74 Morphology of the Fins 



regard these as independently evolved, which would imply that 

 fins of this type are of a very perfect character, and in that 

 case we may be indeed surprised at their so complete disap- 

 pearance in the more highly developed forms, which followed 

 later on." 



As to Gegenbaur's theory it is urged that no form is known 

 in which a gill-septum develops into a limb during the growth 

 of the individual. The main thesis, according to Professor 

 Kerr, "that the archipterygium was derived from gill-rays, is 

 supported only by evidence of an indirect character. Gegen- 

 baur in his very first sugestion of his theory pointed out, as a 

 great difficulty in the way of its acceptance, the position of the 



FIG. 60. Pleuracanthus decheni (Goldfuss). (After Dean.) 



limbs, especially of the pelvic limbs, in a position far removed 

 from that of the branchial arches. This difficulty has been 

 entirely removed by the brilliant work of Gegenbaur's followers, 

 who have shown from the facts of comparative anatomy and 

 embryology that the limbs, and the hind limbs especially, ac- 

 tually have undergone, and in ontogeny do undergo, an extensive 

 backward migration. In some cases Braus has been able to 

 find traces of this migration as far forward as a point just 

 behind the branchial arches. Now, when we consider the 

 numbers, the enthusiasm, and the ability of Gegenbaur's dis- 

 ciples, we cannot help being struck by the fact that the only 

 evidence in favor of this derivation of the limbs has been that 

 which tends to show that a migration of the limbs backwards 

 has taken place from a region somewhere near the last bran- 

 chial arch, and that they have failed utterly to discover any 

 intermediate steps between gill-rays and archipterygial fin. 

 And if for a moment we apply the test of common sense we 

 cannot but be impressed by the improbability of the evolution 

 of a gill-septum, which in all the lower forms of fishes is fixed 



