of the Paired Limbs of Vertebrates. 235 



in connection with clambering about a solid substratum. If this 

 had been so we should look upon the limb as a modified external 

 gill : the limb girdle with Gegenbaur as a modified branchial arch. 



This theory of the Vertebrate paired limb seems to me I con- 

 fess to be a more plausible one on the face of it than either of the 

 two which at present hold the field 1 . If untrue it is so danger- 

 ously 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 endeavoured to establish the homologies of the elements 

 of the pentadactyle limb with those of any type of fully-formed 

 fin. 



Lastly this view naturally takes to itself the only important 

 mass of actual facts which have been adduced to support the 

 Gegenbaur view, viz. : those which have to do with demonstrating 

 the backward migration of the limbs. 



1 The difficulty of most weight which this view has to face, that we do not 

 know of the occurrence of a cartilaginous axis in any undoubted external gills 

 seems to me by no means insuperable. Such a supporting axis has been de- 

 veloped in the barbels of Xenopus which (if not homodynamous with the external 

 gills !) is probably a comparatively recently evolved structure. 



