Page:Popular Science Monthly Volume 60.djvu/571

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THE EVOLUTION OF FISHES.

fied vertebræ, 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 Berycidæ, 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 structure 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 doubtless 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 conditions 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