III. _Mountains suffer themselves to be slowly worn away in order that the face of the earth may be renewed; in other words, they die that we, and all created things, may live._ The reader will find a full account of the methods by which these results are accomplished in chapters v. and vii., and therefore we must not anticipate this part of the subject. Let it suffice for the present to say that this destruction of the hills is brought about by the action of heat and cold, of rain and frost, of snow and ice, and the thousand streams that flow down the mountain-sides. It is with soils that we are chiefly concerned at present. Try to think for a moment of the literally _vital_ consequences which follow from the presence of good rich soils over different parts of the earth, and ask whether it would be possible for civilised races of men to flourish and multiply as they do if it were not for the great fertile valleys and plains of the world. Mountain races are neither rich nor powerful. Man exists mainly by cultivation of the soil; and among mountains we only find here and there patches that are worthy of the labour and expenditure of capital involved in cultivation. But in the great plains, in the principal river-valleys of the world, and among the lesser hill-ranges it is different. The _lowlands_ are the fertile regions. All great and powerful nations of the world are children of the plains. It was so in the past; it will be so in the future, unless men learn to feed on something else than corn, milk, and flesh, which is not very likely. The Egyptians, the earliest civilised race of which we have satisfactory records, dwelt in the fertile valley and delta of the Nile. They clearly perceived the value of this great river to themselves, and worshipped it accordingly. They knew nothing of its source in the far-away lakes of Central Africa; but they knew truly, as Herodotus tells us, that Egypt was "the gift of the Nile," for the alluvial soil of its delta has been formed by the yearly floods of that great river, as its waters, laden with a fine rich mud, spread over its banks, and for a time filled the valley with one sheet of water. The Assyrians and Babylonians had their home in the valley of the Euphrates and Tigris. The Chinese, too, have their great rivers. Russia is well watered by powerful rivers. The most populous parts of the United States of America are watered by the great Mississippi, and the other rivers which flow into it. England, Germany, and France are furnished with well-watered plains.