244 READINGS IN BIOLOGICAL SCIENCE 



since neolithic man started civilization on the flood plains of the Nile and 

 in Mesopotamia. Other times have been worse than this, bad as it is. On 

 the whole there has been advance. 



We need a longer perspective even than that of the last six thousand 

 years, a perspective that science alone can give. The master historian is 

 the geologist; he deals with time on a scale which dwarfs ordinary history. 

 To him, as to the Creator, a thousand years are but as yesterday when it 

 is past and as a watch in the night. He speaks of a hundred million years 

 with the same nonchalance that the ordinary historian displays in handling 

 centuries; and if he misses by a million or even ten millions when he is 

 talking big, he sees no need of apologizing. 



That the earth is old, very old, has been known for the last two centuries, 

 ever since the birth of geology as a science. But how old in years? Re- 

 cently the discovery of radioactivity has led to a new and apparently 

 reasonably accurate method of estimating geological time. Uranium by 

 the loss of helium passes into lead. The rate of such loss has been deter- 

 mined in the laboratory. By comparing the amounts of uranium and 

 uranium-derived lead in certain granites we can find the age of the granites, 

 and a minumum age for the sedimentary rocks in which they have been 

 intruded. Thus are obtained the data for the following table, which shows 

 the periods into which the geologist divides the past history of the earth, 

 with their respective lengths: 



These figures are approximations at best; still, we shall probably not go 

 far wrong in fitting our thinking to this schedule. It gives us some two 

 billion years for the age of the earth as recorded in the rocks. Back of 

 that is an indefinitely long, early planetary period of which the geologist 

 has no record. 



II 



Time as mere duration is uninteresting. It is what happens in time that 

 matters. The length of geological time has been emphasized not on its own 

 account, but because probably during the whole of that time there has 

 been life on the earth, slowly, very slowly developing into the variety 



