216 Adaptations of Fishes 



tion, we should find that in cold waters, deep waters, dark 

 waters, fresh waters, and inclosed waters the strain would be 

 less, the relapses to less complex organization more frequent, 

 the numbers of vertebrae would be larger, while the individual 

 vertebrae would become smaller, less complete, and less per- 

 fectly ossified. 



This in a general way is precisely what we do find in exam- 

 ining the skeletons of a large variety of fishes. 



The cause of the increased numbers of vertebrae in cold waters 

 or extratropical waters is as yet unknown. Several guesses have 

 been made, but these can scarcely rise to the level of theories. 

 To ascribe it to natural selection, as the present writer has done, 

 is to do little more than to restate the problem. 



As a possible tentative hypothesis we may say that the 

 retention of the higher primitive traits in the tropics is due to 

 continuous selection, the testing of individuals by the greater 

 variety of external conditions. The degeneration of extra- 

 tropical fishes may be due to isolation and cessation or reversal 

 of selection. Thus fresh waters, the arctic waters, the oceanic 

 abysses are the "back woods" of fish life, localities favorable 

 to the retention of primitive simplicity, equally favorable 

 to subsequent degeneration. Practically all deep-sea fishes are 

 degenerate descendants of shore fishes of various groups. Monot- 

 ony and isolation permit or encourage degeneration of type. 

 Where the struggle for existence is most intense the higher struc- 

 tures will be retained or developed. Among such facts as these 

 derived from natural selection the cause of the relation of tem- 

 perature to number of vertebrae must be sought. How the 

 Cretaceous berycoids first acquired their few vertebrae and the 

 high degree of individual specialization of these structures we 

 may not know. The character came with the thoracic ventrals 

 with reduced number of rays, the ctenoid scales, the toothless 

 maxillary, and other characters which have long persisted in 

 their subsequent descendants. 



An exception to the general rule in regard to the number of 

 vertebrae is found in the case of the eel. Eels inhabit nearly all 

 seas, and everywhere they have many vertebras. The eels of 

 the tropics are at once more specialized and more degraded. 

 They are better eels than those of northern regions, but, as the 



