The History of Things 63 



in a century or a foot in ten centuries), and the 

 answer varied from twenty-six millions to six 

 hundred and eighty millions of years. They tried 

 other methods, such as computing the time re- 

 quired by the sea to become as salt as it is, and 

 they reached other results. The biologists also 

 had their ringer in the pie, and made a modest de- 

 mand for a slice of time sufficient to account for 

 the evolution of living creatures, which some sup- 

 posed would require a hundred million years, and 

 others more, and others less. In short, both ge- 

 ologists and biologists drew without stint upon the 

 bank of time, until the physicists reminded them 

 that their credit was not quite unlimited. Argu- 

 ing from the rate of cooling of the earth and sun 

 and other insecure data, the physicists, notably 

 Professor Tait and Lord Kelvin, refused to allow 

 more than ten to twenty millions of years. Under 

 pressure, the grant was afterwards increased to 

 forty or even a hundred millions, which showed 

 how flexible the calculations were. Within the 

 last few years, however, since the discovery of 

 radio-activity, since it became known that the 

 earth is not self-cooling, but self-heating, the 

 physicists have become willing to grant the ge- 

 ologists and biologists as much time as they want, 

 say a thousand million years! All this uncer- 

 tainty has been mainly due to the insecure data, 

 which no amount of sound mathematics and ac- 



