266 The Outline of Science 



val stuff from which they were all originally derived infinitely 

 long ago? Is there an evolution in the inorganic world which 

 may be going on, parallel to that of the evolution of living things; 

 or is organic evolution a continuation of inorganic evolution? 

 We have seen what evidence there is of this inorganic evolution 

 in the case of the stars. We cannot go deeply into the matter 

 here, nor has the time come for any direct statement that can 

 be based on the findings of modern investigation. Taking it 

 altogether the evidence is steadily accumulating, and there are 

 authorities who maintain that already the evidence of inorganic 

 evolution is convincing enough. The heavier atoms would ap- 

 pear to behave as though they were evolved from the lighter. 

 The more complex forms, it is supposed, have evolved from the 

 simpler forms. Moseley's discovery, to which reference has been 

 made, points to the conclusion that the elements are built up one 

 from another. 



8 



Other New Views 



We may here refer to another new conception to which the 

 discovery of radio-activity has given rise. Lord Kelvin, who 

 estimated the age of the earth at twenty million years, reached 

 this estimate by considering the earth as a body which is grad- 

 ually cooling down, "losing its primitive heat, like a loaf taken 

 from the oven, at a rate which could be calculated, and that the 

 heat radiated by the sun was due to contraction." Uranium and 

 radio-activity were not known to Kelvin, and their discovery 

 has upset both his arguments. Radio-active substances, which 

 are perpetually giving out heat, introduce an entirely new factor. 

 We cannot now assume that the earth is necessarily cooling down ; 

 it may even, for all we know, be getting hotter. At the 1921 

 meeting of the British Association, Professor Rayleigh stated that 

 further knowledge had extended the probable period during 

 which there had been life on this globe to about one thousand 



