ORIGIN OF THE FINS OF FISHES. 541 



Mivart, Byder, Dean, Boulenger and others, and now generally accepted 

 by most morphologists as plausible, is this: that "The paired limbs are 

 persisting and exaggerated portions of a fin-fold once continuous, which 

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

 similar phylogenetic relation as do the separate dorsal and anal fins to 

 the once continuous median fin-fold." 



'This view, in its modern form, was based by Balfour on his obser- 

 vation that in the embryos of certain Elasmobranchs the rudiments of 

 the pectoral and pelvic fins are at a very early period connected together 

 by a longitudinal ridge of thickened epiblast — of which indeed they are 

 but exaggerations. In Balfour's own words referring to these observa- 

 tions : ' If the account just given of the development of the limb is an 

 accurate record of what really takes place, it is not possible to deny that 

 some light is thrown by it upon the first origin of the vertebrate limbs. 

 The facts can only bear one interpretation, viz., that the limbs are the 

 remnants of continuous lateral fins. ' 



' ' A similar view to that of Balfour was enunciated almost synchro- 

 nously by Thacher and a little later by Mivart — in each case based on 

 anatomical investigation of Selachians — mainly relating to the remark- 

 able similarity of the skeletal arrangements in the paired and unpaired 

 fins." 



A third theory is suggested by Mr. J. Graham Kerr (Cambridge 

 P/iilos. Trans., 1899), who has given us the best recent summary of the 

 theories on this subject. Mr. Kerr agrees with Gegenbaur as to the 

 primitive nature of the archipterygium, but believes that it is derived, 

 not from the gill-septum but from an external gill. Such a gill is well 

 developed in the young of all the living sharks, Dipnoans and Crossop- 

 terygians, and in the latter types of fishes it has a form strikingly simi- 

 lar to that of the archipterygium, although without bony or cartilaginous 

 axis. 



We may now take up the evidence in regard to each of the different 

 theories, using largely the language of Kerr, the paragraphs in quotation 

 marks being taken from his paper. We may first consider Balfour's 

 theory of the lateral fold. 



Balfour's Theory of the Lateral Fold. 



"The evidence in regard to this view may be classed under three 

 heads, as Ontogenetic, Comparative Anatomical, and Paleontological. 

 The ultimate fact on which it was founded was Balfour 's discovery that 

 in certain Elasmobranch embryos, but especially in Torpedo (parcoatis), 

 the fin rudiments were, at an early stage, connected by a ridge of epi- 

 blast. I am not able to make out what were the other forms in which 

 Balfour found this ridge, but subsequent research, in particular by 



