The same moral is inculcated by the study of every other order of Tertiary monodelphous _Mammalia_. Each of these orders is represented in the Miocene epoch: the Eocene formation, as I have already said, contains _Cheiroptera, Insectivora, Rodentia, Ungulata, Carnivora,_ and _Cetacea_. But the _Cheiroptera_ are extreme modifications of the _Insectivora_, just as the _Cetacea_ are extreme modifications of the Carnivorous type; and therefore it is to my mind incredible that monodelphous _Insectivora_ and _Carnivora_ should not have been abundantly developed, along with _Ungulata_, in the Mesozoic epoch. But if this be the case, how much further back must we go to find the common stock of the monodelphous _Mammalia_? As to the _Didelphia_, if we may trust the evidence which seems to be afforded by their very scanty remains, a Hypsiprymnoid form existed at the epoch of the Trias, contemporaneously with a Carnivorous form. At the epoch of the Trias, therefore, the _Marsupialia_ must have, already existed long enough to have become differentiated into carnivorous and herbivorous forms. But the _Monotremata_ are lower forms than the _Didelphia,_ which last are intercalary between the _Ornithodelphia_ and the _Monodelphia_. To what point of the Palaeozoic epoch, then, must we, upon any rational estimate, relegate the origin of the _Monotremata_? The investigation of the occurrence of the classes and of the orders of the _Sauropsida_ in time points in exactly the same direction. If, as there is great reason to believe, true Birds existed in the Triassic epoch, the ornithoscelidous forms by which Reptiles passed into Birds must have preceded them. In fact there is, even at present, considerable ground for suspecting the existence of _Dinosauria_ in the Permian formations; but, in that case, lizards must be of still earlier date. 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.