XII 



PHYLUM CHORD ATA 



existence to a thin, transparent, 

 superficial layer, which acts as a 

 prism : in such feathers the colour 

 changes according to the relative 

 position of the Bird and of the 

 eye of the observer with regard to 

 the source of light. 



There is also infinite variety in 

 the general coloration of Birds. In 

 many the colouring is distinctly 

 protective, harmonising with the 

 environment, and even changing 

 with the latter, as in the Ptarmigan, 

 which is greyish-brown in summer, 

 white in winter, the former hue help 

 ing to conceal the Bird among 

 herbage, the latter on snow. Fre- 

 quently, as in Pheasants and Birds 

 of Paradise, the female alone is 

 protectively coloured, while the male 

 presents the most varied and bril- 

 liant tints, enhanced by crests, plumes 

 or tufts of feathers on the wings, 

 elongated tail, &c., &c. These have 

 been variously explained as " court- 

 ship colours " for attracting the 

 female ; as due simply to the ex- 

 uberant vitality of the male Bird, or 

 as helping to keep the number of 

 males within proper limits by render- 

 ing them conspicuous to their 

 enemies. Such ornaments as the 

 bars and spots on the wings and 



Man. Zool. 



FIG. 275. Casuarius (Casso- 

 wary). Feather, showing 

 after-shaft and disconnected 

 barbs. (From Headley.) 



I I 



