DEFINITION OF BIRDS. 61 



are any living members of the class of Birds from any known Reptiles, the characters of the 

 two groups converge in geologic history so closely, that the presence of feathers in the fonner 

 class, and their absence from the latter, is one of the most positive differences we have found. 

 The oldest known birds are from the Jurassic rocks of Europe, and the Cretaceous beds of 

 Xorth America. These birds had teeth, and various other strong peculiarities of structure, 

 wliich no living members of the class have retained. 



AVES, or the Class of Birds, may be distinguished from other Sauropsida, for all that 

 is known to the contrary, 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 ; the circu- 

 lation is completely double ; the heart is perfectly four-chambered ; there is but one (the right) 

 aortic arch, and only one pulmonary artery springs from the heart ; the aortic and the pulmo- 

 nary artery have each three semilunar valves. The limgs are fixed and moulded to the cavity 

 of the chest, and some of the air-passages run through them to admit air to other parts of the 

 body, as itnder 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 distinguished from the hind or pelvic pair by being modified into wings, 

 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 tei-minal part of the limb is compressed and reduced, 

 bearing never more than three digits, only two of wliich ever have claws, and no claw.« 

 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 exceptions), 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 developed from two to five or more centres of ossification. The sacral ver- 

 tebrae proper have no expanded ribs abutting against the ilia ; the ilia, or haunch-bones, are 

 greatly prolonged forward ; the socket for the head of the femur, ov 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 vnXh. its fellow in a ventral symphysis (except in Struthio and Rhea). 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 shnilarly unites 

 with the bones of the instep, or irietatarsus. There are never more than four metatarsal bones, 

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

 plete 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, the duck-billed platypus of 

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

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

 the Batrachians (fro^, toads, newts, etc.) and so with the Fishes, Ichthyopsida. In this Verte- 

 brate series. Birds constitute what is called a highly specialized group ; that is to say, a very par- 

 ticular off-shoot, or, more literally, a side-issue, of the Vertebrate genealogical tree, which in 

 the present geological era has become developed into very numerous (about 10,000) species, 



