G ANATOMY OF VERTEBRATES. 



The modification of the tegiimentary covering characteristic of 

 the present class is to be regarded rather as dependent upon, 

 than occasioning, this high degree of internal temperature, which 

 requires for its due maintenance against the agency of external 

 cold an adequate protection of the surface of the body by means 

 of non-conducting down and imbricated feathers ; and this warm 

 clothing is more especially required to meet the sudden variations 

 of temperature to which the bird is exposed, when soaring in the 

 liigher regions of air and stooping to the earth, during rapid and 

 extensive flights. 



The generative product is excluded from the oviduct in an 

 undeveloped state, inclosed, in a liquid form, within a calcareous 

 case or shell. Collision of tw^o brittle coco's in transitu is obviated 

 by the female organs being developed only on the left side of the 

 body. The ovum is subsequently perfected by means of incuba- 

 tion^ for which action the bird is especially adapted by its high 

 degree of animal heat. 



Birds form the best characterised, most distinct, and natural 

 class in the whole animal kingdom, perhaps even in organic 

 nature. They present a constancy in their mode of generation 

 and in their tegumentary covering, which is not met with in any 

 other of the vertebrate classes. Xo species of Bird ever deviates, 

 like the whales among Mammals, the serpents among Reptiles, 

 and the eels among Fishes, from the tetrapodous type charac- 

 terisino; the vertebrate division of animals. 



The anterior extremities are constructed according to that plan 

 Avhich best adapts them for the actions of flight ; and although, in 

 some few instances, the developement of the wings proceeds not 

 so far as to enable them to act upon the surrounding atmosphere 

 with sufficient power to overcome the counteracting force of 

 gravity ; yet, in these cases they assist, by analogous motions, 

 the posterior extremities : either, as in the ostrich, by beating 

 the air while the body is carried swiftly forward by the action of 

 the powerful legs ; or, as in the penguin, by striking the water 

 after the manner of fins, and by the resistance of the denser 

 medium carrying the body through the water in a manner analo- 

 gous to that by Avhich the birds of flight are borne through the 

 air. In a few exceptions, as the cassowary and apteryx, the wings 

 are outwardly represented by a few quills or a small claw In no 

 instance do the anterior extremities take any share in stationary 

 support or in prehension. 



Birds are therefore biped, and the operations of taking the 

 food, cleansing the plumage, &c., are almost exclusively performed 



