THE PIGEON AS A TYPE OF BIRDS. 597 



The eggs have much yolk and hard calcareous shells. The 

 segmentation is meroblastic and discoidal. The allantois is 

 chiefly respiratory, though it helps in absorbing the nutritive 

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

 ivaste 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. 

 The diet of seeds, the wooing of mates, the feeding of 

 the young by both parents, are well known. 



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 nostrils, 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 

 or rectrices serve as a brake, and help a little in steering. 

 A distinct tuft of feathers borne by the thumb is called the 

 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 or 

 plumules, but among the ordinary contour feathers or penna? 

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

 terminal barbs. In herons and some other birds some of 

 the down-feathers are covered with dusty powder (powder- 

 down), formed from the brittle ends of the barbs. Apart 

 from their use in flight, the feathers, being bad conductors 

 of heat, serve to sustain the high temperature of the bird. 

 There is usually pigment in feathers, and the coloration 



