Fig. 267.—Cardinal-fish, a perch-like fish, Apogon semilineatus Schlegel. Misaki, Japan.
Fig. 268.—Summer Herring, Pomolobus æstivalis (Mitchill). Potomac River. Family Clupeidæ.
Another line of descent apparently distinct from that of the herring and salmon extends through the characins to the loach, carps, catfishes, and electric eel. The fishes of this series have the anterior vertebræ 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.
Fig. 269.—Fish with jugular ventral fins, Bassozetus catena Goode & Bean. Family Brotulidæ. Gulf Stream.
Fig. 270.—A specialized bony fish, Trachicephalus uranoscopus. Family Scorpænidæ. From Swatow, China.
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. In these 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.