THE FISHES 173 



skeleton, which is much more specialized with the bony 

 fishes. 



Those who study the fossils on the rocks tell us that 

 the first fishes were very simple, and many believe that 

 their skeleton, like that of the little growing fish, consisted 

 only of a notochord. Many of these old forms died out 

 long ago, while others gradually changed in one way and 

 another to adapt themselves to their surroundings, the con- 

 stant need of adaptation having resulted in the multitude 

 of present-day types. Some, such as the lamprey, have 

 probably changed relatively only to a slight extent ; others, 

 like the sharks and skates, are much more altered; and 

 the bony fishes are far from their original low estate, 

 though their development has been rather toward a greater 

 specialization for aquatic life than an advance upward. 

 The little fish in its growth from the egg thus repeats the 

 history of its ancestral development ; but as though in 

 haste to reach the adult condition, it omits many impor- 

 tant details. Moreover, the record in the rocks is not 

 complete, and we have many things yet to learn of the 

 ancient fishes and their development from age to age to 

 the present day. 



