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 process of natural selection it will become highly developed and highly specialized. Some later changes in conditions will render this specialization useless or even harmful for at least a part of the species possessing it. The structure then undergoes degeneration, and in many cases it is brought to a lower estate than before the original changes. An example of this may be taken from the loricate or mailed-cheek fishes. One of the primitive members of this group is the rockfish known as priestfish (_Sebastodes mystinus_). In this fish the head is weakly armed, covered with ordinary scales. A slight suggestion of cranial ridges and a slight prolongation of the third suborbital constitute the chief suggestions of its close affinity with the mailed-cheek fishes. In other rockfishes the cranial ridges grow higher and sharper. The third suborbital extends itself farther and wider. It becomes itself spinous in still others. Finally it covers the whole cheek in a coat of mail. The head above becomes rough and horny and at last the whole body also is enclosed in a bony box. But while this specialization reaches an extraordinary degree in forms like _Agonus_ and _Peristedion_, it begins to abate with _Cottus_, and thence through _Cottunculus_, _Psychrolutes_, _Liparis_, and the like, and the mailed cheek finds its final degradation in _Parliparis_. In this type no spines are present anywhere, no hard bone, no trace of scales, of first dorsal, or of ventral fins, and in the soft, limp structure covered with a fragile, scarf-like skin we find little suggestion of affinity with the strong rockfish or the rough-mailed _Agonus_. Yet a study of the skeleton shows that all these loricate forms constitute a continuous divergent series. The forms figured constitute only a few of the stages of specialization and degradation which the members of this group represent. [Illustration: FIG. 160.--European Sculpin, _Myoxocephalus scorpius_ (Linnæus). Cumberland Gulf, Arctic America] [Illustration: FIG. 161.--Sea-raven, _Hemitripterus americanus_ (Gmelin). Halifax, Nova Scotia.] Some of the features of the habits and development of certain fresh-water fishes are mentioned in the following chapter. [Illustration: FIG. 162.--Lumpfish, _Cyclopterus lumpus_ (Linnæus). Eastport, Maine.] The degeneration of the eye of the blind fishes of the caves of the Mississippi Valley, _Amblyopsis_, _Typhlichthys_, and _Troglichthys_, have been very fully studied by Dr. Carl H. Eigenmann. According to his observations "The history of the eye of _Amblyopsis spelæus_ may be divided into four periods: [Illustration: FIG. 163.--Sleek Sculpin, _Psychrolutes paradoxus_ (Günther). Puget Sound.]