Periodic Mutations 711 



ation of tlie secular cooling of the earth, as de- 

 duced from the increasing temperature in deep 

 mines, he concluded that the entire age of the 

 earth must have been more than twenty and less 

 than forty millions of years, and probably much 

 nearer twenty than forty-. His views have been 

 much criticised by other physicists, but in the 

 main they have gained an ever-increasing sup- 

 port in the way of evidence. New mines of 

 greater depth have been bored, and their tem- 

 peratures have proved that the figures of Lord 

 Kelvin are strikingly near the truth. George 

 Darwin has calculated that the separation of 

 the moon from the earth must have taken place 

 some fifty-six millions of years ago. Geikie has 

 estimated the existence of the solid crust of the 

 earth at the most as a hundred million years. 

 The first appearance of the crust must soon have 

 been succeeded by the formation of the seas, 

 and a long time does not seem to have been re- 

 quired to cool the seas to such a degree that life 

 became possible. It is very probable that life 

 originally commenced in the great seas, and that 

 the forms which are now usually included in the 

 plankton or floating-life included the very first 

 living beings. According to Brooks, life must 

 have existed in this floating condition during 

 long primeval epochs, and evolved nearly all the 

 main branches of the animal and vegetable king- 



