers including iridescence are due to 

 structure and pigment. There is no pigment in white feathers. Reflected light 

 rays strike obliquely against the dried cell membranes and when there is no 

 pigment, no rays are absorbed — all are shattered and the surface appears 

 white. The microscopic bubbles in well-beaten albumen or "white of egg" are 

 white for the same reason; in this respect a white feather and meringue are 

 nearly related. A blue feather is like the white one except that the cells contain 

 the dark pigment melanin (Fig. 36.5), Rays of reflected light strike obliquely 



Fig. 36.3. Feather tracts of a cuckoo (Geococcyx calif ornicus) . Feathers do 

 not develop equally on all parts of the body except in primitive birds such as 

 penguins. (After Shufeldt. Courtesy, Rand: The Chordates. Philadelphia, The 

 Blakiston Co., 1950.) 



