{5}The additional or spring moult affects the smaller feathers only, while it is still doubtful how far changes of colour are clue 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-suit being changed first to a greyish habit and then to a white; in Penguins the feathers of the wing come off in flakes.[8] 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 "analogous," 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 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 {6}present a considerable number of processes or projections, which protect certain blood-vessels, and serve for the attachment of the muscles which turn the flexible neck. The dorsal vertebrae follow, and some not unfrequently coalesce with each other, but this is always so with the sacrals, and in nearly all existing Birds with the terminal portion of the caudals, which are fused together to form a "pygostyle" or upright triangular plate to carry the tail-feathers.[9] _Archaeopteryx_, so far as is known, stands alone in having all the caudal vertebrae free.