Grass was now spreading over the world, and with this extension arose some huge graminivorous brutes of which no representative survives to-day. Such were the Uintatheres and the Titanotheres. And in pursuit of such beasts came great swarms of primitive dogs, some as big as bears, and the first cats, one in particular (_Smilodon_), a small fierce-looking creature with big knife-like canines, the first sabre-toothed tiger, which was to develop into greater things. American deposits in the Miocene display a great variety of camels, giraffe camels with long necks, gazelle camels, llamas, and true camels. North America, throughout most of the Cainozoic period, appears to have been in open and easy continuation with Asia, and when at last the glaciers of the Great Ice Age, and then the Bering Strait, came to separate the two great continental regions, the last camels were left in the old world and the llamas in the new. In the Eocene the first ancestors of the elephants appear in northern Africa as snouted creatures; the elephant’s trunk dawned on the world in the Miocene. One group of creatures is of peculiar interest in a history that is mainly to be the story of mankind. We find fossils in the Eocene of monkeys and lemurs, but of one particular creature we have as yet not a single bone. It was half ape, half monkey; it clambered about the trees and ran, and probably ran well, on its hind legs upon the ground. It was small-brained by our present standards, but it had clever hands with which it handled fruits and beat nuts upon the rocks and perhaps caught up sticks and stones to smite its fellows. It was our ancestor. § 4 Through millions of simian generations the spinning world circled about the sun; slowly its orbit, which may have been nearly circular during the equable days of the early Eocene, was drawn by the attraction of the circling outer planets into a more elliptical form. Its axis of rotation, which had always heeled over to the plane of its orbit, as the mast of a yacht under sail heels over to the level of the water, heeled over by imperceptible degrees a little more and a little more. And each year its summer point shifted a little further from perihelion round its path. These were small changes to happen to a one-inch ball, circling at a distance of 330 yards from a flaming sun nine feet across, in the course of a few million years. They were changes an immortal astronomer in Neptune, watching the earth from age to age, would have found almost imperceptible. But from the point of view of the surviving mammalian life of the Miocene, they mattered profoundly. Age by age the winters grew on the whole colder and harder and a few hours longer relatively to the summers in a thousand years; age by age the summers grew briefer. On an average the winter snow lay a little later in the spring in each century, and the glaciers in the northern mountains gained an inch this year, receded half an inch next, came on again a few inches....