14 David Starr Jordan 



may also bear in mind the fact that those types of fishes (soft- 

 rayed and anacanthine) which are properly characterized by 

 increased numbers of vertebrae predominate in the fresh waters, 

 the deep seas, and in arctic and antarctic regions. On the 

 other hand the spiny-rayed* fishes are in the tropics largely in 

 the majority. 



In this paper, I wish to consider these generalizations and 

 the extent to which each is true. I propose to refer all of them 

 to the same group of causes. In fact all of them may be com- 

 bined into one statement, that in general all other fishes have 

 a large number of vertebrae as compared with the shore-fishes 

 of the tropics. The cause of the reduction in number of the 

 vertebrae must therefore be sought in conditions peculiar to 

 the tropical seas. If in any case an increase in the number of 



*For the purpose of the present discussion, we may regard the ordin- 

 ary fishes, exclusive of sharks, ganoids, eels, and other primitive or 

 aberrant types as forming three categories : (i) The soft-rayed or Phy- 

 sostomous fishes, with no true spines in the fins, with an open duct 

 to the air-bladder, the ventral fins abdominal (the pelvis being attached 

 only by the flesh and remote from the shoulder-girdle), cycloid scales, 

 etc. (2) The spiny-rayed or Acanthopterygian fishes, having usually 

 spines in the dorsal and other fins, no duct to the air-bladder, the skel- 

 ton firm, the ventrals attached by the pelvis to the shoulder-girdle, the 

 shoulder-girdle joined to the skull, and the scales usually ctenoid or 

 otherwise peculiar. The vertebrae among spiny-rayed fishes are larger, 

 and therefore generally fewer in number, and their appendages (shoul- 

 der-girdle, gill arches, ribs, interspinal bones, etc. , ) are more specialized. 

 The spiny-rayed fishes are usually regarded as the most specialized or 

 " highest " in the scale of development. The question of whether, on 

 the whole, they are " higher " or ' ' lower " as compared with sharks and 

 other primitive types is ambiguous, because various ideas are associated 

 with these words " high " and " low." It is certain, however, that the 

 spiny-rayed fishes deviate farthest from the primitive stock, and that the 

 qualities that distinguish fishes as a group are most intensified. In other 

 words, it is in the spiny-rayed fishes that the process of " ichthyizatiou" 

 or fish-forming has gone farthest. A third category would comprise the 

 Anacanthines (cods, flounders, etc.), fishes anatomically similar to the 

 spiny-rayed forms, but without spines to their fins, with weaker skele- 

 tons and smaller and more numerous vertebrse. They are " degenerate" 

 or more "generalized" offshoots from the spiny- rayed types, as the eels 

 are from some soft-rayed type. 



