426 OSBURN 



appearance of those in the pectoral. This again shows their 

 adaptive nature. 



(5) In [higher vertebrates which have taken up aquatic life 

 we have well-known examples of extra or supernumerary digits 

 formed adaptively on the post-axial side of the limb. This 

 in the whales is proved to have taken place by a longitudinal 

 division of the fifth digit (Kukenthal '88 and Symington '06). 

 A migration of the fifth digit into a well-marked post-axial 

 position for adaptive purposes is also well illustrated in certain 

 aquatic reptiles and mammals, as I have elsewhere indicated 

 (Osburn '06). 



IV. Kerr ('99) pointed out that the real stumbling-block 

 for those who have found themselves unable to accept the fin- 

 fold theory lies in the fact that the fin-rays do not arise separately 

 and later become fused to form the basalia. However, when 

 we examine the mode of formation of the fin skeleton this diffi- 

 culty disappears. It must be noted that the skeleton structures 

 of the fin are the last of all to develop, and that the fin already 

 has approximately its permanent shape when the skeleton begins 

 to take form. It consequently makes its appearance as an 

 adapted structure, suited to the mechanical needs of the fin 

 at the time it develops. In the unpaired fins, which the gill-arch 

 theorists consider primitively metameric, the process is precisely 

 similar to that in the paired fins. Wherever, in the unpaired 

 fins, the rays have become joined to form basalia, these basalia 

 are present from the first, according to the writer's studies, 

 just as they are in the paired fins, and they are not formed by 

 the fusion of separate rays. There is, then, no more difficulty 

 in accepting the origin of the paired fin basalia from rays than 

 there is in accepting such an origin for the unpaired fins, 

 since both proceed exactly alike. In neither case is there any 

 fusion of once discrete rays to form basalia, but in both the 

 basalia are adaptively formed as such from the first, due to the 

 failure of the mesenchyme to differentiate into smaller elements. 



V. The opponents of the fin-fold theory have lately insisted 

 (Furbringer '02, Braus '04) that the fusion of the muscle-buds 

 described by Mollier ('93) in the paired fins proves the original 

 dysmetamery of the paired fin skeleton, since the fusion to 



