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The same truth is exemplified if we turn to certain great periods of the earth's history as, for example, the Mesozoic epoch. There are groups of reptiles, such as the Ichthyosauria and the Plesiosauria, which appear shortly after the commencement of this epoch, and they occur in vast numbers.

For if every species, without exception, has arisen by minute modifications, it seems incredible that a small percentage of such transitional forms should not have been preserved. This, of course, is especially the case as regards the marine Ichthyosauria and Plesiosauria, of which such numbers of remains have been discovered.

But the Amphibia and Plesiosauria, though not themselves primitive vertebrate types, may be thought by some to have derived their limb-structure by direct descent from such. Tortoises, however, must be admitted to be not only highly differentiated organisms, but to be far indeed removed from primeval vertebrate structure.

Among Vertebrates there is no ordinally distinct fossil fish: there is only one extinct order of Amphibia the Labyrinthodonts; but there are at least four distinct orders of Reptilia, viz. the Ichthyosauria, Plesiosauria, Pterosauria, Dinosauria, and perhaps another or two.

The same truth is exemplified if we turn to certain great periods of the earth's history as, for example, the Mesozoic epoch. There are groups of reptiles, such as the Ichthyosauria and the Plesiosauria, which appear shortly after the commencement of this epoch, and they occur in vast numbers.

And if the very small differences which are observable between the Crocodilia of the older Mesozoic formations and those of the present day furnish any sort of approximation towards an estimate of the average rate of change among the Sauropsida, it is almost appalling to reflect how far back in Palaeozoic times we must go, before we can hope to arrive at that common stock from which the Crocodilia, Lacertilia, Ornithoscelida, and Plesiosauria, which had attained so great a development in the Triassic epoch, must have been derived.

But I am surprised to find the Sirenia left in one group with the Cetacea, and the Plesiosauria with the Ichthyosauria; the ordinal distinctness of these having, to my mind, been long since fully established.

Godwin Austin would say comes next, but probably rocks containing more ammonites, and more ichthyosauria and plesiosauria, with a vast number of other things; and under that I should meet with yet older rocks containing numbers of strange shells and fishes; and in thus passing from the surface to the lowest depths of the earth's crust, the forms of animal life and vegetable life which I should meet with in the successive beds would, looking at them broadly, be the more different the further that I went down.

Or among the Teleostei in what respect is the Beryx of the Chalk more embryonic, or less differentiated, than Beryx lineatus of King George's Sound? Or to turn to the higher Vertebrata in what sense are the Liassic Chelonia inferior to those which now exist? How are the Cretaceous Ichthyosauria, Plesiosauria, or Pterosauria less embryonic, or more differentiated, species than those of the Lias?

Among Vertebrates there is no ordinally distinct fossil fish: there is only one extinct order of Amphibia the Labyrinthodonts; but there are at least four distinct orders of Reptilia, viz. the Ichthyosauria, Plesiosauria, Pterosauria, Dinosauria, and perhaps another or two.