THE MASTERY OF THE AIR 243 



scales were replaced by feathers, no one knows 

 how, the primitive birds probably became 

 arboreal, and served a long apprenticeship as 

 parachutists, launching themselves from tree 

 to tree, until at last they learned to soar aloft. 



It is all uncertain, but it is not unreasonable 

 to suppose that before birds became true fliers, 

 they were swift runners of spare build, with 

 light bones, a strong heart, very rich blood, 

 a hot skin, a power of keeping up an almost 

 constant body-temperature, a very good 

 digestion, a fine brain, and the further great 

 advantage that the flapping of the wings, even 

 before true flight was fully attained, helped the 

 breathing. A bird's body is a bundle of 

 fitnesses, well suited for flight, but it is in- 

 teresting to inquire whether the excellent 

 qualities of birds may not have been acquired 

 before they became fliers. But it is difficult to 

 do more than inquire ; we cannot roll back the 

 ages and see. We are not even sure whether 

 the Running Birds of to-day (the African 

 Ostrich, the South American Rhea, the Austra- 

 lasian Emu and Cassowary, and the Kiwi of 

 New Zealand) are the descendants of rather 

 primitive birds which never attained to flight, 

 or of flying birds which have lost their flying 



