4. The discussions touching the _Eozoon,_ which commenced in 1864, have abundantly justified the fourth proposition. In 1862, the oldest record of life was in the Cambrian rocks; but if the _Eozoon_ be, as Principal Dawson and Dr. Carpenter have shown so much reason for believing, the remains of a living being, the discovery of its true nature carried life back to a period which, as Sir William Logan has observed, is as remote from that during which the Cambrian rocks were deposited, as the Cambrian epoch itself is from the tertiaries. In other words, the ascertained duration of life upon the globe was nearly doubled at a stroke. 5. The significance of persistent types, and of the small amount of change which has taken place even in those forms which can be shown to have been modified, becomes greater and greater in my eyes, the longer I occupy myself with the biology of the past. Consider how long a time has elapsed since the Miocene epoch. Yet, at that time there is reason to believe that every important group in every order of the _Mammalia_ was represented. Even the comparatively scanty Eocene fauna yields examples of the orders _Cheiroptera, Insectivora, Rodentia_, and _Perissodactyla_; of _Artiodactyla_ under both the Ruminant and the Porcine modifications; of _Caranivora, Cetacea_, and _Marsupialia_. Or, if we go back to the older half of the Mesozoic epoch, how truly surprising it is to find every order of the _Reptilia_, except the _Ophidia_, represented; while some groups, such as the _Ornithoseclida_ and the _Pterosauria_, more specialised than any which now exist, abounded. There is one division of the _Amphibia_ which offers especially important evidence upon this point, inasmuch as it bridges over the gap between the Mesozoic and the Palaeozoic formations (often supposed to be of such prodigious magnitude), extending, as it does, from the bottom of the Carboniferous series to the top of the Trias, if not into the Lias. I refer to the Labyrinthodonts. As the Address of 1862 was passing through the press, I was able to mention, in a note, the discovery of a large Labyrinthodont, with well-ossified vertebrae, in the Edinburgh coal-field. Since that time eight or ten distinct genera of Labyrinthodonts have been discovered in the Carboniferous rocks of England, Scotland, and Ireland, not to mention the American forms described by Principal Dawson and Professor Cope. So that, at the present time, the Labyrinthodont Fauna of the Carboniferous rocks is more extensive and diversified than that of the Trias, while its chief types, so far as osteology enables us to judge, are quite as highly organised. Thus it is certain that a comparatively highly organised vertebrate type, such as that of the Labyrinthodonts, is capable of persisting, with no considerable change, through the period represented by the vast deposits which constitute the Carboniferous, the Permian, and the Triassic formations.