654 



BIRDS. 



substance of the egg, and acts as a receptacle for the embryo's 

 waste products. 



THE PIGEON (Columba) AS A TYPE OF BIRDS 



The numerous varieties of domesticated pigeon (pouter, 

 fantail, tumbler, etc.) are all descended 

 from the rock-dove, Columba livia, and 

 afford vivid illustrations of variation, and 

 of the results of artificial selection. 

 Certain variations, e.g. in beak or tail, 

 crop up, we know not how ; and similar 

 forms are bred together until a new breed 

 is established. 



External characters. The form of 

 the body, well suited for rapid flight, 

 ceases to be graceful when stripped of 

 its feathers. The cere above the nos- 

 trils, the third eyelid in the anterior 

 upper corner of the orbit, the external 

 opening of the ear concealed by the 

 feathers, the preen gland on the dorsal 

 surface at the root of the tail, and the 

 cloacal aperture, are external features 

 easily recognised. 



The feathers most important in flight 

 are the twenty -three remiges of the 

 wing, divided into eleven primaries 

 borne by the metacarpals and phalanges 

 of the two fingers, and twelve secondaries 

 borne by the ulna. Twelve tail feathers 

 very or rectrices serve as a brake, and help 

 young feather within a little in steering. A distinct tuft of 



its sheath (sh.) ; c., the ,. . , . , .1 ,1 i n j 



core of dermis ; *., the feathers borne by the thumb is called 



external view the S&m *' tne Bastard wing. Covering the bases 



of the large feathers are the coverts, 



wing-coverts and tail- coverts, which belong to the 



series of contour feathers which give shape to the whole 



body. In the pigeon there are no true down-feathers 01 



plumules, but among the ordinary contour feathers or pennae 



there are little hair-like feathers (filoplumes) with only a few 



FIG. 359. After 



Nitzsch. 



