26 HUMAN BIOLOGY 



to what they are now. The evolution of living forms has 

 been continuous, with no trace of a serious general set-back. 

 There have been recurrent epochs of glaciation; but at no 

 time is there evidence that a frigid climate prevailed over 

 the whole world; and it is quite certain that the temperature 

 never rose anywhere near the boiling point, even for a single 

 year. The Sun's radiation of heat, upon which the Earth's 

 surface temperature wholly depends, must have been remark- 

 ably constant all through this long interval. 



But radioactivity tells us more than this about the 

 history of our planet, it enables us to set an upper hmit, 

 as well as a lower, to its age. Uranium still exists in the 

 Earth's crust, widely disseminated in the rocks, though 

 in very small quantities, averaging about one part in 140,000 

 by weight. 



There must have been more uranium in the past, and, 

 in place of that which has changed, we should find an 

 equivalent amount of lead. Now the amount of lead which 

 is now present in the crust is approximately thrice that 

 of the uranium. Aston's recent separation of common lead 

 into isotopes shows that about 30 per cent of this lead has 

 the atomic weight 206. If all the lead of this sort has arisen 

 from disintegration of uranium, a simple calculation shows 

 that the earth would be 4,800,000,000 years old, and to 

 account for any considerably greater age appears to be 

 difficult. 



This seems at first sight perilously like an attempt to 

 date the Creation. But it dates not the origin of matter, 

 but the formation of our planet, of which we will presently 

 speak, pausing only to note with what relatively narrow 

 limits the age of the Earth now appears to be definable, 

 the upper limit being less than three times the lower. Future 

 discoveries may, of course, give reason to change these 

 estimates; but, in the present state of knowledge, the 

 conclusion that the Earth is more than two and less than 

 five billions of years of age appears to be trustworthy. 



The origin of the solar system next requires consideration, 

 and the limitations of space compel an unwelcome brevity. 

 Our system possesses an extraordinary dynamical pecu- 

 liarity. More than 98 per cent of the angular momentum 



