Adaptations of Fishes 217 



eel is a degraded type, they have gone farther in the loss of 

 structures in which this degradation consists. 



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 them- 

 selves 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 compe- 

 tition this degenerate and non-fishlike type, lurking in holes 

 among the rocks, or creeping in the sand; thieves and scaven- 

 gers among fishes. The eels thus fill a place otherwise left un- 

 filled. 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 every- 

 thing. 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 rose- 

 fish. The organs of the latter are there, but each one is re- 

 duced or degenerate, the bones as soft as membranes, the spines 



