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 vertebre. 
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, ete. 
