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Analogy seems to me to be rather in favour of, than against, the supposition that while only Ganoid fishes inhabited the fresh waters of our Devonian land, Amphibia and Reptilia, or even higher forms, may have existed, though we have not yet found them.
Even among the 'Reptilia' the class which exhibits the largest proportion of entirely extinct forms of any one type, that of the 'Crocodilia', has persisted from at least the commencement of the Mesozoic epoch up to the present time with so much constancy, that the amount of change which it exhibits may fairly, in relation to the time which has elapsed, be called insignificant.
But the command regarding animal life had followed the other after a short interval, so that we soon see this developing pari passu with the other groups first the lower marine forms and gradually advancing to the Pisces, Amphibia, Reptilia, and then to Aves, as a special division in the second great design group. Lastly the mammals appear and man.
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 instability tends to right itself in the next generation, or that following. Genius as a sport, as well as sudden degeneration of family stock, the whole problem of mutation, may be closely connected with this tendency. It has been noted that the extinction of species has been preceded by a great increase in their size, for example, the case of the great reptilia of prehistoric time.
Every year tends to bring about a greater uniformity of opinion throughout the zoological world as to the limits and characters of these groups, great and small. At present, for example, no one has the least doubt regarding the characters of the classes Mammalia, Aves, or Reptilia; nor does the question arise whether any thoroughly well-known animal should be placed in one class or the other.
If the interpretation of the gigantic foot-steps in the colored sandstone of North America, as belonging to the cursorial birds, is correct, the first appearance of birds falls in the time between the reptilia and mammalia; otherwise the first mammalia would have appeared before the first birds.
I could have shot the Mahar with ease, for I knew intuitively that she was escaping but I did not. I felt that if she could return to her own kind with the story of her adventures, the position of the human race within Pellucidar would be advanced immensely at a single stride, for at once man would take his proper place in the considerations of the reptilia.
Examples of the second fact that individuals of higher classes or orders in former states of their embryonic development represent an organization which corresponds to the full-grown individuals of the lower classes are: the tail of the human embryo, the gill-arches of the embryos of reptilia, of birds, of mammalia, and of man.
Nowhere about them could Bradley see signs of other Wieroos, nor of those other menaces which he had feared might bring disaster to his plans for escape the huge, winged reptilia that are so numerous above the southern areas of Caspak and which are often seen, though in lesser numbers, farther north.
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