ORIGIN AND DEVELOPMENT OF THE EARTH - 



oceans, and thus the hydrosphere became fully developed. 

 At the same time the erosion of the uplands commenced and 

 sediments began to accumulate in the depressions. 



With the establishment of these great processes, suitable 

 heat, light, moisture, air, and all the other conditions which 

 seem necessary for the existence of life were present, and life 

 probably began. But the origin of the first living things is 

 still among the unsolved problems of science. There is, 

 however, every reason to believe that, whatever their origin, 

 the earliest forms of life were very simple, and probably more 

 like the lowest plants of to-day than like animals. From 

 these early forms all later kinds are thought to have been 

 derived by a vast number of slow changes, probably occupying 

 many tens of millions of years. 



Volcanoes probably appeared on the earth at a compara- 

 tively early stage in its history, long before it had grown to the 

 size of the moon. There is reason to believe that volcanic 

 action gradually increased in prominence and reached its 

 climax after the earth had attained its present size. 



The two theories compared. In many respects these 

 two theories of the earth's origin are directly opposed to each 

 other. Under the Laplacian view the planet was at first 

 larger and hotter than now, and it continually cooled and con- 

 tracted until it became partly or entirely solid. Under the 

 planetesimal theory the earth grew larger by gathering in 

 material from outside, and it was not necessarily ever hotter 

 than now, if as hot. Under the first, the atmosphere has 

 become thinner and poorer, from the time when it was exceed- 

 ingly heavy, dense, and composed largely of steam ; under 

 the second it has grown larger and richer in material, and 

 it was never hot. According to the Laplacian theory the 

 oldest rocks which we might hope to find would be entirely 

 igneous, portions of the original crust which coated the 

 surface of the molten globe ; but under the later hypothesis, 

 we should expect not only the igneous materials derived from 

 volcanic eruptions, but sedimentary deposits as well, in the 



