By the study of the successive beds, more especially of those deposited in the times of continental submergence, we obtain a table of geological chronology which expresses the several stages of the formation of the earth’s crust, from that early time when a solid shell first formed on our nascent planet to the present day. By collecting the fossil remains embedded in the several layers and placing these in chronological order, we obtain in like manner histories of animal and plant life parallel to the physical changes indicated by the beds themselves. The facts as to the sequence we obtain from the study of exposures in cliffs, cuttings, quarries, and mines; and by correlating these local sections in a great number of places, we obtain our general table of succession; though it is to be observed that in some single exposures or series of exposures, like those in the great canons of Colorado, or on the coasts of Great Britain, we can often in one locality see nearly the whole sequence of beds. Let us observe here also that, though we can trace these series of deposits over the whole of the surfaces of the continents, yet if the series could be seen in one spot, say in one shaft sunk through the whole thickness of the earth’s crust, this would be sufficient for our purpose, so far as the history of life is concerned.
The evidence is similar to that obtained by Schliemann on the site of Troy, where, in digging through successive layers of débris, he found the objects deposited by successive occupants of the site, from the time of the Roman Empire back to the earliest tribes, whose flint weapons and the ashes of their fires rest on the original surface of the ground.
Let us now tabulate the whole geological succession with the history of animals and plants associated with it:
| ANIMALS. | SYSTEMS OF FORMATIONS. | PLANTS. | ||
| Age of Man and Mammalia. | Kainozoic. |  | Modern, Pleistocene, Pliocene, Miocene, Eocene. | Angiosperms and Palms dominant. |
| Age of Reptiles. | Mesozoic. | | Cretaceous, Jurassic, Triassic. | Cycads and Pines dominant. |
| Age of Amphibians and Fishes. Age of Invertebrates. | Palæozoic. | | Permian, Carboniferous, Erian, Silurian, Ordovician, Cambrian, Huronian (Upper). | Acrogens and Gymnosperms dominant. |
| Age of Protozoa. | Eozoic. | | Huronian (Lower), Upper Laurentian, Middle Laurentian, Lower Laurentian. | Protogens and Algæ. |
It will be observed, since only the latest of the systems of formations in this table belongs to the period of human history, that the whole lapse of time embraced in the table must be enormous. If we suppose the modern period to have continued for say ten thousand years, and each of the others to have been equal to it, we shall require two hundred thousand years for the whole. There is, however, reason to believe, from the great thickness of the formations and the slowness of the deposition of many of them in the older systems, that they must have required vastly greater time. Taking these criteria into account, it has been estimated that the time-ratios for the first three great ages may be as one for the Kainozoic to three for the Mesozoic and twelve for the Palæozoic, with as much for the Eozoic as for the Palæozoic. This is Dana’s estimate. Another, by Hull and Houghton, gives the following ratios: Azoic, 34·3 per cent.; Palæozoic, 42·5 per cent.; Mesozoic and Kainozoic, 23·2 per cent. It is further held that the modern period is much shorter than the other periods of the Kainozoic, so that our geological table may have to be measured by millions of years instead of thousands.
We cannot, however, attach any certain and definite value in years to geological time, but must content ourselves with the general statement that it has been vastly long in comparison to that covered by human history.
Bearing in mind this great duration of geological time, and the fact that it probably extends from a period when the earth was intensely heated, its crust thin, and its continents as yet unformed, it will be evident that the conditions of life in the earlier geologic periods may have been very different from those which obtained later. When we further take into account the vicissitudes of land and water which have occurred, we shall see that such changes must have produced very great differences of climate. The warm equatorial waters have in all periods, as superficial oceanic currents, been main agents in the diffusion of heat over the surface of the earth, and their distribution to north and south must have been determined mainly by the extent and direction of land, though it may also have been modified by the changes in the astronomical relations and period of the earth, and the form of its orbit.[A] We know by the evidence of fossil plants that changes of this kind have occurred so great as, on the one hand, to permit the plants of warm temperate regions to exist within the Arctic Circle; and, on the other, to drive these plants into the tropics and to replace them by Arctic forms. It is evident also that in those periods when the continental areas were largely submerged, there might be an excessive amount of moisture in the atmosphere, greatly modifying the climate, in so far as plants are concerned.
[A] Croll, “Climate and Time.”
Let us now consider the history of the vegetable kingdom as indicated in the few notes in the right-hand column of the table.