of the Pawed Limbs of Vertebrates. 231 



the character of lateral flexure, and not at all for movements in 

 the sagittal plane — which would be not only difficult to achieve 

 but would tend to alternately compress and extend its spinal cord 

 and its viscera. Such a creature would swim through the water 

 as does a Cyclostome, or a Lepidosiren, or any other elongated 

 vertebrate without special swimming organs. Swimming like 

 this, specialization for more and more rapid movement would 

 mean flattening of the tail region and its extension into an at 

 first not separately mobile median tail fold. It is extremely 

 difficult to my mind to suppose that a new purely swimming 

 arrangement should have arisen involving up and down move- 

 ment and which at its first beginnings while useless as a 

 swimming organ in itself must greatly detract from the efficiency 

 of that which already existed. 



We now return to the Gegenbaur view — that the limb is a 

 modified gill septum. 



Resting on Gegenbaur's discovery already mentioned that the 

 gill rays in certain cases assume an arrangement showing great 

 similarity to that of the skeletal elements of the Archipterygium, 

 it has, so far as I am aware, up to the present time received no 

 direct support whatever, of a nature comparable with that found 

 for the rival view in the fact that in certain forces at all events, 

 the limbs actually do arise in the individual in the way that the 

 theory holds they did in Phylogeny. No one has produced either 

 a form in which a gill septum becomes the limb during ontogeny : 

 or the fossil remains of any form which shows an intermediate 

 condition. 



The portion of Gegenbaur's view which asserts that the bi- 

 serial archipterygial fin is of an extremely primitive character is 

 supported by a large body of anatomical facts 1 , and is rendered 

 further probable by the great frequency with which fins ap- 

 parently 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. 



But the main thesis, and the one which concerns us now, 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 out, as a great difficulty in the 



1 I would recall specially the apparently transitional forms between biserial 

 archipterygium and the fin of present day Selachians met with in the Xenacan- 

 thidae, and in Gladodus neilsoni, where the rays on the mesial side of the axis are 

 in course of disappearance, and in Acanthias with its few mesial rays still per- 

 sisting at the top of the metapterygium and most numerous in the young animal. 



