SKELETON 5 



The additional or ypriiig moult affects the smaller feathers 

 only, while it is still doubttul how far changes of colour are due 

 to a mere dropping off of the fringe of bai'bicels. The decorative 

 plumes of the males of many species are gained at the vernal 

 moult. The double process is certainly not diagnostic of Families 

 or even (renera, except in isolated cases ; as an instance, however, 

 the Larks have one moult, the Pipits and Wagtails two. 



In such cases as Swallows and Diurnal Birds of Prey generally, 

 the plumage is not changed till after the migration ; in the 

 Ptarmigan there is a triple moult, the breeding-suit being 

 changed first to a greyish habit and then to a white ; in Penguins 

 the feathers of the wing come off in apparent flakes.^ 



Skeleton, Digestive Organs, etc. — The plumage, however, 

 though often striking, and of undoul)ted utility as a non-conductor 

 of heat and a protection against wet, plays a subordinate part in 

 determining the relationships of the larger groups of Birds. For 

 this we need the assistance of anatomy, if indeed we do not rely 

 upon it almost entirely. It will be well before starting to state 

 that structures which are morphologically similar, that is, which 

 have a like origin in the embryo, are termed "homologous," while 

 those which perform the same physiological functions are " analo- 

 gous," the word in its strictest sense implying initial diversity. 



Any standard work on Vertebrate Anatomy ought to furnish 

 a concise account of the bony framework or Skeleton of a Bird, 

 but it will be convenient here to follow mainly the treatment of 

 Dr. Gadow, in Prof Newton's Dictionary of Birds, pp. 848-867. 



According to this authority the Axial Skeleton consists of 

 the Skull and Vertebral Column ; the Appendicular Skeleton of 

 the Eibs, the Sternum, the Limbs and their Arches, the Hyoid 

 Apparatus or framework of the tongue, and the Jaws. 



1. The Vertebral Colunm, which protects the Spinal Cord, is 

 composed of a variable number of cervical, dorsal, sacral or pelvic, 

 and caudal vertebrae ; that is, those of the neck, back, loins and 

 tail respectively. The first cervical vertebra, which bears tiie 

 head, articulating with it by a single condyle, is called the Atlas ; 

 the second, on which it turns, the Axis ; the succeeding cervicals 



' In certain of the Tctraonidac the claws are shed in spring; in some Alcidae 

 (Auks) the horny bill-sheath and the outgrowths over the eyes are lost after the 

 breeding season ; the American White Pelican moults a horny projection on the 

 cuhnen after nesting, while the beak of Redpolls is much elongated in summer. 



