VERTEBRATES. Fishes. Reptiles. Birds. Mammalia. T | Present,.........|.........|.........|........|....... E | | | | | R | Pliocene,........|.........|.........|........|....... T | | | | | I | Miocene,.........|.........|.........|........|....... A | | | | | R | Eocene,..........|.........|.........|..True Mammalia. Y | | | | | | | | | S | Cretaceous,......|.........|.........|........|....... E | | | | | C | Jurassic,........|.........|.........|....Marsupials.. O | | | | N | Triassic,........|.........|.......Birds.............. D | | | A | Permian,.........|.........|.......................... R | | | Y | Carboniferous,...|......Reptiles...................... | P | | R | Devonian,........|.................................... I | | M | Silurian,.......Fishes................................ A | R | Azoic. Y | If such discussions were not inappropriate here from their technical character, I think I could show upon combined geological and zoological evidence that the classes which are not present with the others at the beginning, such as Insects among Articulates, or Reptiles, Birds, and Mammalia among Vertebrates, are always introduced at the time when the conditions essential to their existence are established,--as, for instance, Reptiles, at the period when the earth was not fully redeemed from the waste of waters, and extensive marshes afforded means for the half-aquatic, half-terrestrial life even now characteristic of all our larger Reptiles, while Insects, so dependent on vegetable growth, make their appearance with the first forests; so that we need not infer, because these and other classes come in after the earlier ones, that they are therefore a growth out of them, since it is altogether probable that they would not be created till the conditions necessary for their maintenance on earth were established. From a merely speculative point of view it seems to me natural to suppose that the physical and the organic world have progressed together, and that there is a direct relation between the successive creations and the condition of the earth at the time of those creations. We know that all the beings of the Silurian and Devonian periods were marine; the land, so far as it existed in their time, was a great beach, and along those shores, wherever any part of the continents was lifted above the level of the waters, the Silurian and Devonian animals lived. Later, in the marshes