68 The Bird 



ginning brings, of fish, of bird, of man even, soon melted 

 away and there they nod and sway in the watery cur- 

 rents, never to know of the opportunity Nature has 

 snatched from them why, who can tell? 



In adult sharks, the back-bone has become jointed 

 and flexible, and a crude kind of skull is present, but 

 still more important is the presence of four fins which 

 correspond to the four legs of lizards and to the wings 

 and legs of birds. A curious basket-like skeleton pro- 

 tects the delicate gills, and it is probable that this existed 



FIG. 44. Back-bone of Dogfish, with simple cartilaginous vertebrae. 



long before the limbs appeared. All of this is composed 

 of gristly cartilage. In the higher fishes, bone replaces 

 the cartilage, and when the lowly tadpole fish-like at 

 first, swimming about by means of the fin around his 

 tail pushes forth his legs and climbs upon the land, our 

 skeleton is well on its way birdwards.* Reptiles of old 

 took to trees; their back-bones grew less flexible, so that 

 they might safely sail through the air; feathers replaced 



* The actual evolution of birds was of course not through fish, tadpoles, 

 and reptiles as we know them, but by some line of creatures unknown to us 

 forever, and resembling some of these other living Classes at least in the pos- 

 session of gills, scales, etc. 



