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 di- 

 gestion, 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 Australasian 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 



