RIVER OF LIFE 53 



sea and the land. Climbing perches are leaving the streams 

 for the air, and tropical crabs have learned to climb trees. But 

 the great evolutionary changes of the future will pursue their 

 courses over enormous periods of time, just as the great 

 changes of the past have taken million of years to complete. 

 The rest of our story is concerned with the legacies of the past 

 and with the brief second of time which is the present. 



The marine sediments in which we have thus traced the 

 River of Life bring us legacies of more immediate practical 

 value than fossils and the story of evolution we read from them. 

 Along the ancient margins and shallow continental extensions 

 of the seas into which the eddies of the River flowed in the 

 dim past there were plants and animals which lived and died 

 and left behind their chemical traces. These traces, vastly 

 altered, provide us with the fuel whereby the ships and air- 

 planes of today are propelled across the waters of the River. 

 The origin of oil-bearing rocks, the precise alchemy of nature 

 that transformed the original remains of plants and animals 

 into oil, and the ways in which this oil became concentrated 

 in its underground reservoirs are still not fully understood, 

 despite the great economic importance of the problem and the 

 considerable amount of scientific research that has been de- 

 voted to it over the past century. Nevertheless the problem is 

 gradually yielding to investigation, and we may be fairly sure 

 of the broad outlines if not the details of these infinitely slow 

 processes that bring us fuel and energy from the past. 



Between the great mountain-building periods, when branches 

 and eddies of the River circulated over the continents, the 

 future oil-bearing sediments were laid down in the shallow 

 seas. Trapped in the muds and sands of the sediments, the 

 dead bodies of plants and animals that lived and died in the 

 waters above were gradually buried under thousands of feet of 

 rock, where a long series of chemical changes converted them 

 to petroleum. Strangely enough, it is not from fishes, giant 

 reptiles, voracious squids, and belemnites that we believe the 



