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respiratory organs of the Marsipobranchii with those of the amphibians,-it is possible that frogs and salamanders may be directly descended from beings closely analogous to the division of the Marsipobranchii termed Myxine. It is to be hoped that this highly interesting observation may soon be made public. We gather from the general Ontogenesis of the amphibians, that the tailed forms are the most ancient. This is also the case with the oldest amphibian-like animals, the Labyrinthodonts. From their remains (Archegosaurus and others), chiefly contained in the Carboniferous formation, we have learnt that they had incomplete limbs or none, that their ventral side was partially provided with bony plates, the vertebral column fish-like, and that their skull, with some of the characters of the present amphibians, combined others which remind us partly of certain bony Ganoids, and partly of the reptiles which subsequently appeared. Now if in the singularly elongated snake-like Cæcilia, which is however without tail or limbs, some peculiarities of the skull of the Labyrinthodont appear again, we must own our utter ignorance as to the actual progenitors of this, as well as of the two other living orders of the Coecilia and the Batrachians. Here, therefore, we are, as we have said, thrown entirely on the evolutionary history of the individual. By what right we may frame a picture with great probability approaching the truth, the reader may have gathered from our previous chapters.
Among the tailed amphibians, it is not only in Ontogenesis that we see the passage from gill to lungbreathing; the systematic series from the proteus to the triton and the salamander, likewise exhibits this
physiological ascent, linked with various morphological transformations, which may similarly be shown between the ancient and modern specimens of the Labyrinthodonts. The Batrachians, indeed, rise higher in development than the Coecilia; but, as the friend above mentioned informs me, they more nearly approach the Myxine in the construction of the internal gills of their larva. We shall obtain a general view of the reptiles by means of the appended diagram, in which we shall avoid any minute systematic designations.
The class presents a very comprehensive picture, although only four orders now exist, of which two, the lizards and the snakes, are scarcely to be separated. That the snakes, which first appear in the Tertiary period, are a direct offshoot from the lizards, is reduced to a certainty by comparative anatomy and the history of development. In the various families of lizards we see the absence of feet occurring in conjunction with the elongation of the body and the multiplication of the vertebræ; and the modifications peculiar to the skull of the "true" snakes are likewise represented in the systematic series in every gradation, beginning with the skull of the true lizard. We cannot specify the fossil genera in which the transformation was initiated; but in this case a doubt would be only a capricious denial. It is otherwise with the remaining orders, which in the beginnings, hitherto accessible to us, exhibit diversities so decidedly marked, that in none has it been possible to trace a direct descent from any known member of another. Prof. Huxley, a great authority on the anatomy of these animals, says on this subject as follows:
"If we ask, in what manner the earliest representatives of these orders are distinguished from their living or latest known representatives, we shall find, in all cases, that the amount of difference in itself is remarkably small in comparison with the length of time during which the order has existed. So far as I know, there is no fact to show that the later Plesiosauria, or Ichthyosauria, exhibit an advance upon the earlier members of the group. It is not clear that the Dinosauria of the wealden and of the Cretaceous formations are more highly organized than those of the Trias; and even where a
differentiation of structure is to be observed, as in the Lacertilia, or Crocodilia, it goes no further than a modification of the form of the articular surfaces of the vertebræ, or of the degree to which the internal nasal apertures are surrounded by bone. The osteological differences, which alone are exhibited by fossil remains, have doubtless been accompanied by many changes in the organization of the destructible parts of the body; but everything tends to show that the amount of change in the organization of reptiles since their first known appearance upon the earth, is not great in itself; and is wholly insignificant, if we take into consideration the lapse of time, and the changes of the surface of the globe, which are represented by the Mesozoic and Tertiary formations.
"From the point of view of the evolution hypothesis, it is necessary to suppose that the Reptilia have all sprung from a common stock, and I see no justification for the supposition that the rapidity of their divergence from this stock was greater before the epoch of the Trias than it has been since. Consequently, seeing that the approximation of the oldest known representatives of the different orders is so slight, reptiles must have lived before the Trias for a length of time, compared with which that which has elapsed from the Triassic epoch until now is small-in other words, the commencement of the existence of reptiles must be sought in a remote palæozoic epoch."
Comparison thus points us back to ages which afford no record of the actual derivation of this class. Even the Ichthyosauria and Plesiosauria, so frequently mentioned in conjunction, deviate widely from one another
in very essential characters, which refer their supposed common origin to a remote period. We will mention only the fin-like extremities of the former, which are of an obviously piscine type. We are thus thrown back vaguely on such mixed forms as may have been analogous to the Labyrinthodont; nay, the question arises whether the Ichthyosauria alone, or perhaps the Plesiosauria with them, did not diverge from the fishes independently of the other branches of the reptile family; an eventuality which is taken into account in the pedigree at p. 250. A certain resemblance with the skull of the tortoises (Chelonia) is exhibited by that of the Dicynodonta. In them also the jaws, as appears from their shape, were manifestly cased in horny sheaths; but at the same time the upper jaw contained two huge tusks, and it is scarcely possible to imagine a direct transition from the Dicynodonta,. appearing in the Trias, to the more recent tortoise. In some particulars of the skull, as well as in the situation of the posterior nasal apertures, the forms of older crocodiles exhibit an affinity with the lizards, from the older and unknown forms, of which they probably branched off. The winged saurians, or Pterodactyles, may also be a branch of the lizards. They have gained by adaptation several characters, such as the shape and lightness of head, the length, slenderness, and pneumatic character of the tubular bones, which they share with the birds. But it is not in them, but in the division comprising several families which Huxley terms Ornithoscelidæ, or reptiles with the legs of a bird, that we must look for the actual progenitors of the birds. For among them one of the most important characters of the birds is, in