266 A HISTORY OF BIRDS 



six rod-like stalks which apparently serve the purpose neither of 

 use nor ornament. 



In the development of the skull again we find in the very 

 young nestling that the squamosal, in the more primitive forms, 

 is a relatively large bone attached comparatively loosely to the 

 skull wall, and finding no place within the skull, thus approach- 

 ing the reptiles. In the more highly specialised forms the squa- 

 mosal appears to have slowly absorbed the underlying bones 

 so that before the sutures marking the separate bones disappear 

 the squamosal will be found to have forced its way, as it were, 

 to the interior of the skull cavity. 



Though the history of the development of the bird's wing, 

 from the embryonic to the adult organ, shows but few traces 

 of a phase earlier than that seen in the earliest known bird, 

 Archaeopteryx, this is not true of the tail. And this perhaps be- 

 cause in Archaeopteryx this organ had not yet shaken off its 

 reptilian character, being of considerable length and composed 

 of a series of tapering vertebrae, each of which supported a pair 

 of elongated feathers. As time went on these vertebrae 

 became reduced in length, more especially the terminal members 

 of the series, which to-day make up the " ploughshare " element 

 or " pygostyle " which forms the termination of the tail of all 

 living species. This element in the adult supports the tail 

 feathers, as we shall show presently. In the embryo and nest- 

 ling it is found to be made up of several separate vertebrae, 

 varying between six and seven in number; but these have now 

 become reduced to mere rings of bone closely pressed together, 

 and recalling the similar reduction of the neck vertebrae in 

 whales. But the tail of living birds has also undergone reduc- 

 tion in number, by the loss of vertebrae from the free end of 

 the tail, since in Arclijeopteryx eleven vertebrae were concerned 

 ' in the support of the tail feathers, whereas in modern birds not 

 more than seven — -those included in the " pygostyle " — of these 

 remain. The " pygostyle" in the adult is a blade-shaped bone, 

 sometimes developing a disc-shaped plate on its inferior surface 

 — as in the Falcons and Woodpeckers, birds which make much 

 use of their tails, the one as a " brake " in flight, the other as an 

 organ of support in climbing. By the gradual shortening of the 

 bodies of these vertebrae the tail feathers became brought nearer 

 and nearer together, till at last, in place of forming a series of 



