CHAPTER V 



MORPHOLOGY OF THE FINS 



I RIGIN of the Fins of Fishes. — One of the most interest- 

 ing problems in vertebrate morphology, and one of 

 the most important from its wide-reaching relations, is 

 that of the derivation of the fins of fishes. This resolves 

 itself at once into two problems, the origin of the median fins, 

 which appear in the lancelets, at the very bottom of the fish-like 

 series, and the origin of the paired fins or limbs, which are much 

 more complex, and which first appear with the primitive sharks. 

 In this study the problem is to ascertain not what theoreti- 

 cally should happen, but what, as a matter of fact, has happened 

 in the early history of the fish-like groups. That these struc- 

 tures, with the others in the fish body, have sprung from simple 

 origins, growing more complex with the demands of varied 

 conditions, and then at times again simple, through degenera- 

 tion, there can be no doubt. It is also certain that eacii struc- 

 ture must have had some element of usefulness in all its 

 stages. In such studies we have, as Hsckel has expressed it, 

 " three ancestral documents, paleontology, morphology, and onto- 

 geny " — the actual history as shown by fossil remains, the side- 

 light derived from comparison of structures, and the evidence 

 of the hereditary influences shown in the development of the 

 individual. As to the first of these ancestral documents, the 

 evidence of paleontology is conclusive where it is complete. 

 But in very few cases are we sure of any series of details. The 

 records of geology are hke a book with half its leaves torn out, 

 the other half confused, displaced, and blotted. Still each record 

 actually existing represents genuine history, and in paleontology 

 we must in time find our final court of appeal in all matters 

 of biological origins. 



The evidence of comparative anatomy is most completely 

 secured, but it is often indecisive as to relative age and primi- 



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