232 Mr Kerr, Note on Hypotheses as to the Origin 



way of its acceptance, the position of the limbs, especially of the 

 pelvic limbs in a position far removed from that of the branchial 

 arches. This difficulty has been entirely removed by the brilliant 

 work of Gegenbaur's followers, who have shown from the facts of 

 Comparative Anatomy and Embryology that the limbs, and the 

 hind limbs especially actually have undergone and in Ontogenj' 

 do undergo an extensive backward migration. In some cases 

 Braus has been able to find traces of this migration as far 

 forwards as a point just behind the branchial arches. Now when 

 we consider the numbers the enthusiam and the ability of Gegen- 

 baur's disciples, we cannot help being struck by the fact that the 

 only evidence in favour of his derivation of the limbs has been 

 that which tends to show that a migration of the limbs backwards 

 has taken place from a region somewhere near the last branchial 

 arch, and that they have failed utterly to discover any inter- 

 mediate steps between gill rays and archipterygial fin. And if 

 for a moment we apply the test of common sense we cannot but 

 be impressed by the improbability of the evolution of a gill 

 septum, which in all the lower forms of fishes is fixed firmly in 

 the body wall, and beneath its surface, into an organ of loco- 

 motion. 



May I express the hope that what I have said is sufficient to 

 show in what a state of uncertainty our views are regarding the 

 Morphological nature of the paired fins, and upon what an exceed- 

 ingly slender basis rest both of the two views which at present 

 hold the field 1 . 



It is because I feel that in the present state of our knowledge 

 neither of the two views I have mentioned has a claim to any 

 higher rank than that of extremely suggestive speculations that I 

 venture to say a few words for a third view which is avowedly a 

 mere speculation. 



Before proceeding with it I should say that I assume the 

 serial homology of fore- and hind-limbs to be beyond dispute. 

 The great and deep-seated resemblances between them are such 



1 In the above it will be seen that I have confined myself to a consideration of 

 Gegenbaur's view as to the derivation of the skeleton of the free limb. The view 

 to be drawn attention to below involves the acceptance of Gegenbaur's theory of 

 the morphology of the limb girdles — that they are modified branchial arches. For 

 this view there is strong evideuce both in the topographical relations of the 

 shoulder-girdle in fishes, and in the great resemblance to a branchial arch of the 

 segmented shoulder-girdle of the Xenacanthidae. 



I need perhaps hardly point out however that the evidence in favour of the 

 shoulder-girdle being a modified branchial arch derived from the presence on it of 

 a group of vestigial external gills in Protopterus ceased to exist as soon as it 

 became known that their position there was quite a secondary one. This was 

 shown in the first instance by the blood-supply of the three vestigial gills by three 

 successive branchial arches, and corroborated later by the facts of Ontogeny as 

 determined in the allied form Lepidoairen. 



