MODERN science tells us that the earth is but a tiny fragment of the sun, broken off from it countless ages ago, and that the sun itself is but one of innumerable stars scattered here and there through space. When we consider that the volume of the sun is over a million times greater than that of the earth, we realise how tiny is the stage on which human history has been worked out. At first the earth was nothing but molten matter, and countless ages passed before its surface cooled, and the first rocks could form. These rocks, produced by the heat of the inside of the earth, are called igneous (fiery) rocks. Then the steam-clouds condensed into rain, and the water began to deposit the various kinds of matter it carried with it. The deposits, in time, formed another group of rocks, which are called stratified, because they follow one on top of another in regular layers or strata. The arrangement of these layers has, however, been complicated bewilderingly by upheavals and sinkings of the earth's crust. For the interior, eight thousand miles deep, on which this crust rests, is still cooling and contracting, hence volcanoes and earthquakes. All the igneous rocks and half of the stratified rocks preserve no trace of life and are therefore called Azoic (lifeless). Then the greatest of miracles happens, and life appears where there was none before. Preserved in the rocks are found shells and worm-tracks of creatures that arose in shallow tidal waters. These were followed by a higher organism, the fishes, whose backbones (vertebrae) gave them a stronger structure than their invertebrate predecessors. The next step was the invasion of the land by plants and animals, which, by developing new organs to deal with the new conditions of breathing, could live partly on sea and partly on the land. A continuous succession then leads us to the Mesozoic (middle life) age, with its terrifying dinosaurs, huge animals up to one hundred feet in length, bulky but comparatively brainless. But the race was not to the physically strong, and they were replaced by the mammals, who are the leading type of the next Cainozoic (new life) age. The mammals suckle and tend their young, and from this connection of child and parent arises mutual understanding. The brain grows because of the interplay with another brain, and becomes more able to meet changing conditions.