CHAPTER XXV 
Crass AVES—BIRDS 
I, Sub-class ARCHAORNITHES (or Saururce); extinct Archaeopteryx. 
II. Sub-class NEORNITHES. 
1. Division Ratite. ‘* Running Birds.” Ostrich, etc. 
2. Division Odontolee. Extinct Hesperornis. 
3. Division Carinate. ‘Flying Birds” with keeled sternum. 
Birps 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, é.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 their 
mates, in their care of the young, and in the joyousness of 
their mood, often bursting forth in song, 
