DEFINITION OF BIRDS. 61 



converge in geologic history so closely, that the presence of feathers in the avian class, and 

 tlieir absence from the reptilian, is one of tlie most positive differences. The oldest known 

 birds are from the Jurassic rocks of Europe and North America. These birds had teeth, and 

 various other strong peculiarities of structure, which no living members of the class have 

 retained. 



AVES, or the Class of Birds, may be distinguished from other Sauropsida by the 

 following sum of characters : The body is covered with feathers, a kind of skin-outgrowth 

 no other animals possess. The blood is hot; circulation is completely double; the heart is 

 perfectly four-chambered ; there is but one (the right) aortic arch, and only one pulmonary 

 artery springs from tlie heart ; the aortic and the pulmonary artery have each three semilunar 

 valves. The lungs are fixed and moulded to the cavity of the thorax, and some of the air- 

 passages run through them to admit air to other parts of the body, as under the skin and in 

 various bones. Reproduction is oviparous ; the eggs are very large, in consequence of the 

 copious yolk and white ; have a hard chalky shell, and are hatched outside the body of the 

 parent. There are always four limbs, of which the fore or pectoral pair are strongly distin- 

 guished frt)in the hind or pelvic pair by being modified into tvings, fitted for flying, if at all, 

 by means of feathers — not of skin as in the cases of such mammals, reptiles, and fishes as 

 can fly. The terminal part of the limb is compressed and reduced, bearing never more than 

 three digits,* only two of which ever have claws, and no claws being the rule. There are 

 not more than two separate carpals, or wrist-bones, in adult recent birds (with very rare 

 exceptions) ; nor any distinct interclavicular bone. The clavicles are complete (with rare ex- 

 ceptions), and coalesce to form a " wish-bone" or "merry-thought." The sternum, or breast- 

 bone, is large, usually carinate, or keeled, and the ribs are attached to its sides only ; it is 

 devek)ped from two to five or more centres of ossification. The sacral vertebrae proper have 

 no expanded ribs abutting against the ilia ; the ilia, or haunch-bones, are greatly prolonged 

 fi)rward; the socket for the head of the femur, or thigh-bone, is a ring, not a cup; the ischia 

 and puhes are prolonged backward in parallel directions, and neither of these bones ever unites 

 with its fellow in a ventral symphysis (except in Struthio and Bhea). The fibula, or outer 

 bone of the leg, is incomplete below, taking no part in the ankle-joint. The astragalus, or 

 upper bone of the tarsus, unites with the tibia, or inner bone of the leg, leaving the ankle- 

 joint between itself and other tarsal bones, the lower of which latter similarly unites with the 

 bones of the instep, or metatarsus. There are never more than four metatarsal bones, and the 

 same number of digits ; the first or inner metatarsal bone is usually free, and incomplete above ; 

 the other three anchylose (fuse) together, and with distal tarsal bones, as already said, to form 

 a compound tarso-metatarsus. Recent birds, at any rate, have a certain saddle-shape of the 

 ends of the bodies of some vertebrae. Such birds have also no teeth and no fleshy lips; the 

 jaws are covered with horny or leathery integument, as the feet are also, when not feathered. 



The Position of the Class Aves among other Vertebrates is definite. Birds come in 

 the scale of development next below the Class Mammalia, and no close links between Birds 

 and Mammals are known ; the most bird-like known mammal, tlie duck-billed platypus of 

 Australia (Ornithorhynchus paradoxus), being several steps beyond any known bird. Birds 

 are tiie higher one of the two classes of Sauropsida — the lower class, Reptilia, connecting with 

 tlie Batrachians (frogs, toads, newts, etc.) and so with the Fishes, Ichthyopsida. In this Ver- 

 tebrate series. Birds constitute what is called a hifjJily specialized group ; that is to say, a very 

 particular off-shoot, or, more literally, a side-issue, of the Vertebrate genealogical tree, wliich 

 in the present geological era has become developed into very numerous (about 11,500) species, 

 closely agreeing witli one another in the sum of their physical characters. In comparison with 

 other classes of Vertebrates, all birds are mucli alike ; there is a less degree of difference among 



