basins, ridges of elevated land, and broad plateaus intervening between the ridges, and which were at some times under water, and at other times land, with many intermediate phases. The settlement and crumpling of the crust were not continuous, but took place at intervals; and each such settlement produced not only a ridging up along certain lines, but also an emergence of the plains or plateaus. Thus at all times there have been ridges of folded rock constituting mountain-ranges, flat expansions of continental plateau, sometimes dry and sometimes submerged, and deep ocean-basins, never except in some of their shallower portions elevated into land.

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 cañons 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 obtain mann on the site of Troy, where, in digging cessive layers of débris, he found the object successive occupants of the site, from the Roman Empire back to the earliest tribes weapons and the ashes of their fires rest o surface of the ground.

Let us now tabulate the whole geologi with the history of animals and plants assoc

Αι

Cy

Acr

P

It will be observed, since only the late tems of formations in this table belongs to human history, that the whole lapse of tim the table must be enormous. If we suppos period to have continued for say ten thousa each of the others to have been equal to it. quire two hundred thousand years for the w is, however, reason to believe, from the grea the formations and the slowness of the depos

btained by Schlieging through sucjects deposited by the time of the ribes, whose flint est on the original Logical succession ssociated with it:

PLANTS.

Angiosperms and
Palms dominant.

Cycads and Pines dominant.

crogens and Gymnosperms dominant.

Protogens and
Algæ.

st of the systhe period of embraced in e the modern nd years, and we shall rehole. There

t thickness of sition of many

of them in the older systems, that they must have re quired vastly greater time. Taking these criteria int account, it has been estimated that the time-ratios for the first three great ages may be as one for the Kainozoi to three for the Mesozoic and twelve for the Paleozoic with as much for the Eozoic as for the Paleozoic. This i Dana's estimate. Another, by Hull and Houghton, give 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 shorte than the other periods of the Kainozoic, so that ou geological table may have to be measured by millions o years instead of thousands.

We cannot, however, attach any certain and definit value in years to geological time, but must content our selves 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 wher the earth was intensely heated, its crust thin, and its con tinents as yet unformed, it will be evident that the con ditions of life in the earlier geologic periods may hav 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 tha such changes must have produced very great difference of climate. The warm equatorial waters have in al periods, as superficial oceanic currents, been main agent in the diffusion of heat over the surface of the earth, and their distribution to north and south must have beer 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.* We know by the evidence o

fossil plants that changes of this kind ha great as, on the one hand, to permit the p temperate regions to exist within the Arcti on the other, to drive these plants into t to replace them by Arctic forms. It is evi in those periods when the continental area submerged, there might be an excessive am ure in the atmosphere, greatly modifying t so far as plants are concerned.

Let us now consider the history of the v dom as indicated in the few notes in th column of the table.

The most general subdivision of plants i great series of Cryptogams, or those which fest flowers, and produce minute spores ins and Phænogams, or those which possess flo duce seeds containing an embryo of the fut The Cryptogams may be subdivided into three groups:

1. Thallogens, cellular plants not dist guishable into stem and leaf. These are t Lichens, and the Algæ, or sea-weeds.

2. Anogens, having stem and foliage, b lular. These are the Mosses and Liverwort 3. Acrogens, which have long tubular fil cells in their composition, and thus have th attaining a more considerable magnitude. Ferns (Filices), the Mare's-tails (Equisetac Club-mosses (Lycopodiacea), and a curious of aquatic plants called Rhizocarps (Rhizoca The Phænogams are all vascular, but the in the simplicity or complexity of their flo On this ground they admit of a twofold divi

1. Gymnosperms, or those which bear not enclosed in fruits. They are the Pin allies, and the Cycads.

2. Angiosperms, which produce true fruits enclosing the seeds. In this group there are two well-marked subdivisions differing in the structure of the seed and stem. They are the Endogens, or inside growers, with seeds having one seed-leaf only, as the grasses and the palms; and the Exogens, having outside-growing woody stems, and seeds with two seed-leaves. Most of the ordinary foresttrees of temperate climates belong to this group.

On referring to the geological table, it will be seen that there is a certain rough correspondence between the order of rank of plants and the order of their appearance in time. The oldest plants that we certainly know are Algæ, and with these there are plants apparently with the structures of Thailophytes but the habit of trees, and which, for want of a better name, I may call Protogens. Plants akin to the Rhizocarps also appear very early. Next in order we find forests in which gigantic Ferns and Lycopods and Mare's-tails predominate, and are associated with pines. Succeeding these we have a reign of Gymnosperms, and in the later formations we find the higher Phænogams dominant. Thus there is an advance in elevation and complexity along with the advance in geological time, but connected with the remarkable fact that in earlier times low groups attain to an elevation unexampled in later times, when their places are occupied with plants of higher type.

It is this historical development that we have to trace in the following pages, and it will be the most simple and at the same time the most instructive method to consider it in the order of time.