YQUNG BIRDS AND RECORDS OF THE PAST 147 



nostrils were still present, though extremely reduced in 

 size. The process of closing seems to have been brought 

 about in the first place by a gradual reduction in the 

 bore of the tube formed by the ingrowth of the horny 

 sheath, and this tube seems to be gradually closed by 



bristle 



sheath pliig 



BEAK OF AN ADULT AND OF A NESTLING CORMORANT, ILLUSTRATING 

 THE CLOSURE OF THE EXTERNAL NOSTRILS, 



the shedding of the surface of its inner wall until the 

 aperture is finally plugged. But more than this came to 

 light. It soon became apparent that the darters, gannets, 

 pelicans and penguins had all, in like manner, undergone 

 this process of closing up the external nostrils, and that the 

 underlying skeleton had also become involved in this change. 



