ORIGIN OF THE FINS OF FISHES. 547 



at present hold the field. If untrue it is so dangerously plausible as to 

 surely deserve more consideration than it appears to have had. One of 

 the main differences between it and the other two hypotheses is that, 

 instead of deriving the swimming fin from the walking and supporting 

 limb, it goes the other way about. That this is the safer line to take 

 seems to me to be shown by the consideration that a very small and 

 rudimentary limb could only be of use if provided with a fixed point 

 d'appui. Also on this view, the pentadactyle limb and the swimming 

 fin would probably be evolved independently from a simple form of 

 limb. This would evade the great difficulties which have beset those 

 who have endeavored to establish the homologies of the elements of the 

 pentadactyle limb with those of any type of fully-formed fin. ' ' 



Uncertain Conclusions. 



In conclusion, we may say that the evidence of embryology in this 

 matter is inadequate, that of morphology is inconclusive and perhaps 

 the final answer may be given by paleontology. If the records of the 

 rocks were complete they would be decisive. At present we have to 

 decide which is the more primitive of two forms of pectoral fin actually 

 known among fossils. That of Cladoselache is a low, horizontal fold 

 of skin, with feeble rays, called by Cope ptychopterygium. That of 

 Pleuracanthus is a jointed paddle-shaped appendage with a fringe of 

 rays on either side. In the theory of Gegenbaur and Kerr Pleuracan- 

 thus must be, so far as the limbs are concerned, the form nearest the 

 primitive limb-bearing vertebrate. In Balfour's theory Cladoselache is 

 nearest the primitive type from which the other and with it the archi- 

 pterygium of later forms may be derived. 



Boulenger and others question even this, believing that the archi- 

 pterygium in Pleuracanthus and that in N eo-C eratodus and its Dipnoan 

 and Crossopterygian allies have been derived independently from the 

 archipterygium in the primitive sharks. In the one theory, the type 

 of Pleuracanthus would be ancestral to the other sharks, on the one 

 hand, and to Crossopterygians and all higher vertebrates on the other. 

 With the theory of the origin of the pectoral from a lateral fold, 

 Pleuracanthus would be merely a curious specialized offshoot from the 

 primitive sharks, without descendants and without special significance 

 in phylogeny. 



As elements bearing on this decision we may note that the tapering 

 unspecialized diphycercal tail of Pleuracanthus seems very primitive in 

 comparison with the short heterocercal tail of Cladoselache. This 

 evidence, perhaps deceptive, is balanced by the presence on the head of 

 Pleuracanthus of a highly specialized serrated spine, evidence of a far 

 from primitive structure. 



