xiii THE COLOURS OF BIRDS 271 



like the tiles of a roof. The ultimate causation 

 of the physical colour Gadow ascribes to the trans- 

 parent sheaths of keratin covering these compart- 

 ments, which he thinks act like a series of prisms. 

 An important point in connection with these metallic 

 barbules is, that they are so modified that both 

 booklets and folds are completely lost, and therefore 

 there is no connection between the barbules or the 

 barbs. Metallic feathers of this type have therefore 

 a peculiar looseness of texture which is, for example, 

 very obvious in the ornamental feathers of the 

 peacock ; the solidity of the flattened metallic 

 barbules gives, however, to such feathers an appear- 

 ance quite different from that of ordinary downy 

 feathers, in which also the barbs are unconnected. 

 The unconnected nature of the barbs is of especial 

 interest, because it would render the feathers quite 

 unfitted for purposes of flight if the variation were 

 to occur in quill-feathers. In sun-birds it is usually 

 the contour-feathers "which are metallic, rarely the 

 tail-quills, and apparently never the wing-quills. 



Development of Colours. — The types of coloration 

 already described in the sun-birds are seen in the 

 specialised feathers, especially of the male. In the 

 unspecialised feathers, such as the general contour- 

 feathers of the female, we find what may be regarded 

 as the primitive condition. These feathers are of a 

 dull olive colour, and are divided into three regions — a 

 basal downy region usually of an ashy colour, a median 

 slightly V-shaped region in which the barbules have 

 a very close texture and are of a brown colour, an 

 apical region in which the barbules are unconnected, 

 slightly modified, and faintly pigmented with dark 



