Meanwhile in each of these periods there were types involuntarily extending the range of life beyond the limits prevailing in that period; and when ages of extreme conditions prevailed, it was these marginal types which survived to inherit the depopulated world. That perhaps is the most general statement we can make about the story of the geological record; it is a story of widening range. Classes, genera, and species of animals appear and disappear, but the range widens. It widens always. Life has never had so great a range as it has to-day. Life to-day, in the form of man, goes higher in the air than it has ever done before; man’s geographical range is from pole to pole, he goes under the water in submarines, he sounds the cold, lifeless darkness of the deepest seas, he burrows into virgin levels of the rocks, and in thought and knowledge he pierces to the centre of the earth and reaches out to the uttermost star. Yet in all the relics of the Mesozoic time we find no certain memorials of his ancestry. His ancestors, like the ancestors of all the kindred mammals, must have been creatures so rare, so obscure, and so remote that they have left scarcely a trace amidst the abundant vestiges of the monsters that wallowed rejoicing in the steamy air and lush vegetation of the Mesozoic lagoons, or crawled or hopped or fluttered over the great river plains of that time.[16] VII THE AGE OF MAMMALS § 1. _A New Age of Light._ § 2. _Tradition Comes into the World._ § 3. _An Age of Brain Growth._ § 4. _The World Grows Hard Again._ § 5. _Chronology of the Ice Age._ § 1 The third great division of the geological record, the Cainozoic, opens with a world already physically very like the world we live in to-day. Probably the day was at first still perceptibly shorter, but the scenery had become very modern in its character. Climate was, of course, undergoing, age by age, its incessant and irregular variations; lands that are temperate to-day have passed, since the Cainozoic age began, through phases of great warmth, intense cold, and extreme dryness; but the landscape, if it altered, altered to nothing that cannot still be paralleled to-day in some part of the world or other. In the place of the cycads, sequoias, and strange conifers of the Mesozoic, the plant names that now appear in the lists of fossils include birch, beech, holly, tulip trees, ivy, sweet gum, bread-fruit trees. Flowers had developed concurrently with bees and butterflies. Palms were now very important. Such plants had already been in evidence in the later levels of the (American Cretaceous) Mesozoic, but now they dominated the scene altogether. Grass was becoming a great fact in the world. Certain grasses, too, had appeared in the later Mesozoic, but only with the Cainozoic period came grass plains and turf spreading wide over a world that was once barren stone.