CHAPTER XXV 



CLASS AVES BIRDS 



I. Sub-class ARCH^ORNITHES (or Saururae) ; extinct Archceopteryx. 

 II. Sub-class NEORNITHES. 



1. Division Ratitae. " Running Birds." Ostrich, etc. 



2. Division Odontolcse. Extinct Hesperornis. 



3. Division Carinatae. " Flying Birds " with keeled sternum. 



BIRDS share with Mammals the rank of the highest Verte- 

 brates. For although Mammals excel in brain development, 

 and in the closer organic connection between mother and 

 unborn young, it must be allowed that as regards muscles 

 and skeleton, heart and lungs, indeed most of their struc- 

 ture, the two classes are almost equally differentiated. They 

 are not, however, in any way nearly related, but represent 

 quite divergent lines of evolution. They are related to one 

 another indirectly, since they have in all probability a 

 common ancestry among Reptiles. 



Like Insects among Invertebrates, so Birds among 

 Vertebrates are pre-eminently creatures of the air, and the 

 analogies between these two widely separated classes are 

 many, e.g. as regards power of flight, elaborate respiratory 

 system, bright colouring, sexual dimorphism, preferential 

 mating, and parental instincts. The high body temperature 

 of Birds, exceeding that of all other animals, is a physio- 

 logical index of their rapid metabolism or intense activity. 



Compared with lower Vertebrates, Birds show a marked 

 increase of emotional life, as seen in their affection for tneir 

 mates, in their care of the young, and in the joyousness of 

 their mood, often bursting forth in song. 



