CHAPTER XXIV. 



BIRDS. 



Class AvES. 



Birds share with Mammals the rank of the highest Verte- 

 brates. Their muscles and skeleton, heart and lungs — 

 indeed, most of their structural arrangements — are not less 

 differentiated than those of Mammals. Their body tempera- 

 ture, exceeding that of all other animals, is an index to 

 their intense activity. It is only because we recognise in 

 Mammals a higher degree of brain development, and a close 

 organic connection between mother and offspring, that we 

 venture to place them above Birds. It is more important 

 to recognise that these two classes represent markedly 

 divergent lines of progress. Before we discuss the structure 

 of Birds, let us consider some of the characteristics of their 

 life. 



The Life of Birds. 



Life having begun in the waters, slowly gained possession of 

 the dry land and then of the air. Insects among the lower 

 animals, and Birds among the higher, are pre-eminently the 

 creatures of the air ; intensely vivacious, typically beautiful 

 in form and colour, dainty and delightful in their ways. 



In the Birds we observe a marked increase of emotional 

 life, so that their affection for their mates, their care of their 

 young, the joyousness of their life, often bursting forth in 

 song, have become proverbial among us. With their power 

 of flight they are emblems of freedom. 



Flight of Birds. — Most birds use their wings in flight. 

 The feather-covered arms and fingers, the very large 

 breast muscles which raise and depress the wings, the breast- 



