The five great main divisions of the organic history of the earth, or the palæontological history of development, we call the primordial, primary, secondary, tertiary, and quaternary epochs. Each is distinctly characterized by the predominating development of certain animal and vegetable groups in it, and we might accordingly symbolically designate the five epochs, on the one hand by the names of the groups of the vegetable kingdom, and on the other hand by those of the different classes of vertebrate animals. In this case the _first_, or primordial epoch, would be the era of the Tangles (Algæ) and skull-less Vertebrates; the _second_, or primary epoch, that of the Ferns and Fishes; the _third_, or secondary epoch, that of Pine Forests and Reptiles; the _fourth_, or tertiary epoch, that of Foliaceous Forests and of Mammals; finally, the _fifth_, or quaternary epoch, the era of Man, and his Civilization. The divisions or _periods_ which we distinguish in each of the five _long eras_ (p. 14) are determined by the different _systems_ of strata into which each of the five great _rock-groups_ is divided (p. 15). We shall now take a cursory glance at the series of these systems, and at the same time at the populations of the five great epochs. The first and longest division of the organic history of the earth is formed by the _primordial epoch_, or _the era of the Tangle Forests_. It comprises the immense period from the first spontaneous generation, from the origin of the first terrestrial organism, to the end of the Silurian system of deposits. During this immeasurable space of time, which in all probability was much longer than all the other four epochs taken together, the three most extensive of all the neptunic systems of strata were deposited, namely, the _Laurentian_, upon that the _Cambrian_, and upon that the _Silurian_ system. The approximate thickness or size of these three systems together amounts to 70,000 feet. Of these about 30,000 belong to the Laurentian, 18,000 to the Cambrian, and 22,000 to the Silurian system. The average thickness of all the four other rock groups, the primary, secondary, tertiary, and quaternary, taken together, may amount at most to 60,000 feet; and from this fact alone, apart from many other reasons, it is evident that the duration of the primordial period was probably much longer than the duration of all the subsequent periods down to the present day. Many thousands of millions of years were required to deposit such masses of strata. Unfortunately, by far the largest portion of the primordial group of strata is in the metamorphic state (which we shall directly explain), and consequently the petrifactions contained in them—the most ancient and most important of all—have, to a great extent, been destroyed and become unrecognisable. Only in one portion of the Cambrian strata have petrifactions been preserved in a recognizable condition and in large quantities.