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. 
