COLOUR 



the barrel or quill (calamus or scapus) at the base of the tuft 

 of rays have elongated into a principal shaft (rhachis) ; this is 

 generally accompanied by a secondary " aftershaft " (hyporliacliis), 

 originating from the " ventral " side, which in the Emeu and 

 Cassowary rivals the shaft itself in size. On the rhachis a double 

 series of lamellae or barbs are developed, carrying a similar double 

 series of barbules, much as in the down-feather, but the barbules 

 again give rise to barbicels (cilia), which in the distal rows 

 usually terminate in booklets Qiamuli). These catch in the folded 

 margins of the next proximal row, and a firm surface is thus 

 secured. An after-shaft never, and a down-feather rarely, possesses 

 barbicels ; while in some cases by the absence of these and part 

 of the barbules a " disconnected " web and -a " decomposed " feather 

 are formed, as in the decorative tufts of many species. The barbs 

 may even be absent, as in the wing-quills of Cassowaries, the 

 " wires " of Birds of Paradise, the " bristle-feathers " at the gape 

 of Night-jars or the eyelashes of Hornbills. In the hackles of 

 Gallus (Fowl), and the secondaries or even the tail-feathers of 

 Ampelis (Waxwing), the tip of the rhachis is flattened and wax- 

 like ; and similar structures are observable elsewhere. In the 

 newly-hatched young the down is often partly or entirely sup- 

 pressed, but in certain Birds this suppression is temporary, and a 

 thick coat grows after a few days. " Powder-down " feathers are 

 those which never develop beyond the early stage, and continually 

 disintegrate at the tip into bluish- or greyish-white powder ; they 

 occur in the Tinamidae, Ardeidae, Rliinochetidae, Eurypygidae, 

 Mesitidae, Accipitres and Psittaci, in Podargus, Coracias, Lepto- 

 soma, Gymnoderus and Artamus. 



Colour. The colour of Feathers is due to one of three causes. 

 First, an actual pigment * may be present in certain corpuscles, or 

 in diffused solution, and the tint does not then vary according to 

 the incidence of the light. Secondly, it may arise from a pigment 

 overlaid by colourless structures in the form of ridges or imbedded 

 polygonal bodies ; here, if the vanes are scraped or held up to 

 the light, the pigmentary colour alone is visible. 2 Thirdly, the 

 colour may be iridescent or prismatic ; that is, a blackish pig- 



1 Of this nature are zoomelanin (black), zoonerythrin (red), zooxanthin (yellow), 

 turacin (red only known in the Musophagidae], and perhaps turacoverdin (green, 

 from the same family). Brown is produced by a combination of red and black ; 

 white is the appearance due to innumerable air-spaces. 



2 Such are many yellows, oranges, greens and blues. 



