The Story of Evolution 59 



miles. But it grew by drawing planetesimals into itself until it 

 had a diameter of over 8,100 miles at the end of its growing 

 period. Since then it has shrunk, by periodic shrinkages which 

 have meant the buckling up of successive series of mountains, 

 and it has now a diameter of 7,918 miles. But during the shrink- 

 ing the earth became more varied. 



A sort of slow boiling of the internally hot earth often forced 

 molten matter through the cold outer crust, and there came about 

 a gradual assortment of lighter materials nearer the surface and 

 heavier materials deeper down. The continents are built of the 

 lighter materials, such as granites, w^hile the beds of the great 

 oceans are made of the heavier materials such as basalts. In 

 limited areas land has often become sea, and sea has often given 

 place to land, but the probability is that the distinction of the 

 areas corresponding to the great continents and oceans goes back 

 to a very early stage. 



The lithosphere is the more or less stable crust of the earth, 

 which may have been, to begin with, about fifty miles in thickness. 

 It seems that the young earth had no atmosphere, and that ages 

 passed before water began to accumulate on its surface before, 

 in other words, there was any hydrosphere. The water came 

 from the earth itself, to begin with, and it was long before there 

 was any rain dissolving out saline matter from the exposed rocks 

 and making the sea salt. The weathering of the high grounds 

 of the ancient crust by air and water furnished the material which 

 formed the sandstones and mudstones and other sedimentary 

 rocks, which are said to amount to a thickness of over fifty miles 

 in all. 



3 



Making a Home for Life 



It is interesting to inquire how the callous, rough-and- 

 tumble conditions of the outer world in early days were replaced 

 by others that allowed of the germination and growth of that 



