It is not well to push this analogy too far, but perhaps we can find in the comparison of the tropics and the cities some suggestion as to the development of the eel. In the city there is always a class which follows in no degree the general line of development. Its members are specialized in a wholly different way. By this means they take to themselves a field which others have neglected, making up in low cunning what they lack in humanity or intelligence. Thus, among fishes, we have in the regions of closest competition this degenerate and non-fish-like type, lurking in holes among the rocks, or creeping in the sand; thieves and scavengers among fishes. The eels thus fill a place otherwise left unfilled. In their way they are perfectly adapted to the lives they lead. A multiplicity of vertebral joints is useless to the tropical fish, but to the eel strength and suppleness are everything. No armature of fin or scale or bone is so desirable as its power of escaping through the smallest opening. With the elongation of the body and its increase in flexibility there is a tendency toward the loss of the paired fins, the ventrals going first, and afterwards the pectorals. This tendency may be seen in many groups. Among recent fishes, the blennies, the eel-pouts, and the sea-snails furnish illustrative examples. =Degeneration of Structures.=--In the lancelet, which is a primitively simple organism, the various structures of the body are formed of simple tissues and in a very simple fashion. It is probable from the structure of each of these that it has never been very much more complex. As the individual develops in the process of growth each organ goes as it were straight to its final form and structure without metamorphosis or especial alterations by the way. When this type of development occurs, the organism belongs to a type which is primitively simple. But there are other forms which in their adult state appear feeble or simple, in which are found elements of organs of high complexity. Thus in the sea-snail (_Liparis_), small, weak, with feeble fins and flabby skin, we find the essential anatomy of the sculpin or the rosefish. The organs of the latter are there, but each one is reduced or degenerate, the bones as soft as membranes, the spines obsolete or buried in the skin. Such a type is said to be degenerate. It is very different from one primitively simple, and it is likely in its earlier stages of development to be more complex than when it is fully grown. [Illustration: FIG. 158.--Liparid, _Crystallias matsushimæ_ (Jordan and Snyder). Family _Liparididæ_. Matsushima Bay, Japan.] [Illustration: FIG. 159.--Yellow-backed Rockfish, _Sebastichthys maliger_ Jordan and Gilbert. Sitka, Alaska.]