282 



VERTEBRATE ZOOLOGY 



homy sheath of the bill in several separate pieces; large penis; tail 

 functionless; no oil gland on the tail. 



If feathers, wings, air-sacs, hollow bones, high temperature, etc., 

 are, as we have maintained, adaptations for flight, we have no alter- 



FiG. 1.53. — The most important forms of Ijirds' lieaks. a, ffamiiigo, Phaenicop- 

 teriis: b, spoonbill, Plalalc.a; c, yellow bunting, Emheriza: d, thrush, Turdiis; e, 

 falcon, Fnlcn; f, duck, Mergu-a: g, pelican, Pelicanus; h, avocet, Recun'iiostra; i, 

 black skimmer, Rhynchops; k, pigeon, Colmnba; I, shoebill, Bala-iiiceps: in, stork, 

 Anastomus; n, aracari, Pteroglossus; o, stork, Mi/ctcrta; p, tiird of paradise, Fal- 

 cinellus; q, swift, Cypselus. (From Hegner, after Sedgwick's Zoology; a, b, c, d, 

 k, after Naumann; g, i, m, o, after regne animal; 1, after Brehm.) 



native but to conclude that these and other flightless birds have been 

 derived by reverse adaptation from ancestors that were able to fly. 

 Possibly, however, the Ratitpe represent several independent off- 

 shoots from a primitive avian stock, in which the powers of flight had 

 not fully developed, and in which wings ceased to evolve when the 



