2l8 



Adaptations of Fishes 



obsolete or buried in the skin. Such a type is said to be de- 

 generate. It is very different from one primitively simple, and 



FIG. 158. Liparid, Crystallias matsushimce (Jordan and Snyder). Family Lipa- 

 rididce. Matsushima Bay, Japan. 



it is likely in its earlier stages of development to be more complex 

 than when it is fully grown. 



In the evolution of groups of fishes it is a common feature 

 that some one organ will be the center of a special stress, in 

 view of some temporary importance of its function. By the 



FIG. 159. Yellow-backed Rockfish, Sebastichthys maliger Jordan and Gilbert 



Sitka, Alaska. 



process of natural selection it will become highly developed and 

 highly specialized. Some later changes in conditions will ren- 



