The Evolution of Fishes 



457 



organ usually involves degeneration of some other. Extreme 

 specialization of any organ tends to render it useless under other 

 conditions and may be one step toward its final degradation. 



We have thus seen, in hasty review, that the fish-like verte- 

 brates spring from an unknown and possibly worm-like stock, 



FIG. 271. An African Catfish, Chlarias breviceps Boulenger. Congo River. 

 Family Chlariidce. (After Boulenger.) 



that from this stock, before it became vertebrate, degenerate 

 branches have fallen off, represented to-day by the Tunicates 

 and Enteropneustans . We have seen that the primitive verte- 

 brate was headless and limbless and without hard parts. The 

 lancelet remains as a possible direct offshoot from it ; the cyclo- 



FIG. 272. Silverfin, Notropis whipplii (Girard). White River, Indiana, 

 Family Cyprinidce. 



stome with brain and skull is a possible derivative from archaic 

 lancelets. The earliest fishes leaving traces in the rocks were 

 mailed ostracophores. From an unknown but possibly lamprey- 

 like stock sprang the sharks and chimaeras. The sharks de- 

 veloped into rays in one right line and into the highest sharks 

 along another, while by a side branch through lost stages the 

 primitive sharks passed into Crossopterygians, into Dipnoans, 

 or lung-fishes, and perhaps into Ostracophores. All these types 



