THE REPTILES AND THE BIRDS I23 



usually considered as representing different genera, Ar- 

 chaeopteryx (archaios = ancient; pteron = wing) , and 

 Archaeornis (archaios = ancient; ornis = bird). Ar- 

 chaeopteryx has been called the ideal fossil because it so 

 perfectly connects the birds with their reptilian ancestor. 

 Whether one calls Archaeopteryx a reptile or bird makes 

 little difference. It possessed a long reptilian tail com- 

 posed of twenty hzard-hke vertebrae with feathers ar- 

 ranged in pairs along the axis rather than f anwise ( as in 

 the modem birds); the wings' ended in 'hands' with a 

 thumb and three separate metacarpal bones each carry- 

 ing a free finger and each ending in a reptilian claw; 

 and the upper and lower jaws were Hned with reptilian 

 teeth set in individual sockets. The hind legs were, how- 

 ever, avian, and the clawed feet were adapted to arbo- 

 real life. That Archaeopteryx could at least ghde from 

 tree to tree is beyond question, and paleontologists call 

 it a bird and give it a subclass all to itself, partly because 

 of its anatomical affiliations and partly because of its 

 feathers. By this taxonomic identification they imply that 

 it was warm-blooded. If this supposition is correct, it 

 places the beginning of the evolution of warm-blooded- 

 ness in the avian stem in the Jurassic, possibly even in 

 the Triassic period. But despite the fact that they are 

 warm-blooded, the birds are typically reptihan, espe- 

 cially in matters concerning water balance. 



It was one thing for the Pennsylvanian-Permian amni- 

 otes to devise an egg with its own internal environment 

 to sustain the biochemically helpless embryo; it was an- 

 other for the adult to solve the problem of Hving under 

 arid conditions on a minimal quantity of water. The 

 water-permeable skin of the Amphibia had to be re- 

 placed with an impermeable hide: the skins of the early 

 reptiles are poorly preserved in the fossil record but 

 doubtless most of them were variously covered from 

 snout to tail with small scales, scutes, or plates derived 

 from the thickened, comified epidermis in a manner 



