NOTES ON MEDIAN AND PAIHED FINS OP FISH. 343 



arch ill a developing Elasmobraiicli, sucli as is insisted upon 

 by Braus (5), no mere opinion, unsupported by evidence, that 

 the relative position of the girdle has been altered, such as is 

 expi'essed by Furbringer (12), can outweigh these facts. 



The fourth and last objection which we shall urge against 

 the gill-arch theory is one which will probably seem to most 

 zoologists to be the most fatal of all: the theory gives no 

 explanation of the remarkable resemblance borne 

 by the paired fins to the unpaired fins. The resem- 

 blance is not vague and indefinite, it is minute ; it can be 

 followed out in every detail both of their structure and of their 

 development. In no respect is this more striking than in the 

 development and differentiation of the dermal fin-rays in the 

 various groups of fishes. 



All these facts, which clearly support the lateral fold 

 theory, are so many deadly blows aimed at the rival gill- 

 arch theory. Far from being difficulties Avhicli have to be 

 explained away, they become evidence actually in favour of 

 the fundamental likeness of the paired and unpaired fins. 



The Apparent Migration op Fins. 



We have now to account for the apparent migration of 

 limbs from one place to another on the body of vertebrates. 

 Every trunk segment may be said to be capable of 

 producing muscular, nervous, and skeletal ''limb 

 elements'^ of a paired character. This "potentiality^^ is 

 actually called into force in the case of the Rajidas throughout 

 the trunk region, with the exception of a few anterior seg- 

 ments (see Rabl 31, Mollier 24, and especially Braus 3). In 

 Torpedo, for instance, the 4th to the 30th spinal nerves 

 supply the pectoral fin, and the 31st to the 42nd the pelvic 

 fin. In Trygoii the 3rd to the 59th supply the pectoral^ and 

 the 60th to the 71st the pelvic fin (Braus). The same con- 

 clusion is indicated in the case of forms like Pristiurus and 

 Scyllium, where the paired fins are widely separated, by the 

 development of muscle-buds on all the trunk segments (see 



