HKB
presumption that there was also a vast extension in space of the same marsupial forms during that portion of the Secondary epoch which has been termed "the age of reptiles."
As to the class Reptilia, some of the orders which prevailed when the Secondary rocks were formed are confessedly much higher in their organisation than any of the same class now living. If the less perfect ophidians, or snakes, which now abound on the earth had taken the lead in those ancient days among the land reptiles, and the Deinosaurians had been contemporary with Man, there can be no doubt that the progressionist would have seized upon this fact with unfeigned satisfaction as confirmatory of his views. Now that the order of succession is precisely reversed, and that the age of the Iguanodon was long anterior to that of the Eocene Palaeophis and living boa, while the crocodile is in our own times the highest representative of its class, a retrograde movement in this important division of the vertebrata must be admitted. It may perhaps be accounted for by the power acquired by the placental mammalia, when they became dominant, a power before which the class of vertebrata next below them, as coming most directly in competition with them, may more than any other have given way.
For no less than thirty-four years it had been a received axiom in palaeontology that reptiles had never existed before the Permian or Magnesian Limestone period, when at length in 1844 this supposed barrier was thrown down, and Carboniferous reptiles, terrestrial and aquatic, of several genera were brought to light; and discussions are now going on as to whether some remains of an Enaliosaur (perhaps a large Labyrinthodon) have not been detected in the coal of Nova Scotia, and whether certain sandstones near Elgin in Scotland, containing the bones of lacertian, crocodilian, and rhynchosaurian reptiles, may not be referable to the "Old Red" or Devonian group. Still, no traces of this class have yet been detected in rocks as ancient as those in which the oldest fish have been found. [38]
As to fossil representatives of the ichthyic type, the most ancient were not supposed before 1838 to be of a date anterior to the Coal, but they have since been traced back, first to the Devonian, and then to the Silurian rocks. No remains, however, of them or of any vertebrate animal have yet been discovered in the Ordovician strata, rich as these are in invertebrate fossils, nor in the still older Cambrian; so that we seem authorised to conclude, though not without considerable reserve, that the vertebrate type was extremely scarce, if not wholly wanting, in those epochs often spoken of as "primitive," but which, if the Development Theory be true, were probably the last of a long series of antecedent ages in which living beings flourished.
As to the Mollusca, which afford the most unbroken series of geological medals, the highest of that class, the Cephalopoda, abounded in older Silurian times, comprising several hundred species of chambered univalves. Had there been strong prepossessions against the progressive theory, it would probably have been argued that when these cephalopods abounded, and the siphonated gasteropods were absent, a higher order of zoophagous mollusca discharged the functions afterwards performed by an inferior order in the Secondary, Tertiary, and Post-Tertiary seas. But I have never seen this view suggested as adverse to the doctrine of progress, although much stress has been laid on the fact that the Silurian Brachiopoda, creatures of a lower grade, formerly discharged the functions of the existing lamellibranchiate bivalves, which are higher in the scale.