THEORY OF FINS. 19 



Elasmobranchii and the Dipnoi certainly attained their maxi- 

 mum of specialization before the end of the Palaeozoic Epoch. 



The doctrine of the evolution of fins, resulting from the 

 combined researches of embryologists and palaeontologists, may, 

 indeed, now almost rank as a demonstrated principle. Briefly 

 summarized, it resolves itself into the following four proposi- 

 tions : 



(1) .Fishes originally possessed (a) a continuous median 

 .dermal _ fold, and (6) a pair of continuous lateral folds, each 

 supported by a regular scries <>f parallel endoskeletal rods 

 diverging from the axial skeleton. 



(2) These continuous folds (the median in most cases 

 and the lateral fold always) soon became subdivided, with 

 a concomitant reduction in the size and number of their 

 supports. 



(3) Gradual and constant specialization has been 

 marked by the shortening-up of the endoskeletal supports 

 of the resulting fins, and by the concomitant strengthening 

 of the dermal rays. 



(4) In the course of this evolution the endoskeletal 

 fin-supports have eventually lost all direct connection and 

 correlation with the axial skeleton, those of the dorsal and 

 anal median fins becoming correlated instead with the 

 dermal rays, each supporting one of these rays. 



It is necessary to illustrate this doctrine by the few known 

 facts of Paleontology bearing upon the subject; for it will 

 appear later that the only criterion at present available for 

 determining the ordinal position of any fish in its sub-class, is 

 afforded by the degree of specialization of its paired fins. Just 

 as the various modifications of the pentadactyl limb in the 

 Ungulate Mammals the vertebrates which eventually become 

 most completely adapted for progression on land afford the 

 principal means of determining the natural subdivision of the 

 order; so among the greater groups of fishes the vertebrates 

 that become specially adapted for progression in water the 

 successive modifications of the primitive fin-folds form the most 

 obvious clue to the phases through which the various types 

 have passed in the course of their specialization. 



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