ORIGIN OF THE FINS OF FISHES. 543 



Objections to the Theory of the Lateral Fold. 



The objection to the first view is its precarious foundation. Such 

 lateral folds are found only in certain rays, in. which they may be de- 

 veloped as a secondary modification in connection with the peculiar form 

 of these fishes. Professor Kerr observes that this theory must be looked 

 upon and judged : 



"Just as any other view at the present time regarding the nature 

 of the vertebrate limb, rather as a speculation, brilliant and suggestive 

 though it be, than as a logically constructed theory of the now known 

 facts. It is, I think, on this account allowable to apply to it a test of a 

 character which is admittedly very apt to mislead, that of 'common 

 sense.' 



"If there is any soundness in zoological speculation at all I think 

 it must be admitted that the more primitive vertebrates were creatures 

 possessing a notochordal axial skeleton near the dorsal side, with the 

 main nervous axis above it, the main viscera below it, and the great 

 mass of muscle lying in myotomes along its sides. Now such a creature 

 is well adapted to movements of 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 flatten- 

 ing 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 movement, and which, at its first beginnings, while useless 

 as a swimming organ itself, must greatly detract from the efficiency of 

 that which already existed. ' ' 



Objections to Gegenbaur's Theory. 



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 

 cf a nature comparable with that found for the rival view in the fact 

 i: ut, in certain forms 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. 



