230 Mr Kerr, Note on Hypotheses as to the Origin 



The most important evidence for the fold view from the side of 

 Comparative Anatomy is afforded by (1) the fact that the limb 

 derives its nerve supply from a large number of spinal nerves, and 

 (2) the extraordinary resemblance met with between 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 myo- 

 tomes, 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 the limb. 



The similarity in the skeleton is indeed most striking, though 

 its weight as evidence has been recently greatly diminished by 

 the knowledge that the apparently metameric segmentation of the 

 skeletal and muscular tissues of the paired fins is quite secondary 

 and does not at all agree with the metamery of the trunk 1 . What 

 resemblance there is may well be of a homoplastic character when 

 we take into account the similarity in function of the median and 

 unpaired fins, especially in such forms as Raia where the 

 anatomical resemblances are especially striking. 



There is a surprising dearth of paleontological evidence in 

 favour of the view. The two creatures of which most have been 

 made are the Devonian Glimatius, and the Carboniferous Cladose- 

 lache, each of which has been held to support it. In Glimatius 

 there is a remarkable series of spines behind the pectoral fin, but 

 there appears to be little evidence to decide whether these were 

 really fin or free skin spines. In regard to Cladoselache there 

 seems to be much in the criticisms of Jaekel and Semon, who 

 consider that the creature is a highly specialized one and not 

 primitive at all. 



The short summary I have given is I think sufficient to show 

 that the foundation of fact on which the lateral fold view rests is 

 at best a precarious one. It must, so it seems to me, be looked 

 upon and judged, just as any other view at the present time 

 regarding the nature of the Vertebrate limb, rather as a specu- 

 lation, brilliant and suggestive though it be, than as a logically 

 constructed theory of the known facts. It is, I think, on this 

 account allowable to apply it to a test of a character which is 

 admittedly very apt to mislead that of " common sense " : — 



If there is any soundness in zoological speculation at all I 

 think it must be admitted that the more primitive Vertebrates 

 were creatures possessing a notochordal axial skeleton near the 

 dorsal side, with the main nervous axis above it, the main viscera 

 below it, and the great mass of muscle lying in myotomes along 

 its sides. Now such a creature is well adapted to movements of 



1 Cf. Braus, Verhand. Anat. Gessellschaft 12 te Vermmmlung in Kiel, 1898, p. 166. 



