XIII 



PHYLUM CHORDATA 



411 



ficial layer of the feathers in its passage to the eye : no blue or 

 violet pigments occur in feathers, and green pigments are very 

 rare. The beautiful metallic tints of many birds are entirely the 

 result of structure, owing their 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 helping to conceal the Bird among 

 herbage, the latter on snow. Frequently, as 

 in Pheasants and Birds of Paradise, the female 

 alone is protectively coloured, while the male 

 presents the most varied and brilliant tints, 

 enhanced by crests, plumes or tufts of feathers 

 on the wings, elongated tail, &c., &c. These 

 have been variously explained as " courtship 

 colours " for attracting the female ; as due 

 simply to the exuberant vitality of the male 

 Bird ; or as helping to keep the number of 

 males within proper limits by rendering them 

 conspicuous to their enemies. Such ornaments 

 as the bars and spots on the wings and tail 

 of many gregarious birds, such as Plovers, 

 fully exposed only during flight, and often 

 widely different in closely allied species, have 

 been explained as " recognition marks," ser- 

 ving to enable stragglers to distinguish between 

 a flock of their own and of some other 

 species. 



Skeleton. The vast majority of Birds have 

 saddle-shaped or heteroccelous cervical and 

 thoracic vertebrae, but the thoracic vertebrae 

 are opisthoccelous in the Impennes (Penguins), 

 the Gaviae (Gulls), and the Limicolae (Plovers, 

 &c.), while in the Ichthyornithes alone they are 

 biconcave. The spaces between adjacent 

 centra are traversed by a meniscus with a 

 suspensory ligament, as in the Pigeon (p. 374). The number of 

 vertebrae is very variable, especially in the cervical region, where 

 it rises to twenty-five in the Swan and sinks to nine in some 



FIG. 1072. Feather of 

 Casuarius (Casso- 

 wary), showing after- 



