THE FISHES 181 



in every case the head and then the trunk soon become 

 formed, gills arise, the nervous system appears, which is 

 invariably supported by a skeleton in the form of a gristly 

 rod the notochord. In the lower forms of fishes this per- 

 sists throughout life ; but in the sharks and skates it be- 

 comes replaced in the adult by another and higher type of 

 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 of the sharks have probably 

 changed relatively only to a slight extent ; others, like the 

 garpike, are much more altered ; and the bony fishes are 

 far from their original 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 important details. Moreover, the 

 record in the rocks is not complete, and we have many 

 things yet to learn of the ancient fishes and their develop- 

 ment from age to age to the present day. 



