CHIEF CHARACTERISTICS 7 



importance that in the flight of the bird the actual move- 

 ments help respiration, abetting the expiration of air from 

 the lungs. It cannot be a coincidence that in insects, which 

 are the birds of the Invertebrate sub-kingdom, expiration 

 is also the active part of the breathing process, and that 

 the flying should help in this. A quickly running mammal 

 is apt to get " out of breath " ; a quickly flying bird is 

 helped to breathe by the movements of locomotion. 



To this must be added that the lungs, though relatively 

 small, have a complicated internal structure with a large 

 absorptive surface, and that their associated air-sacs allow 

 of a ** double tide " in respiration. When the lungs are 

 compressed the vitiated air passes out by the wind-pipe, 

 but unvitiated air comes in from the reservoirs or air-sacs. 

 In the passive inspiratory process when the lungs regain 

 their normal size, there is likewise a refilling of the air- 

 reservoirs. 



§ 7. Mentality of Birds 



One of the most marked differences between birds and 

 reptiles is in the size of the brain in proportion to the size 

 of the head and the body generally. The crocodile's 

 brain is almost ridiculously small compared with the huge 

 head ; the bird's brain fills a relatively large cranial cavity, 

 and there is a strong development of the cerebrum and the 

 cerebellum. Can any light be thrown on this salient feature, 

 which is consonant with the general improvement of the 

 central nervous system as we ascend the Vertebrate series .'' 



There are several reasons why variations in the direction 

 of enlarged and complexified brain would pay particularly 

 well in birds. The majority are small creatures, of delicate 

 build, with little in the way of weapons or armour, and 

 therefore more dependent than reptiles on their brains or 

 wits. Moreover, the terrestrial life which amphibians 

 practically began (among Vertebrates), which reptiles made 

 more secure, has its own peculiar risks. The terrestrial 

 animal is limited to one plane of locomotion, unless it 

 becomes a burrower, or arboreal, or a flier. It is therefore 



