BIBD. 95 BIBB. waiMitli) presuinalily entered upon a higher step, the feathers becoininj; larf;er nn certain parts of the body, particularly on the fore limbs and tail, so as to begin to act as a parachute and allow of a safe gliding descent from a height. By suc- cessive increase in the stiirness and size of the feathers, and corresponding modilication and strengthening of the skeleton and muscles, the possibility of incipient but real flight was atTorded." Thus far this explanation of primi- tive bird-formation is theoretical; but at this point conies confirmatory evidence in the fossil remains of Archieopteryx (q.v. ) — an actual bird with real feathers and apparently considerable power of flight, that lived at the beginning of Secondary or Mesozoic time. "The faculty of flight, thus acquired, went on improving. The remiges grew stronger and stronger . . . and in proportion as the fore limbs specialized into highly developed wings . . . the tail short- ened and was consolidated, the posterior verte- brie becoming united as a pygostyle. Thus orig- inated . . .. the higher or l)etter 'Birds of Flight.' This type was established in the Cre- taceous Ichthyornis, and includes the vast major- ity of existing birds commonly grouped as Carinats {i.e. with a keeled breastbone — not Ratitae) ; but these only in later times developed their various higher modifications, which were rendered possible by the saving of material and weight." It is important, however, to bear in mind that all birds did not reach the highest degree of faculty in flight, "ilany stopped, as it were, half way," to continue Xewton's sum- mary of Fiirbringer's conclusions, "when a re- trogression of the power already attained took place: or, if the power were reached, it could not be maintained — an easy life and absence of rivalry inducing an increased bulk of the body, until the utmost exertion of muscular strength could no longer sustain it in the air. Thus when this retrograde movement began, occasion was afforded for the dwindling away of the vo- lant power, and hence arose the different types . which are commonly grouped as Eatitse (os- triches, etc.)." 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 homy 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 fnjm involuted pockets in the skin : whereas feathers are produced from pnpillre 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 Pteryi-OSIS; and for the periodical loss and lenewal of plumage, see Moi.Tlxo.) The origin of plumage can only he 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 are first clothed with down alone. In a large class of birdi, mostly of low organization, this down is assumed before the embryo leaves the egg; such are called Dasypa?des, 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 ;jre-cocial, and their i)arents classed as Priecoces. Among the higher birds, as a rule, however, the yoimg are quite naked wheo hatched, and are termed Gymnop.-edes (or Psilo- psedes), and these birds are spoken of as al- trieial (Altrices), because they must feed and care for the young. Origin and Hervice 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. O. 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 Ije attributed the high degree of development attained in mo<l- eru birds as a class. The second great service of plumage is it§ development, on wings and tail, into an instrument of fiight. (For the phe- nomena of flying, see Flight.) Vi'ings and Their Functions. — Ying3. ex- ternally viewed, are formed of large 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 series of oars when the bird moves. It is believed that in the development of the wings (Gadow, Proceed- ings Zoological Society of London, 1888, page 065) 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 are 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 the 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 the 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 alar expanse of between 1 1 and 12 feet in large
