536 POPULAR SCIENCE MONTHLY. 



OEIGIN OF THE FINS OF FISHES. 



By President DAVID STARR JORDAN, 



LELAND STANFORD .IK. UNIVERSITY. 



ONE of the most interesting 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 theoretically 

 should happen, but what, as a matter of fact, has happened in the early 

 history of the fish-like groujjs. That these structures, with the others 

 in the fish body, have sprung from simple origins, growing more com- 

 plex with the demands of varied conditions, and then at times again 

 simple, through degeneration, there can be no doubt. It is also certain 

 that each structure must have had some element of usefulness in all 

 its stages. In such studies we have, as Haeckel has expressed it, 'three 

 ancestral documents, paleontology, morphology and ontogeny,' — 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 like a book with half its leaves torn out, the 

 other half confused, displaced and blotted. 



The evidence of comparative anatomy is most completely secured, 

 but it is often indecisive as to relative age and primitiveness of origin 

 among structures. As to ontogeny, it is, of course, true that through 

 heredity, 'the life history of the individual is an epitome of the life 

 history of the race.' 'Ontogeny repeats phytogeny, ' and phylogeny, or 

 line of descent of organisms and structures, is what we are seeking. 

 But here the repetition is never perfect, never so perfect in fact as 

 Haeckel and his followers expected to find it. The demands of natural 

 selection may lead to the lengthening, shortening, or distortion of 

 phases of growth, just as they may modify adult conditions. The con- 

 ditions of the individual development may, therefore, furnish evidence 

 in favor of certain theories of origins, but they cannot alone furnish the 

 absolute proof. 



