THE ORIGIN OF VERTEBRATE LIMBS 421 



(2) The supposed demonstration of migration in the pelvic 

 fin at an early period is better explained as due to the concen- 

 tration of the fin from a longer basis, as I will show under another 

 heading. 



(3) The migration of the pelvic, as well as of other fins, has 

 been shown biometrically to occur in accord with the shifting of 

 the center of gravity during development (in Cestracion by Dean 

 '02). By the same method it has been proved that the^ pelvic 

 shifts its position in correlation with the dorsals (in Spinax by 

 Punnett '04) . Hence the observed migration becomes merely an 

 adaptive process without any special meaning in phylogeny. 



(4) While we have no direct evidence that any fin has ever 

 migrated backward to any extent, we have abundant proof of 

 the migration of the pelvic fins forward in many Teleosts, in 

 extreme cases to a position in front of, the pectorals. This, 

 also, can be due only to adaptation. 



(5) i No satisfactory reason has ever been offered why a fin 

 when once in the most important place in the body, viz. the 

 pectoral position, should ever have migrated out of it into a 

 region of such minor importance as the pelvic fin occupies in 

 sharks and other primitive fishes. 



II. The argument based upon the collector nerve is negatived 

 at once by the following facts: 



(1) Collector nerves appear also in the unpaired fins, both 

 in the anterior and posterior parts of the fins, as Paul Mayer 

 ('86) showed in Acanthias, Heptanchus, and Centrophorus, and 

 as my own observations show in the first and second dorsals 

 of Cestracion. Yet it is contrary to the Gegenbaurian con- 

 ception of the unpaired fins that they have migrated at all in 

 their phylogeny. 



(2) A small posterior collector is known in the pelvic fins of 

 certain species. This is assumed by the gill-arch theorists to 

 be due to a secondary migration of the fin in a forward direction. 

 However, the results of Punnett's studies ('00) on Mustelus 

 indicate that in this species the pelvic fin of the female migrates 

 farther forward than that of the male and yet has no posterior 

 collector, while that of the male, which indicates less migration, 

 none the less possesses it. 



