III. 

 The Evolution of Fins. 



OF the multitudinous theories relating to the origin and develop- 

 ment of fin-structures in fishes, some may already be transferred 

 to the rank of definite principles, and others are rapidly advancing 

 in probability. The combined results of Comparative Anatomy, 

 Embryology, and Palaeontology have already provided ample 

 material for generalisation in certain directions, and the latest 

 discoveries among the early Palaeozoic Fishes make known several 

 facts in regard to the primitive characters of fins which it seems 

 impossible to misinterpret. 



It is now the prevalent — perhaps universal — belief that the 

 paired fins of fishes, or those which correspond to the four limbs of 

 lung-breathers, are essentially the same in nature as the median fins 

 which fringe the back and tail. This view is supported, not merely 

 by the identity of structure so frequently observed between the paired 

 and median fins, but also by the presence, in certain embryos of 

 sharks and skates, of a transitory ridge of thickened epiblast between 

 the pectoral and pelvic fins on each side, precisely like the original 

 ridge from which the median fin-fold arises. Since the median fins 

 are proved to result from various modifications of a once-continuous 

 fold of skin and its skeletal supports, it is thus not too much to 

 assume that the paired fins have likewise become differentiated from 

 a similar fold, once extending continuously along each side of the 

 trunk. If, therefore, the primitive character of the median fin-fold 

 can be ascertained, and the various phases in its evolution traced, it 

 ought to be possible to discover the mode and order of development 

 of the more modified paired fins. 



Through the numerous discoveries of Elasmobranch skeletons in 

 the Carboniferous and Permian Formations of Europe, recently 

 described, and from existing knowledge of many other early fishes, 

 the primitive arrangement of the supports of the median fin-fold is 

 fortunately well ascertained. There is no longer any doubt that the 

 series of parallel vertical rods of cartilage supporting this fin-fold is 

 directly continuous with the series of neural and haemal arches of the 

 axial skeleton of the trunk. In other words, the endoskeletal fin- 

 supports are identical in origin with the processes of the vertebrae. 

 This is well shown in the Pleuracanthidae, in which the dorsal fin- 

 supports of the caudal region are equal in number to the neural 



