7 



Morphology of the Fins 



the rudiments of the pectoral and pelvic fins are at a very 

 early period connected together by a longitudinal ridge of thick- 

 ened epiblast of which indeed they are but exaggerations. In 

 Balfour's own words referring to these observations: 'If the 

 account just given of the development of the limb is an accu- 

 rate 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 ver- 

 tebrate 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 

 synchronously by Thacher and a little later by Mivart in each 

 case based on anatomical investigation of Selachians mainly 



FIG. 58. Shoulder-girdle of Polypterus bichir. Specimen from the White Nile. 



relating to the remarkable similarity of the skeletal arrange- 

 ments in the paired and unpaired fins." 



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

 bridge Philos. Trans., 1899), who has recently given a 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 Crossopterygians, and in the latter types 

 of fishes it has a form analogous 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 in part the language of Kerr, the para- 



