xviii THE ELEMENTS AND LIFE 367 



oceanic basins ; the proofs of their permanence throughout 

 all geological time ; the probable causes of that permanence ; 

 the necessity of such permanence to preserve the continuity 

 of life-development, not only on the earth as a whole, but on 

 each of the great continents ; and, lastly, how all these 

 phenomena have combined to secure that general uniformity 

 of climatic conditions throughout the whole period of the 

 existence of terrestrial life which was essential to its full 

 and continuous development. There is, I believe, no more 

 curious and important series of phenomena connected with 

 the possibilities of life upon the earth than those described 

 in the chapters above referred to. 



Water as Preparing the Earth for Man 



There remain yet some further relations of water to life 

 which may be here briefly noticed. Among the various 

 agencies that have modelled and remodelled the earth's 

 surface, water has played the most important part. It is to 

 water that we owe its infinite variety, its grandeur, its 

 picturesqueness, its adaptability to a highly varied vegetable 

 and animal life ; and this work has been carried out through 

 its manifold physical and chemical properties. It is in its 

 three states, solid, liquid, and gaseous, that water exerts its 

 most continuous and effective powers ; and it is enabled to 

 do this because, though each of these has its own limited 

 range of temperature, they yet overlap, as it were, and can 

 therefore act in unison. Thus within the narrow limits of 

 temperature adapted to organic life we have both ice and 

 water-vapour as well as liquid water, in almost continuous 

 action. Through dew, mist, and rain, water penetrates 

 every fissure of the rocks ; through the carbonic acid gas 

 dissolved in it, the rocks are slowly decomposed ; by the 

 expansion of water between 39° and 32° F. it freezes in the 

 upper parts of the fissures, and when the temperature continues 

 to fall the further expansion during ice-crystallisation forces 

 the rocks asunder. The most massive rocks at high altitudes 

 are first cracked and fissured by expansion and contraction 

 due to alternations of temperature caused by sun-heat, then 

 decomposed by rain, then fractured by the irresistible force 

 of ice-formation. On a large scale in polar regions, and 





