EXTERNAL FEATURES 15 



phenomenon — quite different from reptilian moulting and 

 more like the shedding of hair. 



As long as the ancestral birds were still provided with 

 teeth, the covering of the jaws with horny scales would be 

 of minor importance. But with the loss of teeth the 

 condition of affairs was changed. Thus, as Lonnberg 

 (1904) maintains, the reptilian rostral scale became greatly 

 enlarged on the upper half of the bill and the reptilian 

 mental on the lower jaw. This illustrates a frequent 

 method of evolution — making a new thing, in this case the 

 sharp-edged horny bill, out of a very old thing, in this 

 case the scales of the reptile's jaws. 



The epidermis as a whole is relatively thin as compared 

 with that of mammals. It has no sweat-glands and there 

 is usually only the preen-gland. In a few cases there are 

 some gland-cells beside the ear-opening. There are 

 abundant tactile nerve-endings (Pacinian corpuscles) in the 

 skin, and these are often specially developed in a swollen 

 patch or cere at the base of the bill. In birds like woodcock 

 and snipe that probe for their food in soft soil or in mud, 

 there are nerve-endings in the delicate skin continued to 

 the tip of the bill. In ducks and the like there are many 

 nerve-endings along the inner margin of the bill. Here it 

 may be noted that the New Zealand Kiwi or Apteryx is 

 the only bird with the nostrils at the tip of the bill ; in 

 other cases they are at the base of the bill, except that in 

 albatrosses, petrels, and their relatives the openings are 

 carried well forward dorsally in a horny tube. In some 

 cases, such as the gannet, the openings of the nostrils are 

 closed up altogether. 



A few other external features must be noticed. There 

 is no ear-trumpet or pinna, though there is a little flap in 

 owls, and the absence of this apparatus for collecting the 

 sound waves may be connected with the mobility of the 

 head. The reduction of the pinna in man as compared 

 with its large size in the donkey may be in part referred to 

 the same correlation, for it is easy for man by adjusting his 

 head to catch and localise faint sounds. Every day in the 



