1 SKELETON 5 
The additional or spring moult affects the smaller feathers 
only, while it is still doubtful how far changes of colour are due 
to a mere dropping off of the fringe of barbicels. 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 Genera, 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-swit being 
changed first to a greyish habit and then to a white; in Penguins 
the feathers of the wing come off in flakes.! 
Skeleton, Digestive Organs, etc.—The plumage, however, 
though often striking, and of undoubted 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 
& 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. 848-8677. 
According to this authority the Axial Skeleton consists of 
the Skull and Vertebral Column; the Appendicular Skeleton of 
the Ribs, the Sternum, the Limbs and their Arches, the Hyoid 
Apparatus or framework of the tongue, and the Jaws. 
1. The Vertebral Column, 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 the 
head, articulating with it by a single condyle, is called the Atlas ; 
the second, on which it turns, the Axis; the succeeding cervicals 
1 In certain of the Tetraonidae the claws are shed in spring ; in some A/eidae 
(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 
culmen after nesting, while the beak of Redpolls is much elongated in summer. 
