10 THE RIDDLE OF MIGRATION 



Individual feathers borne by them (remiges) replace 

 the patagium of the bat. The entire arrangement 

 is not only more effective but more efficient. The 

 feathers of the tail (rectrices) give the bird yet a 

 further advantage. Both bats and birds are thus 

 warm-blooded creatures capable of sustained flight. 

 Birds are undoubtedly the more successful but the 

 two share many advantages in common. Bats, for 

 instance, even as birds, are found on oceanic islands 

 and in territory from which all other higher mam- 

 mals have been excluded simply because barriers, 

 otherwise effective — expanses of water, deserts, 

 mountains, etc. — mean little more than a risk to 

 animals that can traverse the air. But the spec- 

 tacular migrations characteristic of birds are un- 

 known to bats which are, without doubt, the weak 

 brothers of the vertebrate air force. Some species, 

 moreover, like some other mammals, overcome the 

 difficulties of winter by hibernation. 



In addition to the possession of wonderfully per- 

 fect wings, birds are yet further peculiarly adapted 

 to life in the air. There are no doubt limits, both 

 mechanical and hereditary, to size of wing and if 

 really long journeys are to be undertaken a maxi- 

 mum reduction of body weight would obviously be 

 a direct advantage. Such reduction has actually 

 taken place and in various ways. The shafts of the 



