72 ' Morphology of the Fins 



facts of the development of the skeleton cannot be said to 

 support the fold view ; according to it we should expect to find 

 a series of metameric supporting rays produced which later on 

 become fused at their bases. Instead of this we find a longi- 

 tudinal bar of cartilage developing quite continuously, the rays 

 forming as projections from its outer side. 



"The most important evidence for the fold view from the 

 side of comparative anatomy is afforded by (i) the fact that 

 the limb derives its nerve supply from a large number of spinal 

 nerves, and (2) the extraordinary resemblance met with be- 

 tween the skeletal arrangements of paired and unpaired fins. 

 The believers in the branchial-arch hypothesis have disposed of 

 the first of these in the same way as they did the occurrence of 

 interpterygial myotomes, by looking on the nerves received from 

 regions of the spinal cord anterior to the attachment of the limb 

 as forming a kind of trail marking the backward migration of 

 tlie limb. 



"The similarity in the skeleton is indeed most striking, 

 though its weight as evidence has been recently greatly dimin- 

 ished by the knowledge that the apparently metameric segmen- 

 tation of the skeletal and muscular tissues of the paired fins 

 is quite secondary and does not at all agree with the meta- 

 mery of the trunk. What resemblance there is maj^ well be of 

 a homoplastic character when we take into account the simi- 

 larity in function of the median and unpaired fins, especially in 

 such forms as Raja, where the anatomical resemblances are 

 especially striking. There is a surprising dearth of paleonto- 

 logical evidence in favor of this view." 



The objection to the first view is its precarious foundation. 

 Such lateral folds are found only in certain rays, in which they 

 may be developed as a secondary modification in connection 

 with the peculiar form of these fishes. Professor Kerr observes 

 that this theory must be looked upon and judged: "Just as any 

 other view at the present time regarding the nature of the 

 vertebrate limb, rather as a speculation, brilliant and suggestive 

 though it be, than as a logically constructed theory of the 

 now known facts. It is, I think, on this account allowable to 

 apply to it a test of a character which is admittedly very apt to 

 mislead, that of 'common sense.' 



