COLOUR 



the barrel or quill (calamus or sca.pus) at the base of the tuft 

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

 generally accompanied by a secondary " aftershaft '' {hyporhachis), 

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

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

 series of latnettae 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 (hamuli). 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 

 Aiiqiclis ("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, a,nd continually 

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

 occur in the Tinamidae, Ardeidae, Rhinocheti'clae, JEurypygidae, 

 Mesitidcce, Accipitres and Psittaci, in Fodargiis, Corcwias, Lepto- 

 soma, Gymnoclerus and Artctmus. 



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.^ Thirdly, the 

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



^ Of this nature are zoomelanin (black), zoonerythrin (red), zooxantMn (yellow), 

 turacin (red — only known in the Musophagidae), and perhaps turacoverdia (green, 

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

 white is the appearance due to innumerable air-spaces. 



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



