XIII 



PTTYTATM CHORD ATA 



423 



liaving a unitbnii covciiipj, of tbatlieis. Tlio Ratit;i', also, have 



nothing more than tlic merest trace of hooklets on (lie barbviles, so 



that the barbs do not interlock, and the vanes 



of the feathers are downy or hair-like. In 



the Penofuins the winsj-feathers are decjenerate 



and scale-like. 



]\[any Birds are cjuite naked when hatched, 



but in most cases the body is more or less 



completely covered by a temporary crop of 



feathers, the ncstlinij-dmvns, of various forms, 



but always having a short axis, soft, loose 



barbs, devoid of interlocking apparatus, and, 



except in the Emu, having no after-shaft 



{vide infra). They are succeeded, as already 



described, by the permanent feathers. 



Many Birds, such as the Swan, possess 



dotun-feathers or iilumula: throughout life, in- 

 terspersed among and hidden by the contour 



feathers or pcnnce. In -the Heron and some 



other Carinatse are found 'powder-dovm patches 



(Fig. 1060, B, 2J. d. p, p. d. p), areas of downs, 

 the ends of which break off and make a fine 

 dust. Semi-plmms are downs with a well- 

 developed axis : filoplumcs, as we have seen 

 (Fig. 1023, B), have an elongated axis and 

 vestigial vexilkim. 



In many Birds there springs from the under 

 side of the quill, near the superior umbilicus, 

 a second vane, the after-shaft (Fig. 1061), 

 usually smaller than the main shaft, but some- 

 times of equal size. Both among Carinatnr! 

 and RatitiB we find genera with double- 

 shafted feathers and allied forms in which the 

 after-shaft is rudimentary or absent. 



The feathers are always shed or "moulted" 

 at regular intervals, 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 pigments, 

 i.e., are absorption-colours. White, and in 

 some cases yellow, is produced by the total 

 reflection of light from the spongy, air-contain- 

 ing substance of the feather, there being, as in nearly all other 

 natural objects, no such thing as a Avhite pigment. Blue, violet, 

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



D D 2 



Fig. Kir,]. -Feather of 

 Casuarius (Ca.sso- 

 wary), .showing after- 

 shaft and disconnected 

 barbs. (From Headley.) 



