544 POPULAR SCIENCE MONTHLY. 



"The portion of Gegenbaur's view which asserts that the biserial 

 archipterygial fin is of an extremely primitive character is supported 

 by a large body of anatomical facts, and is rendered further probable by 

 the great frequency with which fins apparently of this character occur 

 amongst the oldest known fishes. On the lateral fold view we should 

 have to 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 disappearance 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. Gegenbaur in his very first suggestion of his theory pointed 

 cut, as a great difficulty in the way of its acceptance, the position of the 



POLYPTERUS CONGICUS, A CROSSOPTERYGI AN FISH FROM CONGO RlVER ; YOUNG WITH EXTER- 

 NAL Gills. After Boulenger 



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, actually 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 disciples, we cannot help 

 being struck by the fact that the only evidence in favor of this deriva- 

 tion 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 branchial 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 firmly in the body wall, and beneath its 

 surface, into an organ of locomotion. 



' ' May I express the hope that what I have said is sufficient to show 

 in what a state of uncertainty our views are regarding the morphological 



