THE EVOLUTION OF FISHES. 563 



fied vertebrae, allied more or less remotely to the herring of to-day. In 

 these and other soft-rayed fishes the pelvis still retains its posterior 

 insertion, the ventral fins being said to be abdominal. The next great 

 stage in evolution brings the pelvis forward, attaching it to the shoulder 

 girdle so that the ventral fins are now thoracic as in the perch and bass. 

 If brought to a point in front of the pectoral fins, a feature of specialized 

 degradation, they become jugular as in the cod-fish. In the abdominal 

 fishes the air bladder still retains its rudimentary duct joining it to the 

 oesophagus. 



From the abdominal forms allied to the herring, the huge array of 

 modern fishes, typified by the perch, the bass, the mackerel, the wrasse, 

 the globe-fish, the sculpin, the seahorse and the cod descended in many 

 diverging lines. The earliest of the spine-rayed fishes with thoracic fins 

 belong to the type of Berycidse, a group characterized by rough scales 

 and the retention of the primitive larger number of ventral rays. These 

 appear in the Cretaceous or chalk deposits, and show various attributes 

 of transition from the abdominal to the thoracic type of ventrals. 



Another line of descent apparently distinct from that of the herring 

 and salmon extends through the characins to the loach, carps, cat-fishes 

 and electric eel. The fishes of this series have the anterior vertebrae 

 coossified and modified in connection with the hearing organ, a struc- 

 ture not appearing elsewhere among fishes. This group includes the 

 majority of fresh-water fishes. Still another great group, the eels, have 

 lost the ventral fins and the bones of the head have suffered much 

 degradation. 



The most highly developed fishes, all things considered, are doubt- 

 less the allies of the perch, bass and sculpin. These fishes have lost the 

 air-duct and on the whole they show the greatest development of the 

 greatest number of structures. But they do not represent an excessive 

 degree of specialization. In other groups their traits one after another 

 are carried to an extreme and these stages of extreme specialization give 

 way one after another to phases of degeneration. The specialization 

 of one organ usually involves degeneration of some other. Extreme 

 specialization of any organ tends to render it useless under other con- 

 ditions and may be one step toward its final degradation. 



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

 spring from an unknown and possibly worm-like stock, — that from 

 this stock, before it became vertebrate, degenerate branches have fallen 

 off, represented to-day by the Tunicates and Balanoglossus. We have 

 seen that the primitive vertebrate was headless and limbless without 

 hard parts. The lancelet remains as a possible direct off-shoot from it; 

 the cyclostome with brain and skull is a probable derivative from archaic 

 lancelets. The earliest fishes leaving traces in the rocks were doubtless 

 cyclostomes, limbless, naked lampreys and mailed ostracophores. The 



