124 



ANIMAL BIOLOGY 



volved in flight. Sustained exercise in the air necessitates an 

 exceptionally efficient heart to rush supplies to the various parts 

 of the body, and cooperating lungs that communicate with a 

 system of air spaces among the viscera and in the hollow bones. 

 Withal, the plumage not only provides an efficient heat-retaining 

 coat but the feathers of wings and tail also are well adapted for 

 propulsion and steering. Minimum weight with maximum strength 

 characterizes these living heavier- 

 than-air flying machines. 



Birds usually are arranged in 

 two very unequal divisions, the 

 Ratitae and Carinatae. The first 

 includes a few species without a 

 keel-like breast-bone to support 

 strong wing muscles, as illus- 

 trated by the flightless Apteryx 

 and Ostriches; and the second divi- 



Fig. 84. — A, Kiwi, Apteryx australis; B, Ostrich, Struthio camelus. 



(From Newman, after Evans.) 



sion comprises those with the ' keel ' which is all the rest of the 

 bird population — about twenty thousand species. These are dif- 

 ferentiated by relatively minor anatomical variations, particularly 

 in regard to wings, feet, and horny beaks ensheathing tooth- 

 less jaws, in adaptation to various habitats and ways of life. 

 (Figs. 84-86.) 



A consideration of the classification of the Carinatae would 

 carry us too far into details, but ornithology is of very high 

 interest and greatest economic value. Birds contribute in large 

 measure, both directly and indirectly, to the human food 

 supply: directly as domestic and game birds, and indirectly be- 

 cause they make successful agriculture possible by eating almost 



