504 Comparative Morphology of Chordates 



little specialized. The salamander is less so than a toad, the tuatara 

 less so than a snake. But it is probably fair to say that birds, taking 

 the Class as a whole, possess a higher average degree of specialization 

 than any other vertebrate Class. If this be admitted, then birds — not 

 mammals — are the "highest" vertebrates. Factors other than ana- 

 tomic ones may justify placing mammals at the apex of the vertebrate 

 series — and it must not be overlooked that there may be some class 

 prejudice on the part of the classifier, who acknowledges himself to be 

 not only a vertebrate but a mammal. 



The various organs of an animal are dependent upon one another 

 to such an extent that specialization of one organ is necessarily ac- 

 companied by appropriate modification of the organs related to it. 

 In birds specialization for flight involves nearly every part of the 

 animal. The immediate agency which keeps a bird aloft is impact of 

 wings against air. The parts of the wing which directly encounter the 

 air are feathers. Arrangement of the essential flight-feathers in a 

 linear series calls for great modification of a pentadactyl pectoral limb. 

 Walking, running, and the vigorous leap required in "taking ofT" for 

 flight devolve upon the pelvic limbs, which are, accordingly, highly 

 modified. The powerful muscles of wings and legs create extraordinary 

 strains. These must be sustained by the skeleton of the trunk, which 

 is in various ways so specialized as to give it great rigidity. Loss of 

 flexibility in the trunk is attended by reduction and modification of 

 muscles. The great output of energy in the excessively vigorous and 

 long-sustained activities of flight demands a generous supply of oxygen. 

 This is provided by lungs, whose efficiency is highly increased by an 

 elaborate system of air reservoirs so connected as to give the lungs a 

 "through draft." In reptiles the aerated blood from the lungs, in pass- 

 ing through the heart, is contaminated by "impure" blood, but in 

 birds the heart and arteries are arranged so that the highly oxygenated 

 blood from the lungs passes undiluted into the general circulation. 

 Sustained flight requires rapid liberation of energy, which necessitates 

 rapid metabolism in the muscles. In general and within limits, metabo- 

 lism is accelerated by rise of temperature. Heat is a by-product of 

 metabolism, whose final phase is oxidation. Conservation of this heat, 

 making the bird "warm-blooded," facilitates the maintenance of a 

 high metabolic rate. The most important factor in the conservation of 

 heat is prevention of loss from the surface of the body. The plumage is 

 a highly effective heat insulator. 



It is to be noted that the series of successively related structures 

 mentioned in the preceding paragraph begins with feathers as the 

 immediate mechanical agency in flight and ends with feathers as con- 

 servators of energy, facilitating the rapid metabolism necessary for 



