EXTERNAL FEATURES. Though a strictly scien tific arrangement might call for the more basal internal structure to be first considered, it suits present purposes better to give first an account of the exterior—the plumage and epidermal ap pendages so characteristic of birds.
Plumage.—Feathers are horny products of the epidermal cells of the skin, but very different from mammalian hairs or reptilian scales. Scales, in the herpetological sense, are pieces of the skin itself, originating as folds of the cuti cle; and hairs arise frmn involuted pockets in the skin: whereas feathers arc produced from papilla of the cuticle. They grow so as to form a coat over the whole body, called plumage. (For the structure and characteristics of a feather considered alone, see FEATHER; for the peculiarities of their growth in 'tracts.' see PrEan,osts; and for the periodical loss and renewal of plumage, see MOLTING.) The origin of plumage can only be surmised. It may well have been that chance variation which marked the earliest divergence from a reptilian stock toward the bird type. It has been conjectured that the primitive plumage was in the form of down, and the probability of this is shown in the fact that at present infant birds arc first clothed with down alone. In a large class of birds, mostly of low organization. this down is assumed before the embryo leaves the egg; such arc called Dasypedes, and as most of them are sufficiently advanced to begin at once to run about and pick up a living without parental help, they are said to be pre-cocial, and their parents classed as Pracoces. Among the higher birds, as a rule, however, the young are quite naked when hatched, and are termed Gymnopedes (or Psilo pfedes), and these birds are spoken of as al tricial (Altrices), because they must feed and care for the young.
Origin and Service of Plumage.—The first and essential service of the feathers is as a coat, pro tecting the body against cold and atmospheric changes. In the view of Prof. 0. C. Marsh, who did more than any one else to collect and reason upon the facts of the origin and development of birds, the acquirement and gradual increase of the feather coat was accompanied by a steadily augmented warmth of blood, and a proportional increase in its activity. The greater activity would result in a more perfect circulation, and this in a steady improvement of general bodily ability and power. If this view be correct, to the acquirement of plumage may be attributed the high degree of development attained in mod ern birds as a class. The second great service of plumage is its development, on wings and tail, into an instrument of flight. (For the • phe nomena of flying, see FLIGHT.) Wings and Their Functions.—Wings. ex ternally viewed, are formed of largo quill or flight feathers (remiges), which grow from the posterior margin of the arms and hands in a plane (when outstretched) horizontal with the long axis of the body; they overlie one another at the outer edge so as to slide much beneath one another when the wing is closed, form an aero plane when distended and quiet, and a serielk of oars when the bird moves. It is believed that in the development of the wings (Gadow, Proceed ings Zoological Society of London, 188S, page 665) and powers of flight in birds, those feathers nearest the body were the first to become service able; but now the most important remiges are those near the outer margin of the wing. These
form a set (usually ten, but in some groups nine, and in a few cases eleven), which spring from the bones of the hand and arc called primaries: they are long and comparatively narrow, and the shaft is near to the anterior margin. In side these. springing from the ulna, is a set of lesser and more equally proportioned quills called secondaries, or better, cubitals: these are vari able in numlx-r, the humming-bird having only six, while the albatross has forty. The fleshy parts of the wing are covered with rows of feathers called coverts, which gradually diminish into the general `scapular' plumage upon and overlying tile shoulder. Wing-feathers vary in length, strength, and shape, according to the mode of life and power of flight in different birds; and there is consequently great variety aMong birds in respect to the relative size, shape, and power of time wings themselves. The extreme of length, both absolutely and relatively, is found among the oceanic birds, whose lives may be said to be spent upon the wing. The albatross and the condor have wings two and one-half times the total length of the body, reaching an afar expanse of between I I and 1• feet in large specimens, and nearly the same proportions hold for other pelagic wanderers, as the tropic-birds; while in such birds as swifts and hummers the wing may largely exceed the body-length. Such wings belong to birds which range over vast spaces of ocean, or remain poised for long periods in the upper air, as do the vultures and diurnal hawks, or that feed upon flying insects, and so have need of extraordinary sustaining power, en durance, and ability for occasional extreme speed and agility. Such wings are always slender, and sabre-like in outline, when extended, the primaries being usually much longer than the secondaries, narrow and curved, but very strong and far overlapped; those of the diurnal birds of prey are peculiarly narrowed, or 'whit tled away' on the inner vane toward the tip. From this extreme, wings grade down in shape to the 'round' or 'short' type characterizing the terrestrial and arboreal birds generally, or those which seek their food in limited areas of quiet water. Here the wing may be considerably shorter than the bOdy, and its feathers broad, the primaries little, if at all, exceeding the sec ondaries in length—all broadly and evenly vaned, so that the breadth of the wing approaches its length. Such are adapted to quick starting and short, agile 'flitting' flights, rather than to pro longed sustention or swift darting; yet they do not lack power, for many short-winged birds make journeys of hundreds of miles without rest ing (see IV IGRATION ) and others are capable of extremely swift motion; but none of them can soar, as do sea-birds and birds of prey. nor equal these in volant grace. All these, moreover, trust for safety rather to hiding than to fleeing from danger.