HAI THIS IS GIRISH
The story of the earth from the beginning of the Cambrian period to the present day was long ago divided by geologists into four great eras. The periods we have already covered—the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian—form the Primary or Palaeozoic Era, to which the earlier Archaean rocks were prefixed as a barren and less interesting introduction. The stretch of time on which we now enter, at the close of the Permian, is the Secondary or Mesozoic Era. It will be closed by a fresh upheaval of the earth and disturbance of life-conditions in the Chalk period, and followed by a Tertiary Era, in which the earth will approach its modern aspect. At its close there will be another series of upheavals, culminating in a great Ice-age, and the remaining stretch of the earth's story, in which we live, will form the Quaternary Era.
In point of duration these four eras differ enormously from each other. If the first be conceived as comprising sixteen million years—a very moderate estimate—the second will be found to cover less than eight million years, the third less than three million years, and the fourth, the Age of Man, much less than one million years; while the Archaean Age was probably as long as all these put together. But the division is rather based on certain gaps, or "unconformities," in the geological record; and, although the breaches are now partially filled, we saw that they correspond to certain profound and revolutionary disturbances in the face of the earth. We retain them, therefore, as convenient and logical divisions of the biological as well as the geological chronicle, and, instead of passing from one geological period to another, we may, for the rest of the story, take these three eras as wholes, and devote a few chapters to the chief advances made by living things in each era. The Mesozoic Era will be a protracted reaction between two revolutions: a period of low-lying land, great sea-invasions, and genial climate, between two upheavals of the earth. The Tertiary Era will represent a less sharply defined depression, with genial climate and luxuriant life, between two such upheavals.
The Mesozaic ("middle life") Era may very fitly be described as the Middle Ages of life on the earth. It by no means occupies a central position in the chronicle of life from the point of view of time or antiquity, just as the Middle Ages of Europe are by no means the centre of the chronicle of mankind, but its types of animals and plants are singularly transitional between the extinct ancient and the actual modern types. Life has been lifted to a higher level by the Permian revolution. Then, for some millions of years, the sterner process of selection relaxes, the warm bosom of the earth swarms again with a teeming and varied population, and a rich material is provided for the next great application of drastic selective agencies. To a poet it might seem that nature indulges each succeeding and imperfect type of living thing with a golden age before it is dismissed to make place for the higher.
The Mesozoic opens in the middle of the great revolution described in the last chapter. Its first section, the Triassic period, is at first a mere continuation of the Permian. A few hundred species of animals and hardy plants are scattered over a relatively bleak and inhospitable globe. Then the land begins to sink once more. The seas spread in great arms over the revelled continents, the plant world rejoices in the increasing warmth and moisture, and the animals increase in number and variety. We pass into the Jurassic period under conditions of great geniality. Warm seas are found as far north and south as our present polar regions, and the low-lying fertile lands are covered again with rich, if less gigantic, forests, in which hordes of stupendous animals find ample nourishment. The mammal and the bird are already on the stage, but their warm coats and warm blood offer no advantage in that perennial summer, and they await in obscurity the end of the golden age of the reptiles. At the end of the Jurassic the land begins to rise once more. The warm, shallow seas drain off into the deep oceans, and the moist, swampy lands are dried. The emergence continues throughout the Cretaceous (Chalk) period. Chains of vast mountains rise slowly into the air in many parts of the earth, and a new and comparatively rapid change in the vegetation—comparable to that at the close of the Carboniferous—announces the second great revolution. The Mesozoic closes with the dismissal of the great reptiles and the plants on which they fed, and the earth is prepared for its new monarchs, the flowering plants, the birds, and the mammals.