78 Morphology of the Fins 



inherited from a common ancestor and consequently of great 

 antiquity in the vertebrate stem." 



As to the third theory, Professor Kerr suggests tentatively 

 that the external gill may be the structure modified to form the 

 paired limbs. Of the homology of fore and hind limbs and 

 consequently of their like origin there can be no doubt. 



The general gill-structures have, according to Kerr, "the 

 primary function of respiration. They are also, however, pro- 

 vided with an elaborate muscular apparatus comprising elevators, 

 depressors, and adductors, and larvae possessing them may be 

 seen every now and then to give them a sharp backward twitch 

 They are thus potentially motor organs. In such a Urodele as 

 Amblystoma their homologues on the mandibular arch are used 

 as supporting structures against a solid substratum exactly as 

 are the limbs of the young Lepidosiren. 



"I have, therefore, to suggest that the more ancient Gna- 

 thostomata possessed a series of potentially motor, potentially 



FIG. 62. Polypterus congicus, a Crossopterygian fish from the Congo River. Young, 

 with external gills. (After Boulenger.) 



supporting structures projecting from their visceral arches; it 

 was inherently extremely probable that these should be made 

 use of when actual supporting, and motor appendages had to be 

 developed in connection with clambering about a solid sub- 

 stratum. 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 

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

 of the two which 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 be- 

 tween it and the other two hypotheses is that, instead of deriving 



