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, Ref. 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 féld 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 
