396 THE WORLD OF LIFE 



tial to its full and continuous development. There is, I be- 

 lieve, no more curious and important series of phenomena con- 

 nected 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 vet some further relations of water to life 

 which may be here briefly noticed. Among the various agen- 

 cies 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 v/ork 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 over- 

 lap, as it w^ere, 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 decom- 

 posed by rain, then fractured by the irresistible force of ice- 

 formation. On a large scale in polar regions, and everywhere 

 at great altitudes, snowfields and permanent glaciers are 

 formed, which not only carry down enormous quantities of 

 debris on their surfaces or embedded in their substance, but 

 with the help of that which is carried along the valley-floors 

 they rest on, and by the enormous weight of the ice itself often 



