6 THE BIOLOGY OF BIRDS 



reptilian scales and the development of birds' feathers. 

 Both arise as flattened outgrowths of the epidermis which 

 become cornified ; both are imbedded in the dermis and 

 fed by a basal core of dermis. But a reptile's scale is not 

 moulted as a whole as a feather is, only the outer layer of 

 the epidermis covering the scales ; and, although birds 

 usually show scales on their feet, there is not any transitional 

 form between bird-scale and bird-feather. But we must 

 return to this subject. 



§ 6. Adaptations Accessory to Flight 



When flight had become an actuality, the achievement 

 would be followed up by adaptation after adaptation within 

 the body. The skeleton would become more lightly built ; 

 there would be fusion here and suppression there ; the wing 

 would be perfected by reduction of parts ; the breastbone 

 would get its keel ; the tail would be reduced ; the neck 

 would be lengthened ; the beak would become a hand ; the 

 balancing of the aerial boat would be adjusted ; the useless 

 muscles would dwindle and the useful ones would grow 

 in strength ; the feathers would be differentiated into 

 pinions, contours, and the like. In short, there are many 

 characteristic features of birds which must be interpreted 

 as adaptations conspiring in the direction of improving 

 flight. It is natural to think of these as being gradually 

 added on after the problem of flight had been solved. 

 They are accessories rather than pre-conditions. 



Another peculiarity of birds, which may have been a 

 pre-condition of flight or a subsequent accessory, concerns 

 the respiration. The lungs are firmly fastened dorsally to 

 the ribs and they are inexpansible. They are compressed 

 by the pressure of the ribs in particular, and more generally 

 by the springy basketwork formed by the thoracic vertebrae 

 above, the breastbone below, and the ribs between. The 

 active part of respiration is breathing out or expiration, 

 whereas in mammals, as every one knows, the active part is 

 breathing in or inspiration. Now it seems of radical 



