TILE BIRD'S PEACE IN NATURE. Birds are classified between the reptiles, regarded as in terior to them, and the mammals, regarded as superior in general organization. Birds differ from mammals, broadly. in being clothed with feathers instead of hairs, and in the absence of milk-gla nds. and by sundry differences in anat omy and methods of existence, such as the hatch ing externally of eggs, and the devotion of the fore limbs to flight. They differ from reptiles in having a covering of feathers instead of scales; a complete double circulation of warm blood; no more than three digits in the manus, long legs, etc.
Affinity with Reptiles.—The differences last noted are, however, of much less importance than those which separate them from mammals, and the structural resemblance is so close that some anatomists, notably Huxley, have included rep tiles and birds in a single group, the Sauropsida, comparable to Fishes or Mammals. and com pleting, with them, the three divisions of the Ve•tebrata. This grouping was founded upon the fact that birds and reptiles were alike in being oviparous or ovoviviparous; in having a cloaca; in the incompleteness of the diaphragm, and of a corpus callosum in time brain; in having only one occipital condyle; in the presence of a movable quadrate bone and other peculiarities of the skull: and in the fact that the ankle-joint is between two sets of tarsal bones. The close relationship thus implied has been confirmed by the disclosures of paleontology, which show that birds have a reptilian ancestry, and are an off shoot of the same stock as modern reptiles.
Development of the Class.—For details of the geological evidenre of the Origin and evolution of birds, the reader is referred to the articles Bum, Fos:gm; and EVOLUTION. It will suffice here to sum up the matter as Prof. A. Newton, following Fiirbringer, does in his Dictionary of Birds (London, 1893-96). Birds, since they spring from reptiles, must have begun with toothed forms of small or moderate size, with long tails and four lizard-like feet, having well fo•med claws, while their bodies were clothed with a very primitive sort of down. To them succeeded forms wherein down developed into feathers, and the fore and hind limbs differed in build—the former becoming organs of pre hension (as is still the case in some young births), and the latter the chief instruments of progression. Then followed a dinosaur-like transformation of the pelvis and legs, and a grad ual coalescence of the ankle-bones, enabling birds more and more to walk erect. These early rep
tilian birds were flightless and terrestrial, or at most climbed trees. "Among those which pos sessed this habit, the befeathering (which as yet had, like the hair of mammals, served only for warmth) presumably entered upon a higher step. the feathers becoming larger on 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 stiffness and size of the feathers, and corresponding modification and strengthening of the skeleton and muscles, the possibility of incipient but real flight was afforded." Thus far this explanation of primi tive bird-formation is theoretical; but at this point comes confirmatory evidence in the fossil remains of Archaeopteryx (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 bra becoming united as a pygostyle. Thus orig inated . . the higher or better 'Birds of Flight.' This type was established in the Cre taceous Iehthyornis, and includes the vast major ity of existing birds commonly grouped as Carinatfc (i.e. with a keeled breastbone — not Ratita) ; 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. "Many stopped, as it were, half way," to continue Newton's sum mary of Furbringer's conclusions, 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 TO lant power, and hence arose the different types , which are commonly grouped as Ratita (os triches, etc.)."