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 
