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 developed 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 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 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 gillrays 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 that, 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.