134 Biology in America 



applies to the ordinary or "contour" feather. The ''down" 

 and "liair" feathers differ in development, although all have 

 essentially the same structure. Hairs are also developed 

 fundamentally in the same way as the feathers, with a dermal 

 papilla, or core, at the base and a horny shaft, which however 

 is solid and not hollow as in the feather. 



Both of these structures, being derived from the horny 

 layer of the skin, are believed to be modifications of the horny 

 reptilian scale, which in its turn probably owes its origin to 

 the epidcniial layer or enamel of the placoid scale of the shark, 

 from which also has evolved the enamel of the mammalian 

 tooth. 



But to return to the original question of the origin of the 

 birds. The theory of the Plungarian palaeontologist, Nopsca, 

 supposes the bird to have arisen from a long-legged, long- 

 tailed, short-armed running reptile, which as it ran flopped 

 its arms to aid its motion, on somewhat the same principle 

 that a man uses his arms in a race. If some of the scales along 

 the posterior angle of the arm and along either side of the 

 tail were to enlarge, they might readily aid the forerunner 

 of the bird in its motion and by further enlargement and 

 modification give rise to feathers, and the arm become a wing, 

 and the reptile a bird. 



Another theory, advocated by Osborn, and more recently 

 by Beebe, assumes an arboreal reptile as the ancestral bird. 

 This creature is supposed to have been gifted with four wings 

 instead of two and a long tail, which it used much as a flying 

 squirrel uses its tail in sailing from tree to tree. AVitli loss 

 of the hind pair of wings and strengthening and improvement 

 of the front pair, the sailing reptile became a flying bird. 

 In support of this theory Beebe adduces a veiy interesting 

 fact. lie points out that in the newly hatched bird there is 

 a row of quills running along the outer side of the leg, in 

 such a position that, if developed, they would produce a 

 miniature wing. And further, just as in the case of the 

 "secondaries" (the smaller of the flight feathers in a bird's 

 wing) there develops above these quills and alternating with 

 them a second row of quills, which if developed would produce 

 "covert" feathers. Similar tufts of feathers occurred in 

 Archffiopteryx, which is strong evidence for Beebe 's theory, 

 for as we have already seen, higher animals tend to repeat 

 in an abbreviated way the structure of their ancestors. 



Yot others adopt a compromise theory and assume that 

 while ArcluL'opteryx lived in trees, using his wings as well as 

 his feet for grasping the branches, yet his flight was not 

 merely a sailing one, but that the wings were actively used 

 for this purpose. 



