The Fauna of British India, including Ceylon and Burma/Birds/Volume 1/Aves

by Edward Charles Stuart Baker

Birds are distinguished from all other vertebrates by their covering of feathers. Though related to the Reptiles, they differ in being warm-blooded—a feature which is correlated with a four-chambered heart, in which the chambers are completely separated, thus preventing the intermixture of arterial and venous blood which obtains among the lower vertebrates. Of the right and left aortic arches present in the Reptiles, only the right persists in Birds and the left in Mammals. The skull, which presents no sutures in the adult, possesses but a single occipital condyle and the jaws are produced into a beak ensheathed in horn, whilst in more primitive, extinct species, they were armed with teeth. The lower jaw is a complex of several bones, but the right and left rami are never separable as in Reptiles and many Mammals. Proximally the mandible articulates with the skull, after the reptilian fashion, by means of a quadrate bone. The fore-limb has become transformed into a "wing," and the sternum, in accordance with the requirements of flight, has taken on the form of a broad, oblong plate, usually provided with a median keel for the attachment of the pectoral muscles, which have become excessively developed. In the hip-girdle the three elements of the pelvis have become fused. The ilium has become greatly elongated, and is closely applied to the vertebral column, preventing all movement between the vertebræ within its grip. As a consequence, these vertebræ, which include more or fewer of the lumbar, the sacral and a variable number of post-sacrals, have become welded together to form a synsacrum. In the hind-limb the proximal row of tarsa's have become fused with the shaft of the tibia to form a "tibio-tarsus," while the distal row have fused with the metatarsals to form a tarso-metatarsus. On this account the ankle-joint is "intertarsal" as in many reptiles. Three of the four surviving metatarsals have fused to form a solid, cylindrical shaft or "cannon-bone" as in Dinosaurs, while the fourth has become reduced to a mere nodule of bone supporting the hallux. In many species the hallux has become reduced to a mere vestige, and, in some, it has disappeared altogether, whilst in the Ostrich (Struthio) but two toes remain. With the reptiles on the one hand, and the primitive mammals Echidna and Ornithorhynchus on the other, birds agree in being oviparous.

Hitherto most systems of classification have been founded on living birds only, and have therefore to some extent failed in their purpose. Birds have been commonly divided into two great groups or sub-classes, Ratitæ and Carinatæ, according to the presence or absence of a median keel to the sternum. But these groups, though accepted by Blanford and Oates, are very unsatisfactory, since in some flightless Carinatæ the sternum has become reduced to the Ratite condition.

Taking into consideration birds both living and extinct, we have two well-defined sub-classes, the Archæornithes and Neornithes.

Under view of the skull of a Raven. vo, vomer; mxp, maxillo-palatine process; pa, palatine; ptg, pterygoid; q, quadrate; b.sph, basi-sphenoid; sph.r, sphenoidal rostrum.

The first is reserved for the Archæopteryx with an elongated reptilian chain of caudal vertebræ, each bearing a pair of rectrices and having the jaws armed with teeth. The sub-class Neornithes ncludes all living birds in which the vertebræ supporting the rectrices have become so abbreviated that the tail feathers have to be arranged fan-wise on either side of a fused mass of bones known as the "pygostyle."

As regards the Neornithes, the palate affords a much more satisfactory basis of division than the sternum. According to this, living birds are divisible into two further groups, the Palæognathæ and Neognathæ, the former in substitution for the Ratitæ and the latter for the Carinatæ.

In the Palæognathæ the vomer is large, and articulates by squamous suture with the pterygoid, while the palatine is applied to the outer margin of the vomero-pterygoid articulation.

In the Neognathæ the palatines have shifted inwards, under the vomero-pterygoid articulation, to meet one another in the median line. The pterygoids, in early post-embryonic life, undergo a striking process of segmentation, inasmuch as that portion of their shafts which rests upon the proximal end of the palatine snaps off, as it were from the main shaft, and fuses with the palatine. Later, at the point of fracture a cup-and-ball joint is formed, affording the strongest possible contrast with the squamous suture found in the Palæognathæ.

Where the vomer still retains some semblance of its former size, its proximal bifurcated end may just reach the extreme tip of the anterior end of the pterygoid, but it now depends for its support not upon the pterygoid, but upon the palatine, as, for example, in the Penguins. But among the Neognathæ the vomer displays a striking series of stages in degeneration, becoming more and more divorced from the pterygoid, until it finally assumes the form of a minute nodule of bone, and at last, in the Gallinæ, it becomes a mere spicule of bone held by a few tendinous fibres to the anterior border of the expanded ends of the palatines, and in some, as in the Falconidæ for example, vanishes altogether. If nothing were known of the early post-embryonic developmental stages of the Neognathine vomer, it would have been impossible to divine that the Neognathine was a direct derivative from the Palæognathine palate.

These two orders, the Palæognathæ and Neognathæ, must be divided further, for the Class Aves, in the course of its evolution, has split up into a vast number of different forms. The genetic relation of these forms or types to one another, and the precise affinities of the individual members of the various groups, should as far as possible find expression in any system of classification. These divisions may be known as Orders, which are again divided into Sub-Orders, Families, Genera and Species.