FINS : THEIR USES. 67 



serving to stiffen these balancing membranes, and 

 obviously make them more effective. At about 

 this time, however, there vras a tendency to 

 diminish the extent of the lateral or side mem- 

 branes, and this tendency became more and more 

 pronounced till certain portions entirely dis- 

 appeared leaving four separate or detached folds 

 or lobes. In these four separate lobes we have the 

 origin of the paired fins, the two immediately 

 behind the head becoming the pectoral and the 

 two near the tail the pelvic fins (fig. 7, B.). 



We have seen already that the fins of modern 

 fishes act primarily as balancing organs ; in 

 addition they serve also as accessory steering 

 organs. For this purpose they have become 

 freely movable in various directions. In this 

 mobility we have one great distinction between 

 the fixed, newly isolated balancing organs, the 

 evolution of which we have just traced, and the 

 freely movable fin of the modern fish. How 

 did this mobility come about 1 To this question 

 we have at present no definite answer. We 

 may, however, endeavour to trace the improve- 

 ments which accompanied this mobility. One of 

 these improvements was the blending together of 

 certain of the supporting rods of cartilage to 

 form a central and definite axis, and the arrange- 

 ment of certain of the remaining rods on either 

 side of this in the form of rays, as shown in the 

 accompanying fig. 7, CD. The size of the fin 

 became next enlarged by the addition of hair- 

 like fibres outside the rays, as in fig. 7, F. This 

 form of fin can be studied to-day in the lung-fish 

 Ceratodus. 



