410 



ZOOLOGY 



SECT. 



have seen (Fig. 1034, B), have an elongated axis and vestigial 

 vexillum. 



In many Birds there springs from the under side of the quill, 

 near the superior umbilicus, a second vane, the after-shaft (Fig. 1072), 

 usually smaller than the main shaft, but sometimes of equal size. 

 Both among Carinatse and Ratitse we find genera with double- 

 shafted feathers and allied forms in which the after-shaft is rudi- 

 mentary or absent. 



The feathers are always shed or " moulted " at regular intervals, 



cv.a-pt 



FIG. 1071. A, pterylosis of Gypaetus (Bearded Vulture) ; B, of Ardea (Heron), a!, pt, 

 wing-tract ; c. pt, head-tract ; cd. pt, caudal tract ; cr. pt, crural tract ; e!>. apt. cervical 

 space ; hu. pt, humeral tract ; lat. apt, lateral space ; p. d. p., p. d. p', powder-down patches ; 

 sp. pt, spinal tract ; y. apt, ventral space ; w. pt, ventral tract. 



as a rule annually. The old feathers drop out and new ones are 

 formed from the same pulps. 



The colours of feathers present great variety. Black, brown, red, 

 orange, and yellow colours are due to the presence of definite pig- 

 ments, i.e. are absorption-colours. White, and in some cases 

 yellow, is produced by the total reflection of light from the spongy 

 air-containing substance of the feather, there being, as in nearly 

 all other natural objects, no such thing as a white pigment. Blue, 

 violet, and in some cases green are produced by the light from a 

 brown pigment becoming broken up as it passes through the super- 



