PAIRED FINS 



41 



The foremost pair appears anteriorly not far behind the 

 gill region : from its position it has certainly the more im- 

 portant mechanical function in balancing the fish's length 

 — on this account becoming more widely modified in form 

 and function as the pectoral fins. The hinder pair, or ven- 

 tral fins, though in the plane of the pectorals, has a more 

 ventral position, the hinder borders converging in the 

 region of the anus. The ventral fins are certainly placed 

 in the most motionless region of the fish : they are little 

 affected by either the lateral or upward movements of the 

 body ; and remain accordingly smaller in size and simpler 

 in structure than the pectoral fins. That there may have 

 existed in primitive fishes a third (post-ventral) pair of fins 

 is by no means improbable (cf. T. J. Parker, Rcf. p. 244), 

 although its presence has not as yet been satisfactorily 

 demonstrated. 



The paired fins thus appear to have been derived from a 

 continuous dermal fold, similar in every way to that giving 

 rise to the vertical fins. They appear, moreover, to have 

 undergone the same mode of evolution in their structures 

 as have the dorsal or anal fins. The unpaired fin fold as it 

 passed forward on the ventral side of the body may primi- 

 tively have forked in the anal region, and given rise on 

 either side to a lateral fold. In these might next appear 

 an anterior and posterior pair of lappets, — pectoral and 

 ventral fins, — whose positions would be determined by 

 mechanical needs, and whose size would increase as the 

 intervening and useless portion of the dermal fold disap- 

 peared. In the subsequent history of pectoral and ventral 

 fins, supporting elements, actinotrichia, radials, and basals, 

 would arise in the same way as in the unpaired fins, and a 

 similar metamorphosis of the fin form would take place, 

 owing; to the concrescence of these elements and to the 



