378 ZOOLOGY sect. 



ventrally, an<l a scapula '(^v.^ which resembles tliat of tlic Rliynchocephalia : 

 a clavicle is ne\'er pi-esent. In the pelvis the ilium, wliich usually does not 

 articulate witli the spinal column, is a rocl-shape<l bone : the ischiuni and pubis 

 resemble tliose of the Lizards. The bones of botli fore- and hind-limljs are 

 short ; there are live digits in each. The teeth are conical, ])ointcd, and 

 ankylosed by expanded bases to the summits of tlie maxilhe and pterygoids. 

 Dermal scutes have been ol)served in one genus. 



The remains of Pythonomor])ha have l)een found only in certain beds belong- 

 ing to the Cretaceous period in l^jurope, North America, and New Zealand. 



CLASS v.— AVES. 



In many respects Birds are the most liighly specialised of 

 Craniata. As a class they are adapted for aerial life ; and almost 

 every part of their organisation is modified in accordance with 

 the unusual enviroment. The non-conducting covering of 

 feathers ; the modification of the fore-limbs as wings, of the 

 sternum and shoulder-girdle to serve as origins of the great 

 Aving-muscles, and of the pelvic girdle and hind-limbs to enable 

 them to support the entire weight of the body on the surface of the 

 ground ; the perfection of the respiratory system, producing a 

 higher temperature than in any other animals : all these peculiari- 

 ties are of the nature of adaptations to flight. Add to them the 

 absence, in all existing Birds, of teeth, the loss of the left aortic 

 arch, and of the right ovary and oviduct, the specialised character 

 of the brain, the poorly developed olfactory organs, and the extra- 

 ordinarily large and perfect eyes, and we ha,ve a series of strongly- 

 marked characteristics such as distinguish hardly any other class. 

 Moreover, the organisation of existing Birds is, in its essential 

 features, singularly uniform, the entire class presenting less 

 diversity of structure than many single orders of Fishes, 

 Amphibians, and Reptiles. 



1. Example of the Class. — The Common Pigeon (Colnmha 



liv'ia, var. domestica). 



The Common or Domestic Pigeon is known under many varieties, 

 which differ from one another in size, proportions, coloration, 

 details in the arrangements of the feathers, and in many points of 

 internal anatomy. The Pouters, Carriers, Fantails, and Tumblers 

 may be mentioned as illustrating extreme forms. All these 

 varieties have, however, been produced by artificial selection, 

 that is, by breeders selecting, generation after generation, the 

 Birds which most nearly attained to some artificial standard of 

 perfection, breeding from them alone, and killing off the inferior 

 strains. The ancestral species from which the domestic breeds 

 have in this way been evolved, is the Rock Pigeon (Cohimha livia), 



