Mr Ken-, Note on Hypotheses as to the Origin, etc. 227 



Note on Hypotheses as to the Origin of the Paired Limbs of 

 Vertebrates. By Mr J. Graham Kerr. 



[Received 9 December, 1899.] 



In a footnote to my paper on the " External Features in the 

 Development of Lepidosiren 1 " I have referred to the view that the 

 paired limbs of Vertebrates may have arisen in Phylogeny from 

 External Gills. 



As this view does not appear to have been clearly formulated 

 before it seems advisable to do so now, and to attempt its justi- 

 fication, for, as 1 hope will appear from the sequel such a view has 

 in its favour sufficient probability to entitle it to rank on at least 

 an equal footing with either of the two rival hypotheses which at 

 present hold the field 2 . 



In order to bring this out satisfactorily it seems necessary to 

 state in a few words each of the rival hypotheses referred to, and 

 then to attempt a short critical examination of the points in its 

 favour and those against it. 



I need hardly say that this examination makes no claim to 

 thoroughness, I only produce sufficient arguments to show that 

 the position of each of the two at present predominant views is a 

 very insecure one. 



The two views referred to are these : — 



(1) that the paired fins are persisting and exaggerated 

 portions of an at one time continuous fin-fold which stretched 

 along each side of the body, and to which they bear an exactly 

 similar phylogenetic relation, so do the separate dorsal and anal 

 fins to the once continuous median fin-fold. 



1 Phil. Trans. Roy. Soc. 



2 Jardine long ago (1841: Ann. and Mag. Nat. Hist., Vol. vn., p. 24) referred 

 to the vestigial external gills of the adult Protopterus as reduced limbs. 



Goeldi (Proc. Zool. Soc. 1898, p. 857) after discussing the fore-limb in Lepi- 

 dosiren as supporting the Gegenbaur hypothesis refers to "one other possibility, 

 that the so-called fore-limb of Lepidosiren is in fact not yet a true anterior 

 extremity, but a persistent rudimentary external branchia." 



Clemens, in his general discussion on external gills (Anat. Hefte Abt. i., Bd. v., 

 p. 141), has a passage which may mean that he had a similar view to mine in his 

 mind. 



It is of importance to consider each of the three passages referred to, in relation 

 to its context. 



